presentation of digestive system

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Human digestive system interactive

The lips and cheeks, the roof of the mouth, the floor of the mouth.

• Salivary glands
• Blood and nerve supply
• Stomach contractions
• Gastric mucosa
• Gastric secretion
• Absorption and emptying
• Contractions and motility
• Rectum and anus
• Gross anatomy
• Microscopic anatomy
• Acinar cells
• Islet cells
• General features of digestion and absorption
• Carbohydrates
• Fat-soluble vitamins
• Vitamin B 12
• Intestinal gas
• Production and secretion of peptides
• Somatostatin
• Cholecystokinin
• Gastric inhibitory peptide
• Intestinal glucagon
• Neurotensin
• Pancreatic polypeptide
• Vasoactive intestinal peptide
• Substance P
• Endorphins and enkephalins
• Prostaglandins
• The gastrointestinal tract as an organ of immunity
• Embryonic development
• Evolutionary development

human digestive system

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• Mayo Clinic - See How Your Digestive System Works
• Healthline - Digestive Health Basics
• LiveScience - Digestive System: Facts, Function and Diseases
• Patient - The digestive system
• Cleveland Cliinic - Digestive System
• National Institute of Diabetes and Digestive and Kidney Diseases - Your Digestive System & How it Works
• WebMD - The Digestive System: How It Works
• University of Minnesota Libraries - Human Biology - The Digestive System
• digestive system - Children's Encyclopedia (Ages 8-11)
• digestive system - Student Encyclopedia (Ages 11 and up)
• Table Of Contents

• What are the main organs involved in the human digestive system?
• How does the process of digestion begin in the mouth?
• What role does the stomach play in digestion?
• How do the small and large intestines contribute to the digestive process?
• How does the digestive system interact with other body systems?
• What is the function of the liver in the digestive process?
• How does the pancreas contribute to digestion?
• What are common digestive disorders and their causes?
• How does the nervous system regulate digestion?

human digestive system , system used in the human body for the process of digestion . The human digestive system consists primarily of the digestive tract , or the series of structures and organs through which food and liquids pass during their processing into forms that can be absorbed into the bloodstream. The system also consists of the structures through which wastes pass in the process of elimination and of organs that contribute juices necessary for the digestive process.

In order to function properly, the human body requires nutrients. Some such nutrients serve as raw materials for the synthesis of cellular materials, while others help regulate chemical reactions or, upon oxidation, yield energy. Many nutrients, however, are in a form that is unsuitable for immediate use by the body; to be useful, they must undergo physical and chemical changes, which are facilitated by digestion.

Structures and functions of the human digestive system

The digestive tract begins at the lips and ends at the anus . It consists of the mouth , or oral cavity, with its teeth , for grinding the food, and its tongue , which serves to knead food and mix it with saliva ; the throat, or pharynx ; the esophagus ; the stomach ; the small intestine , consisting of the duodenum , the jejunum, and the ileum ; and the large intestine , consisting of the cecum , a closed-end sac connecting with the ileum, the ascending colon, the transverse colon, the descending colon, and the sigmoid colon , which terminates in the rectum . Glands contributing digestive juices include the salivary glands , the gastric glands in the stomach lining, the pancreas , and the liver and its adjuncts—the gallbladder and bile ducts. All of these organs and glands contribute to the physical and chemical breaking down of ingested food and to the eventual elimination of nondigestible wastes. Their structures and functions are described step by step in this section.

Mouth and oral structures

Little digestion of food actually takes place in the mouth. However, through the process of mastication , or chewing, food is prepared in the mouth for transport through the upper digestive tract into the stomach and small intestine, where the principal digestive processes take place. Chewing is the first mechanical process to which food is subjected. Movements of the lower jaw in chewing are brought about by the muscles of mastication (the masseter, the temporal , the medial and lateral pterygoids, and the buccinator). The sensitivity of the periodontal membrane that surrounds and supports the teeth, rather than the power of the muscles of mastication, determines the force of the bite.

Mastication is not essential for adequate digestion. Chewing does aid digestion, however, by reducing food to small particles and mixing it with the saliva secreted by the salivary glands. The saliva lubricates and moistens dry food, while chewing distributes the saliva throughout the food mass. The movement of the tongue against the hard palate and the cheeks helps to form a rounded mass, or bolus , of food.

The lips, two fleshy folds that surround the mouth, are composed externally of skin and internally of mucous membrane , or mucosa. The mucosa is rich in mucus-secreting glands, which together with saliva ensure adequate lubrication for the purposes of speech and mastication.

The cheeks, the sides of the mouth, are continuous with the lips and have a similar structure. A distinct fat pad is found in the subcutaneous tissue (the tissue beneath the skin) of the cheek; this pad is especially large in infants and is known as the sucking pad. On the inner surface of each cheek, opposite the second upper molar tooth, is a slight elevation that marks the opening of the parotid duct, leading from the parotid salivary gland, which is located in front of the ear. Just behind this gland are four to five mucus-secreting glands, the ducts of which open opposite the last molar tooth.

The roof of the mouth is concave and is formed by the hard and soft palate . The hard palate is formed by the horizontal portions of the two palatine bones and the palatine portions of the maxillae, or upper jaws. The hard palate is covered by a thick, somewhat pale mucous membrane that is continuous with that of the gums and is bound to the upper jaw and palate bones by firm fibrous tissue. The soft palate is continuous with the hard palate in front. Posteriorly it is continuous with the mucous membrane covering the floor of the nasal cavity. The soft palate is composed of a strong, thin, fibrous sheet, the palatine aponeurosis, and the glossopalatine and pharyngopalatine muscles. A small projection called the uvula hangs free from the posterior of the soft palate.

The floor of the mouth can be seen only when the tongue is raised. In the midline is a prominent, elevated fold of mucous membrane ( frenulum linguae ) that binds each lip to the gums, and on each side of this is a slight fold called a sublingual papilla , from which the ducts of the submandibular salivary glands open. Running outward and backward from each sublingual papilla is a ridge (the plica sublingualis) that marks the upper edge of the sublingual (under the tongue) salivary gland and onto which most of the ducts of that gland open.

The gums consist of mucous membranes connected by thick fibrous tissue to the membrane surrounding the bones of the jaw. The gum membrane rises to form a collar around the base of the crown (exposed portion) of each tooth. Rich in blood vessels, the gum tissues receive branches from the alveolar arteries; these vessels, called alveolar because of their relationship to the alveoli dentales, or tooth sockets, also supply the teeth and the spongy bone of the upper and lower jaws, in which the teeth are lodged.

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Digestive system

Author: Lorenzo Crumbie, MBBS, BSc • Reviewer: Francesca Salvador, MSc Last reviewed: November 02, 2023 Reading time: 30 minutes

Extending from the mouth to the anus , the digestive tract is one of the largest systems in the human body. It contains organs that regulate food intake, its digestion and absorbtion of the useful materia that it contains. In addition to this, the digestive system also eliminates the waste products from food and products from various endogenous metabolic processes.

In a nutshell, the digestive tract has the tumultuous responsibility of converting large chunks of food into their constituent micro-molecules that will subsequently be used to build and repair the body.

This article will discuss the anatomy of the digestive system.

To improve your understanding of digestive system terminology, take a closer look at some commonly used roots, prefixes and suffixes in the digestive system in this video:

Trigger and initiation

Mastication, swallowing and physical digestion, chemical digestion, buccal cavity, bowel mnemonic, salivary glands, gallbladder, arterial supply, innervation, venous drainage, examination of the digestive system.

The digestive system can be broken down into two major components:

• There is the primary digestive tract , which functions mainly as a conduit and storage pathway. This portion is needed in order to move food contents along the tract (peristalsis) so that absorption of nutrients and excretion of undigested substances can occur. The tract also allows for segmentation of food at different stages of digestion. This is important because some enzymes produced in one part of the tract (e.g. pepsin in the stomach) will not function optimally in another part of the tract (e.g. the jejunum).
• The other component is the accessory digestive tract . This group of organs are responsible for the synthesis and secretion enzymes to facilitate chemical digestion.

This article aims to give an overview of the anatomy and function of the digestive tract. Details regarding neurovascular supply, histology , and clinically significant points regarding each segment of the digestive tract can be found in the respective articles. It will also briefly mention a few elements of the abdominal examination.

Give it a shot and test yourself on the main organs of the digestive system!

Functions of the digestive system

The function of the digestive system truly begins within the brain . Whenever the body’s energy stores (i.e. blood glucose, protein, or fat stores) fall below a set point, the hunger centres of the hypothalamus are activated. These centres regulate satiety (fullness) and appetite in order to maintain energy homeostasis. This signals to the brain that there is a need to obtain food. Keep in mind, however, that this is not the only source of hunger stimulation, as there are elements of pleasure and comfort that can be derived from ingesting food.

Monogastric organisms like humans have two kinds of digestive processes occurring in the digestive tract – mechanical and chemical digestion. Once the desired food is obtained, the digestive process begins in the mouth with mechanical digestion . Here the teeth are used to cut, tear, and grind chunks of food into smaller particles. This process of mastication involves the alternating action of the muscles of mastication (namely, the superficial and deep masseter , the pterygoids , and the temporalis muscles ).

Mastication is actually a reflex action that is stimulated once food is present in the mouth. At that point, there is inhibition of the muscles of mastication which results in a fall of the mandible. This causes distention of the muscles of mastication, resulting in reflex contraction of the muscle fibres; thus raising the mandible. That action causes apposition of the upper and lower rows of teeth, crushing the food that is between them. The cycle is repeated until the food particles can be rolled into a bolus .

As the tongue rolls the chewed food into a bolus, the salivary glands secrete saliva to moisten the bolus in order for it to pass smoothly to the stomach. Also note that some amount of chemical digestion also occurs in the mouth, as saliva contains the enzyme amylase , which breaks down some carbohydrates in the mouth.

Once the bolus is prepared, then swallowing will be initiated. This is another intricate reflex arc that involves the action of afferent and efferent tracts of several cranial nerves being relayed to and from the nucleus tractus solitarius and nucleus ambiguus of the brainstem .

The end result is that these nerves send motor signals to the tongue, which moves the bolus against the hard and soft palate, then into the oropharynx (which is also under brainstem regulation). The bolus then continues inferiorly towards the laryngopharynx and the swallowing reflex is initiated at the esophagus . All the actions of swallowing up to this point were under voluntary control ; however, the rest of the action is carried out by involuntary peristaltic contractions that travel in a craniocaudal fashion. At the level of the pharynx:

• Superiorly, the bolus is prevented from entering the nasopharynx by the actions of Passavant’s ridge . This structure is formed by the joint actions of the palatopharyngeal sphincters, the superior constrictor muscles, salpingopharyngeus and the muscles of the soft palate.
• The epiglottis closes off the larynx to prevent food from entering the airway. The vocal cords are also adducted as an additional protective measure.

At the level of the esophagus :

• There is relaxation of the cricopharyngeal sphincter and the bolus enters the proximal esophagus. The presence of the bolus causes distention of the myenteric plexus within the walls of the esophagus, initiating the primary esophageal peristaltic wave.
• The continued presence of the food stimulates the secondary peristaltic waves in a craniocaudal direction.

These waves, along with the action of gravity, move the bolus toward the lower esophageal sphincter at a rate of 4 cm/s. At rest, the sphincter has a high tone. However, the presence of the bolus aids relaxation of the lower esophageal sphincter, and food is able to enter the stomach. Here, the majority of the chemical digestion will take place.

Once the bolus enters the stomach , there is regulated release of a variety of enzymes that facilitate chemical digestion. Some of these enzymes also stimulate the accessory digestive organs to release their enzymes to aid in digestion. In addition to chemical digestion (particularly of proteins), the stomach also functions as:

• A storage point , which gradually releases its contents into the small intestines , to allow adequate time for further digestion and absorption.
• A mixer : the mode of contraction and arrangement of the stomach mucosa results in further mixing of the food contents to form chyme.
• A conduit : essentially passing food from the esophagus to the small intestines.
• Immunological defence : the acidic pH of the stomach aids in dissolving invading pathogens before they are able to cause an infection.
• Other micronutrients : iron (Fe), vitamin B12 ,and folate absorption are heavily regulated by the stomach.

The stomach can be functionally divided into proximal and distal motor pumps, which stores food content and pumps chyme along the conduit, respectively. Portions of chyme are passed into the pylorus and into the small intestines .

Once chyme has entered the first part of the duodenum, it activates the neurohormonal axis which promotes the release of bile (from the liver and gallbladder ) and other enzymes from the pancreas. The peristaltic waves continue to move the chyme along the intestinal tract. The intricate folding of the intestines facilitates absorption of nutrients from the chyme. Majority of the nutrients are absorbed within the small intestines. The remnants are passed through the unidirectional ileocecal valve into the cecum.

As the peristaltic waves continue into the colon, the chyme continues to move along the tract. Further absorption of electrolytes and water from the remaining chyme occurs and the chyme is then converted into stool, which is stored in the rectum. As the rectum becomes distended, the stretch receptors signal to the brain that promotes defecation . While the internal anal sphincters are under autonomic regulation, the external anal sphincters are under voluntary control. Therefore the individual may resist the urge to defecate until an appropriate time and place is identified.

Organs of the digestive system

The digestive tract is also referred to as the alimentary canal. It is a tubular continuum that is segmented into dilated regions. These dilated regions are often separated by thickened regions of the wall that form sphincters . This prevents unintended mixing of the contents in the respective segments.

The buccal cavity marks the opening of the digestive tract. It is made up by the oral vestibule (space between the inner cheeks and the teeth) and the oral cavity proper (behind the teeth). The buccal cavity also contains the teeth and tongue. It is limited anteriorly by the lips and teeth and posterolaterally by the palatopharyngeal arches. Posteriorly the buccal cavity opens into the oropharynx. The roof is formed by the hard palate (at the anterior two-thirds) and the soft palate (at the posterior third), and the floor of the oral cavity also contains the tongue.

The teeth can be subdivided into incisors (designed for cutting), canines (designed for tearing), premolars , and molars  (designed for grinding). The molars and premolars have complementary surfaces that also aid in grinding. The tongue consists of intrinsic and extrinsic muscles. It is also populated  with taste buds that facilitate gustatory sensation.

Posterior to the oral cavity proper is the oropharynx . This is the middle part of the pharynx that communicates superiorly with the nasopharynx and inferiorly with the laryngopharynx. The walls of the oropharynx are formed by the superior and middle pharyngeal constrictor muscles . Anterolaterally, the palatopharyngeal folds form a demarcation between the oral cavity proper and the oropharynx.

The base of the tongue also serves as another landmark in the anteroinferior aspect of the oropharynx. The mucosa of the walls also contains numerous mucosa associated lymphatic tissue (MALT) . It can be separated from the nasopharynx by the muscles of Passavant’s ridge and the supporting structures of the soft palate during deglutition.  

The tubular conduit responsible for transferring food from the oropharynx to the stomach is the esophagus. It can be divided into three parts:

• abdominal components

The total length of the muscular tube is 25 cm, commencing at the cricopharyngeus at the lower border of the cricoid cartilage at the sixth cervical vertebrae (C6) . It journeys posteriorly to the trachea in the neck along its caudal course. It then travels through the superior, then posterior mediastinum alongside the thoracic vertebrae. It pierces the diaphragm at the tenth thoracic vertebra (T10). The remaining 2.5 cm of the esophagus is the abdominal part. It transitions into the stomach at the gastroesophageal junction, where the physiological lower esophageal sphincter exists.

Within the abdominal cavity, the esophagus enters the stomach. This is a dilated area of the alimentary canal that participates in both mechanical and chemical digestion. It is divided into four main parts, namely the:

Laterally, there is a greater curvature , and medially there is a lesser curvature . There are two distinct notches on the stomach. The first is the cardiac incisura formed at the lateral border of the cardioesophageal junction. The other is the less pronounced incisura angularis found distally at the caudal end of the lesser curvature (at the pyloric antrum). The pylorus is the distal aspect of the stomach that is thickened. It acts as a physiological sphincter that regulates the passage of chyme from the stomach to the beginning of the small intestines.

The duodenum marks the beginning of the small intestines. It is roughly 20 – 25 cm long, extending from the pylorus to the ligament of Treitz . Not only is the duodenum the shortest part of the small intestines, but it is also the widest. It can be subdivided into four parts based on its geometrical orientation. The first part is known as pars superioris (the superior part); it is roughly 2 – 3 cm long and travels above the head of the pancreas. The second part is pars descendens which commences behind the neck of the gallbladder. It travels about 8 – 10 cm lateral to the head to the pancreas.

The inferior duodenal flexure (where pars descendens begins to turn) marks the transition of the second part of the duodenum to the third part – pars horizontalis . It travels for roughly 10 cm before it begins to curve upwards into the final segment of the duodenum, the pars ascendens (which is only 2.5 cm long).

The transition from the duodenum to jejunum occurs at the ligament of Treitz . The difference in the luminal diameter of the jejunum and duodenum is an important distinguishing feature. The duodenum is significantly wider than the jejunum.

However, it may be more challenging to distinguish between the jejunum and ileum as there are no external anatomical landmarks to guide. The key distinguishing features are as follows:

• The external diameter of the jejunum (4 cm) is greater than that of the ileum (3.5 cm).
• The internal diameter of the ileum (2 cm) is also smaller than that of the jejunum (2.5 cm).
• The walls of the jejunum appear thicker than that of the ileum.
• Additionally, the jejunum appears more hyperaemic than the ileum because it has a more extensive vascular supply.
• The luminal surface of the jejunum is significantly folded into plicae circulares that are more numerous and appear deeper than anywhere else within the digestive tract. Furthermore, the plicae circulares become less abundant distally within the ileum.
• Finally, the luminal mucosa of the ileum has more prominent mucosa-associated lymphoid tissue (MALT) than the jejunum.

The ileum terminates at the ileocecal valve ; which marks the transition from the small intestines to the large intestines . The ileocecal valve is a one way structure that prevents reflux of the bolus from the large intestines to the small intestines.

The proximal end of the large intestines – also known as the colon – is formed by a dilated cul-de-sac known as the cecum . There is also a vermiform appendix attached at variable parts of the cecum. The colon is divided into:

• sigmoid parts

Externally, the colon has a segmented appearance due to the haustrations that are present on the luminal surface of the conduit. The muscular layers of the colon are concentrated into three muscular bands known as taenia coli ; which travel along the length of the colon. The three taenia are:

• Taenia libera is the free taenia that is found at the antimesenteric surface of the colon; which is located on the anterior surface of the colon.
• Taenia omentalis is located posterolaterally and is attached to the omentum of the large intestines.
• Taenia mesocolic is found at the midpoint between the taenia libera and the mesenteric attachment on the colon.

Importantly, the cecum does not have the prominent haustrations seen on the rest of the colon. As the ascending colon travels from the right iliac fossa superiorly, it transitions to the transverse colon at the hepatic (right colic) flexure. The transverse colon travels across from the left to the right hypochondriac regions . It turns caudally at the splenic (left colic) flexure to form the descending colon . As the descending colon travels from the left hypochondrium to the left iliac fossa, it transitions into the sigmoid colon . This distal segment enters the pelvic inlet and terminates at the rectosigmoid junction at the level of the third sacral vertebra (S3).

Unlike the preceding colon, the rectum is circumferentially walled by smooth muscles . It does not have haustrations and is devoid of taenia coli. This distal continuation of the large intestines functions of a reservoir for stool, prior to excretion. It terminates at the level of the sacrococcygeal curvature . It passes over the pelvic diaphragm to form the anorectal junction .

The portion of the alimentary tract between the stomach and the anal canal is known as  bowel  and is formed of the following components:

You can remember this easily if you use the mnemonic  D ow J ones I ndustrial A verage C losing S tock R eport, where the first letter of each word corresponds to the first letter of a bowel component!

The final passageway through which undigested food and exfoliated mucosa will exit the body is called the anal canal. It continues from the anorectal junction and passes through the loop formed by the puborectalis muscle, which swings the anal canal anteriorly. Distally, the mucosa of the anal canal transitions from the columnar epithelium with goblet cells found throughout the colon, to the squamous epithelium of the perianal skin. This point is referred to as the anal verge .  

Organs of the accessory digestive system

The organs of the accessory digestive system have the principal role of synthesizing and secreting digestive enzymes to further break down food into nutrients. The salivary glands are paired structures in the oral cavity that secrete saliva and other enzymes that mix with the masticated food to form the bolus. There are three major salivary glands in the oral cavity:

• Parotid glands
• Submandibular glands
• Sublingual glands

Another important accessory digestive organ is the liver. It is located in the right upper quadrant of the abdomen , beneath the right hemidiaphragm. The liver has two anatomical lobes, but eight functional segments. In addition to producing bile to digest fats, all of the nutrients absorbed from the small intestines enter the liver via the hepatic portal venous system .

From the liver, the nutrients are integrated into various catabolic processes and sent throughout the body. Most of the bile made by the liver is stored in the gallbladder . This muscular, sac-like organ that resides on the posterior surface of the liver drains its contents into the extrahepatic biliary tree following a fatty meal.

Finally, the pancreas is a retroabdominal organ that also provides enzymes for digestion. The head of the pancreas is found within the  C-shaped loop of the duodenum. The body extends superolaterally, behind the gastric antrum. The tail of the pancreas terminates at the hilum of the spleen .

Neurovascular supply and venous drainage

The vast majority of the digestive system is supplied by the abdominal aorta . In the abdominal cavity, the aorta gives off three major branches that, in succession, supply the derivatives of the foregut, midgut and hindgut:

• Celiac trunk , which after a short course further splits three into major branches; left gastric , common hepatic and splenic arteries . Via these branches, the celiac trunk supplies the foregut, including the abdominal part of the esophagus, stomach, upper 1/3 of the duodenum, liver, spleen, and pancreas.
• Superior mesenteric artery , which gives off the inferior pancreaticoduodenal , middle colic , right colic , ileocolic arteries , jejunal and ileal branches, and the marginal artery of Drummond . These branches supply the midgut, which includes the distal 2/3 of the duodenum, jejunum, ileum, cecum, appendix, ascending colon and proximal 1/3 of the transverse colon.
• Inferior mesenteric artery , which gives off the left colic , sigmoid, and superior rectal arteries . These branches provide blood to the hindgut, supplying the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and anus.

The digestive system receives both sympathetic and parasympathetic innervation. Sympathetic innervation stems from the abdominopelvic splanchnic nerves . These nerves arise from the sympathetic trunk and include the thoracic and lumbar splanchnic nerves . The splanchnic nerves synapse with prevertebral ganglia (celiac, aorticorenal, superior mesenteric, and inferior mesenteric ganglia) which project postganglionic fibers to innervate the organs of the digestive system. Sympathetic innervation inhibits peristalsis, constricts blood vessels and redirects blood from the digestive system to the skeletal muscles and contracts the anal sphincters.

The parasympathetic supply to the digestive system stems from the vagus nerve (CN X) and pelvic splanchnic nerves . The vagus nerve supplies the digestive tract from the esophagus to the transverse colon, while the pelvic splanchnic nerves supply the descending colon, sigmoid colon and rectum. Parasympathetic innervation increases peristalsis, promotes secretion of digestive juices, relaxes the internal anal sphincter and contracts the rectum.

There are two venous systems that drain the organs of the digestive tract; portal venous system and the systemic venous system .

In the portal venous system, the nutrient-rich blood from the digestive tract is drained by the hepatic portal vein into the liver to be filtered and detoxified. The hepatic portal vein is formed by the merger of the  superior mesenteric vein and the splenic vein .

After being processed by the liver, the blood is carried into the systemic venous system by the hepatic veins , which drain into the inferior vena cava .

Gastrointestinal pathology ranks high among the chief presenting complaints of patients presenting to both outpatient and emergency settings. Obtaining adequate historical details will aid the clinician in narrowing down the pathology to a particular part of the digestive system. Once the history is complete, the clinical examination is conducted to confirm, rule-in or rule-out any diagnosis. The examination follows the rules of inspection, palpation, percussion, and auscultation.

Adequate exposure of the patient for an abdominal exam should allow visibility from the nipple line to mid thigh. Inspect the abdomen for:

• distention (either symmetrical or asymmetrical)
• distended superficial veins (caput medusae)
• cutaneous discoloration
• visible peristalsis

On inspection, also ask the patient to cover their mouth and cough while the examiner observes the abdominal wall. A visible cough impulse (localized distention of the abdominal wall due to raised intra-abdominal pressure from coughing) suggests that there is a hernia at that site.

Palpation should be performed to assess for any masses as well as to determine the surface texture of the liver. The examiner may also determine if there is an enlargement of the kidneys or spleen at this time as well. The abdominal cavity has the potential to store a large volume of interstitial fluid. Large volumes of fluid can be detected during palpation by eliciting a fluid thrill. Smaller volumes can be picked up during percussion with the shifting dullness technique. Percussion also helps the examiner to confirm the size and location (if enlarged) of the liver and spleen. Be sure to auscultate the abdomen for bowel sounds and bruits. The abdominal exam is concluded with a digital rectal and genital examination.

References:

• Ahima, Rexford S., and Daniel A. Antwi:  "Brain Regulation Of Appetite And Satiety." Endocrinology And Metabolism Clinics Of North America, vol 37, no. 4, 2008, pp. 811-823. Elsevier BV, doi:10.1016/j.ecl.2008.08.005.
• Burnand, Kevin G, and Norman L Browse:  Browse's Introduction To The Symptoms & Signs Of Surgical Disease. 5th ed., Boca Raton, FL, CRC Press, Taylor & Francis Group, 2015,.
• Guyton, Arthur C, and John E Hall:  Textbook Of Medical Physiology. 11th ed., Philadelphia, PA, Elsevier Saunders, 2007,.
• Standring, S., Borley, N. and Gray, H.:  (2008). Gray's Anatomy. 42nd ed. [Edinburgh]: Churchill Livingstone/Elsevier, pp.1191-1195.

Illustrators:

• Digestive system (Small intestine) - Begoña Rodriguez
• Hypothalamus (Ventromedial nucleus - lateral-left view) - Paul Kim
• Masseter muscle - Paul Kim
• Nucleus ambiguus (lateral-left view) - Paul Kim
• Esophagus (lateral-left view) - Yousun Koh
• Circular layer of muscular coat of stomach (ventral view) - Rebecca Betts
• Duodenum (ventral view) - Samantha Zimmerman
• Oral cavity proper (ventral view) - Paul Kim
• Palatopharyngeal muscle (dorsal view) - Yousun Koh
• Greater curvature of the stomach (ventral view) - Irina Münstermann
• Descending part of duodenum (ventral view) - Begoña Rodriguez
• Jejunum (ventral view) - Irina Münstermann
• Taenia mesocolica (ventral view) - Begoña Rodriguez
• Rectum (ventral view) - Samantha Zimmerman
• Sublingual gland - Paul Kim
• Liver (ventral view) - Begoña Rodriguez
• Duodenum and jejunum during a surgical intervention - Prof. Carlos Suárez-Quian
• Cecum, appendix and ascending colon in a cadaver - Prof. Carlos Suárez-Quian
• Pharynx (cadaveric dissection) - Prof. Carlos Suárez-Quian

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23.1 Overview of the Digestive System

Learning objectives.

By the end of this section, you will be able to:

• Identify the organs of the alimentary canal from proximal to distal, and briefly state their function
• Identify the accessory digestive organs and briefly state their function
• Describe the four fundamental tissue layers of the alimentary canal
• Contrast the contributions of the enteric and autonomic nervous systems to digestive system functioning
• Explain how the peritoneum anchors the digestive organs

The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process ( Figure 23.2 ).

As is the case with all body systems, the digestive system does not work in isolation; it functions cooperatively with the other systems of the body. Consider for example, the interrelationship between the digestive and cardiovascular systems. Arteries supply the digestive organs with oxygen and processed nutrients, and veins drain the digestive tract. These intestinal veins, constituting the hepatic portal system, are unique; they do not return blood directly to the heart. Rather, this blood is diverted to the liver where its nutrients are off-loaded for processing before blood completes its circuit back to the heart. At the same time, the digestive system provides nutrients to the heart muscle and vascular tissue to support their functioning. The interrelationship of the digestive and endocrine systems is also critical. Hormones secreted by several endocrine glands, as well as endocrine cells of the pancreas, the stomach, and the small intestine, contribute to the control of digestion and nutrient metabolism. In turn, the digestive system provides the nutrients to fuel endocrine function. Table 23.1 gives a quick glimpse at how these other systems contribute to the functioning of the digestive system.

Digestive System Organs

The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body. Accessory digestive organs, despite their name, are critical to the function of the digestive system.

Alimentary Canal Organs

Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = “to nourish”) is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the alimentary canal is to nourish the body. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. Both the mouth and anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and nourish the body’s “inner space.”

Accessory Structures

Each accessory digestive organ aids in the breakdown of food ( Figure 23.3 ). Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. Together, these are called accessory organs because they sprout from the lining cells of the developing gut (mucosa) and augment its function; indeed, you could not live without their vital contributions, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts.

Histology of the Alimentary Canal

Throughout its length, the alimentary tract is composed of the same four tissue layers; the details of their structural arrangements vary to fit their specific functions. Starting from the lumen and moving outwards, these layers are the mucosa, submucosa, muscularis, and serosa, which is continuous with the mesentery (see Figure 23.3 ).

The mucosa is referred to as a mucous membrane, because mucus production is a characteristic feature of gut epithelium. The membrane consists of epithelium, which is in direct contact with ingested food, and the lamina propria, a layer of connective tissue analogous to the dermis. In addition, the mucosa has a thin, smooth muscle layer, called the muscularis mucosae (not to be confused with the muscularis layer, described below).

Epithelium —In the mouth, pharynx, esophagus, and anal canal, the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen, the space inside the alimentary canal. Interspersed among its epithelial cells are goblet cells, which secrete mucus and fluid into the lumen, and enteroendocrine cells, which secrete hormones into the interstitial spaces between cells. Epithelial cells have a very brief lifespan, averaging from only a couple of days (in the mouth) to about a week (in the gut). This process of rapid renewal helps preserve the health of the alimentary canal, despite the wear and tear resulting from continued contact with foodstuffs.

Lamina propria —In addition to loose connective tissue, the lamina propria contains numerous blood and lymphatic vessels that transport nutrients absorbed through the alimentary canal to other parts of the body. The lamina propria also serves an immune function by housing clusters of lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte clusters are particularly substantial in the distal ileum where they are known as Peyer’s patches. When you consider that the alimentary canal is exposed to foodborne bacteria and other foreign matter, it is not hard to appreciate why the immune system has evolved a means of defending against the pathogens encountered within it.

Muscularis mucosae —This thin layer of smooth muscle is in a constant state of tension, pulling the mucosa of the stomach and small intestine into undulating folds. These folds dramatically increase the surface area available for digestion and absorption.

As its name implies, the submucosa lies immediately beneath the mucosa. A broad layer of dense connective tissue, it connects the overlying mucosa to the underlying muscularis. It includes blood and lymphatic vessels (which transport absorbed nutrients), and a scattering of submucosal glands that release digestive secretions. Additionally, it serves as a conduit for a dense branching network of nerves, the submucosal plexus, which functions as described below.

The third layer of the alimentary canal is the muscularis (also called the muscularis externa). The muscularis in the small intestine is made up of a double layer of smooth muscle: an inner circular layer and an outer longitudinal layer. The contractions of these layers promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal. In the most proximal and distal regions of the alimentary canal, including the mouth, pharynx, anterior part of the esophagus, and external anal sphincter, the muscularis is made up of skeletal muscle, which gives you voluntary control over swallowing and defecation. The basic two-layer structure found in the small intestine is modified in the organs proximal and distal to it. The stomach is equipped for its churning function by the addition of a third layer, the oblique muscle. While the colon has two layers like the small intestine, its longitudinal layer is segregated into three narrow parallel bands, the tenia coli, which make it look like a series of pouches rather than a simple tube.

The serosa is the portion of the alimentary canal superficial to the muscularis. Present only in the region of the alimentary canal within the abdominal cavity, it consists of a layer of visceral peritoneum overlying a layer of loose connective tissue. Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia. These tissues serve to hold the alimentary canal in place near the ventral surface of the vertebral column.

Nerve Supply

As soon as food enters the mouth, it is detected by receptors that send impulses along the sensory neurons of cranial nerves. Without these nerves, not only would your food be without taste, but you would also be unable to feel either the food or the structures of your mouth, and you would be unable to avoid biting yourself as you chew, an action enabled by the motor branches of cranial nerves.

Intrinsic innervation of much of the alimentary canal is provided by the enteric nervous system, which runs from the esophagus to the anus, and contains approximately 100 million motor, sensory, and interneurons (unique to this system compared to all other parts of the peripheral nervous system). These enteric neurons are grouped into two plexuses. The myenteric plexus (plexus of Auerbach) lies in the muscularis layer of the alimentary canal and is responsible for motility , especially the rhythm and force of the contractions of the muscularis. The submucosal plexus (plexus of Meissner) lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food (see Figure 23.3 ).

Extrinsic innervations of the alimentary canal are provided by the autonomic nervous system, which includes both sympathetic and parasympathetic nerves. In general, sympathetic activation (the fight-or-flight response) restricts the activity of enteric neurons, thereby decreasing GI secretion and motility. In contrast, parasympathetic activation (the rest-and-digest response) increases GI secretion and motility by stimulating neurons of the enteric nervous system.

Blood Supply

The blood vessels serving the digestive system have two functions. They transport the protein and carbohydrate nutrients absorbed by mucosal cells after food is digested in the lumen. Lipids are absorbed via lacteals, tiny structures of the lymphatic system. The blood vessels’ second function is to supply the organs of the alimentary canal with the nutrients and oxygen needed to drive their cellular processes.

Specifically, the more anterior parts of the alimentary canal are supplied with blood by arteries branching off the aortic arch and thoracic aorta. Below this point, the alimentary canal is supplied with blood by arteries branching from the abdominal aorta. The celiac trunk services the liver, stomach, and duodenum, whereas the superior and inferior mesenteric arteries supply blood to the remaining small and large intestines.

The veins that collect nutrient-rich blood from the small intestine (where most absorption occurs) empty into the hepatic portal system. This venous network takes the blood into the liver where the nutrients are either processed or stored for later use. Only then does the blood drained from the alimentary canal viscera circulate back to the heart. To appreciate just how demanding the digestive process is on the cardiovascular system, consider that while you are “resting and digesting,” about one-fourth of the blood pumped with each heartbeat enters arteries serving the intestines.

The Peritoneum

The digestive organs within the abdominal cavity are held in place by the peritoneum, a broad serous membranous sac made up of squamous epithelial tissue surrounded by connective tissue. It is composed of two different regions: the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelopes the abdominal organs ( Figure 23.4 ). The peritoneal cavity is the space bounded by the visceral and parietal peritoneal surfaces. A few milliliters of watery fluid act as a lubricant to minimize friction between the serosal surfaces of the peritoneum.

Disorders of the...

Digestive system: peritonitis.

Inflammation of the peritoneum is called peritonitis. Chemical peritonitis can develop any time the wall of the alimentary canal is breached, allowing the contents of the lumen entry into the peritoneal cavity. For example, when an ulcer perforates the stomach wall, gastric juices spill into the peritoneal cavity. Hemorrhagic peritonitis occurs after a ruptured tubal pregnancy or traumatic injury to the liver or spleen fills the peritoneal cavity with blood. Even more severe peritonitis is associated with bacterial infections seen with appendicitis, colonic diverticulitis, and pelvic inflammatory disease (infection of uterine tubes, usually by sexually transmitted bacteria). Peritonitis is life threatening and often results in emergency surgery to correct the underlying problem and intensive antibiotic therapy. When your great grandparents and even your parents were young, the mortality from peritonitis was high. Aggressive surgery, improvements in anesthesia safety, the advance of critical care expertise, and antibiotics have greatly improved the mortality rate from this condition. Even so, the mortality rate still ranges from 30 to 40 percent.

The visceral peritoneum includes multiple large folds that envelope various abdominal organs, holding them to the dorsal surface of the body wall. Within these folds are blood vessels, lymphatic vessels, and nerves that innervate the organs with which they are in contact, supplying their adjacent organs. The five major peritoneal folds are described in Table 23.2 . Note that during fetal development, certain digestive structures, including the first portion of the small intestine (called the duodenum), the pancreas, and portions of the large intestine (the ascending and descending colon, and the rectum) remain completely or partially posterior to the peritoneum. Thus, the location of these organs is described as retroperitoneal .

Interactive Link

By clicking on this link you can watch a short video of what happens to the food you eat, as it passes from your mouth to your intestine. Along the way, note how the food changes consistency and form. How does this change in consistency facilitate your gaining nutrients from food?

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Access for free at https://openstax.org/books/anatomy-and-physiology-2e/pages/1-introduction

• Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix
• Publisher/website: OpenStax
• Book title: Anatomy and Physiology 2e
• Publication date: Apr 20, 2022
• Location: Houston, Texas
• Book URL: https://openstax.org/books/anatomy-and-physiology-2e/pages/1-introduction
• Section URL: https://openstax.org/books/anatomy-and-physiology-2e/pages/23-1-overview-of-the-digestive-system

© Jun 13, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.

Overview of the Digestive System

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Introduction

The digestive system , also known as the gastrointestinal system , regulates food intake, enables the digestion and absorption of nutrients and enables waste elimination . 1

This digestive system is responsible for transforming the variety of foods consumed into usable energy and building blocks for bodily functions, growth and repair . 1,2

The primary digestive tract is the pathway that allows for food to be moved along the tract, digested, absorbed and waste excreted. 2

The accessory digestive tract refers to the group of organs that synthesise, and secrete enzymes that allow for the chemical breakdown of food products. 2

Organs of the digestive system

Digestion begins in the mouth with mastication (chewing). Food is broken down in a bolus . 1,2

The pharynx pushes the food bolus from the mouth to the oesophagus , whilst protecting the airway. 1,2

The oesophagus is a muscular tube connecting the pharynx and the stomach , allowing for the conduction of the food bolus. 1,2

It has two sphincters, the upper and the lower oesophageal sphincters . The upper sphincter allows food to enter the oesophagus, whereas the lower sphincter controls the entry of the food bolus into the stomach.

The stomach is a reservoir for food that is consumed before digestion begins. Food is mechanically churned and mixed with gastric acid forming chyme . The pyloric sphincter connects to the duodenum enabling the controlled release of chyme into the duodenum for further digestion and absorption. 1,2

Small intestine

The small intestine is divided into three structural sections: 1,2

• Duodenum : approximately 25cm in length and surrounds the head of the pancreas. Chyme enters the duodenum, as do digestive enzymes produced by the pancreas and bile from the liver. Glands in the duodenum secrete bicarbonate, which neutralises the acidic stomach.
• Jejunum : approximately 2.5 metres long, containing large amounts of villi that increase its surface area. The majority of absorption of nutrients occurs in the jejunum.
• Ileum : the last section of the small intestine and is approximately 3 metres long. Vitamin B12, bile acids and any remaining nutrients are absorbed here.

Large intestine

The large intestine is the final part of the digestive system. The main function is to remove water from waste material and form faeces for excretion. 1,2

The large intestine is made up of:

• Caecum : the first part of the large intestine that connects with the ileum. The appendix is located in the caecum. 
• Colon : divided into ascending, transverse, descending and sigmoid sections. In the colon, water, electrolytes and vitamins are absorbed, compacting the faecal matter.
• Rectum : provides a storage area for faeces before defecation. The physical stretch of the rectal walls leads to the urge to defecate.
• Anus : the canal that faeces exits the body through. There are two sphincters, the internal and external anal sphincters. When the urge to defecate occurs, the internal anal sphincter relaxes, allowing faeces through. Being under somatic control, the external sphincter will relax when defecation is appropriate!

Accessory organs

Salivary glands.

Located in the oral cavity, salivary glands secrete saliva . Saliva contains some proteins and enzymes that lubricate the mouth and begin the chemical digestion of food. 1,2

The liver produces bile , which further enables chemical digestion by allowing for fat emulsification . Via the biliary system, bile enters the small intestine in the duodenum. 1,2

The pancreas produces digestive enzymes , bicarbonate and electrolytes that enter the duodenum via the pancreatic duct. These act to help neutralise stomach acid and break down proteins, fats and carbohydrates. 1,2

Gallbladder

When bile is produced but not immediately needed, it flows to the gallbladder , where is concentrated for storage. It is then released as required for digestion. 1,2

Clinical relevance: Cholecystectomy

Gallstones are common, and some patients require the removal of their gallbladder (a  cholecystectomy ).   Following a cholecystectomy, the patient is no longer able to store and concentrate bile . As the liver can only produce a certain about of bile at a given time, bile becomes a finite resource, and with no stores to draw upon in the event of a fatty meal, fat digestion can be affected , particularly in the first few months after the procedure. 

Symptoms of fat malabsorption after cholecystectomy include nausea, vomiting, abdominal pain, malodourous diarrhoea (steatorrhea) and increased flatulence. 

Fats should not be avoided but slowly reintroduced into a patient’s diet post-procedure to ascertain tolerance levels. Complete avoidance of fats will likely lead to vitamin deficiency and low caloric intake . 1

Neurovascular supply

Arterial supply.

Arterial supply of the digestive system is via branches of the abdominal aorta : 2

• Coeliac trunk : supplies liver, stomach, spleen, pancreas and proximal third of duodenum
• Superior mesenteric artery : supplies distal two-thirds of duodenum, jejunum, ileum, caecum, appendix, ascending colon and proximal third of the transverse colon
• Inferior mesenteric artery : supplies the distal half of the transverse colon, descending colon, sigmoid colon, rectum and anus

The arterial supply of the digestive system is divided embryologically . The foregut is supplied by the coeliac trunk, the midgut by the superior mesenteric artery and the hindgut by the inferior mesenteric artery.

Venous drainage

The digestive system is drained by the portal venous system and the systemic venous system . 1,2

Nutrient-rich blood is drained from the intestines by the hepatic portal vein to the liver, where it is filtered.

After processing by the liver, the blood is carried to the systemic system by hepatic veins, which then drain into the inferior vena cava .

Innervation

Swallowing and the anal sphincter are under somatic control. However, the rest of the tract is under autonomic control : 2

• Sympathetic innervation is via the abdominopelvic splanchnic nerves and acts to inhibit peristalsis, contract anal sphincters, constrict blood vessels and redirect blood flow to skeletal muscles when needed.
• Parasympathetic innervation occurs via the vagus nerve and pelvic splanchnic nerves. This innervation acts to increase peristalsis, increase digestion, and relax the internal anal sphincter.

Dr Chris Jefferies

Text references.

• Moore, K.L., Dalley, A.F. and Agur, A.M., 2018.  Clinically oriented anatomy . Lippincott Williams & Wilkins.Vancouver
• Boron, W.F. and Boulpaep, E.L., 2016.  Medical physiology E-book . Elsevier Health Sciences.

Image references

• Figure 1. OpenStax College. Components of the Digestive System. License: [ CC BY ]

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Your Digestive System & How it Works

On this page:

What is the digestive system?

Why is digestion important, how does my digestive system work, how does food move through my gi tract, how does my digestive system break food into small parts my body can use, what happens to the digested food, how does my body control the digestive process, clinical trials.

The digestive system is made up of the gastrointestinal tract—also called the GI tract or digestive tract—and the liver , pancreas , and gallbladder. The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus . The hollow organs that make up the GI tract are the mouth, esophagus , stomach, small intestine, large intestine, and anus. The liver, pancreas, and gallbladder are the solid organs of the digestive system.

The small intestine has three parts. The first part is called the duodenum. The jejunum is in the middle and the ileum is at the end. The large intestine includes the appendix , cecum, colon , and rectum. The appendix is a finger-shaped pouch attached to the cecum. The cecum is the first part of the large intestine. The colon is next. The rectum is the end of the large intestine.

Bacteria in your GI tract, also called gut flora or microbiome, help with digestion . Parts of your nervous and circulatory systems also help. Working together, nerves, hormones , bacteria, blood, and the organs of your digestive system digest the foods and liquids you eat or drink each day.

Digestion is important because your body needs nutrients from food and drink to work properly and stay healthy. Proteins , fats , carbohydrates , vitamins , minerals , and water are nutrients. Your digestive system breaks nutrients into parts small enough for your body to absorb and use for energy, growth, and cell repair.

• Proteins break into amino acids
• Fats break into fatty acids and glycerol
• Carbohydrates break into simple sugars

MyPlate offers ideas and tips to help you meet your individual health needs .

Each part of your digestive system helps to move food and liquid through your GI tract, break food and liquid into smaller parts, or both. Once foods are broken into small enough parts, your body can absorb and move the nutrients to where they are needed. Your large intestine absorbs water, and the waste products of digestion become stool . Nerves and hormones help control the digestive process.

The digestive process

Food moves through your GI tract by a process called peristalsis. The large, hollow organs of your GI tract contain a layer of muscle that enables their walls to move. The movement pushes food and liquid through your GI tract and mixes the contents within each organ. The muscle behind the food contracts and squeezes the food forward, while the muscle in front of the food relaxes to allow the food to move.

Mouth. Food starts to move through your GI tract when you eat. When you swallow, your tongue pushes the food into your throat. A small flap of tissue, called the epiglottis, folds over your windpipe to prevent choking and the food passes into your esophagus.

Esophagus. Once you begin swallowing, the process becomes automatic. Your brain signals the muscles of the esophagus and peristalsis begins.

Lower esophageal sphincter. When food reaches the end of your esophagus, a ringlike muscle—called the lower esophageal sphincter —relaxes and lets food pass into your stomach. This sphincter usually stays closed to keep what’s in your stomach from flowing back into your esophagus.

Stomach. After food enters your stomach, the stomach muscles mix the food and liquid with digestive juices . The stomach slowly empties its contents, called chyme , into your small intestine.

Small intestine. The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine, and push the mixture forward for further digestion. The walls of the small intestine absorb water and the digested nutrients into your bloodstream. As peristalsis continues, the waste products of the digestive process move into the large intestine.

Large intestine. Waste products from the digestive process include undigested parts of food, fluid, and older cells from the lining of your GI tract. The large intestine absorbs water and changes the waste from liquid into stool. Peristalsis helps move the stool into your rectum.

Rectum. The lower end of your large intestine, the rectum, stores stool until it pushes stool out of your anus during a bowel movement .

Watch this video to see how food moves through your GI tract .

As food moves through your GI tract, your digestive organs break the food into smaller parts using:

• motion, such as chewing, squeezing, and mixing
• digestive juices, such as stomach acid, bile , and enzymes

Mouth. The digestive process starts in your mouth when you chew. Your salivary glands make saliva , a digestive juice, which moistens food so it moves more easily through your esophagus into your stomach. Saliva also has an enzyme that begins to break down starches in your food.

Esophagus. After you swallow, peristalsis pushes the food down your esophagus into your stomach.

Stomach. Glands in your stomach lining make stomach acid and enzymes that break down food. Muscles of your stomach mix the food with these digestive juices.

Pancreas. Your pancreas makes a digestive juice that has enzymes that break down carbohydrates, fats, and proteins. The pancreas delivers the digestive juice to the small intestine through small tubes called ducts.

Liver. Your liver makes a digestive juice called bile that helps digest fats and some vitamins. Bile ducts carry bile from your liver to your gallbladder for storage, or to the small intestine for use.

Gallbladder. Your gallbladder stores bile between meals. When you eat, your gallbladder squeezes bile through the bile ducts into your small intestine.

Small intestine. Your small intestine makes digestive juice, which mixes with bile and pancreatic juice to complete the breakdown of proteins, carbohydrates, and fats. Bacteria in your small intestine make some of the enzymes you need to digest carbohydrates. Your small intestine moves water from your bloodstream into your GI tract to help break down food. Your small intestine also absorbs water with other nutrients.

Large intestine. In your large intestine, more water moves from your GI tract into your bloodstream. Bacteria in your large intestine help break down remaining nutrients and make vitamin K . Waste products of digestion, including parts of food that are still too large, become stool.

The small intestine absorbs most of the nutrients in your food, and your circulatory system passes them on to other parts of your body to store or use. Special cells help absorbed nutrients cross the intestinal lining into your bloodstream. Your blood carries simple sugars, amino acids, glycerol, and some vitamins and salts to the liver. Your liver stores, processes, and delivers nutrients to the rest of your body when needed.

The lymph system, a network of vessels that carry white blood cells and a fluid called lymph throughout your body to fight infection, absorbs fatty acids and vitamins.

Your body uses sugars, amino acids, fatty acids, and glycerol to build substances you need for energy, growth, and cell repair.

Your hormones and nerves work together to help control the digestive process. Signals flow within your GI tract and back and forth from your GI tract to your brain.

Cells lining your stomach and small intestine make and release hormones that control how your digestive system works. These hormones tell your body when to make digestive juices and send signals to your brain that you are hungry or full. Your pancreas also makes hormones that are important to digestion.

You have nerves that connect your central nervous system—your brain and spinal cord—to your digestive system and control some digestive functions. For example, when you see or smell food, your brain sends a signal that causes your salivary glands to "make your mouth water" to prepare you to eat.

You also have an enteric nervous system (ENS)—nerves within the walls of your GI tract. When food stretches the walls of your GI tract, the nerves of your ENS release many different substances that speed up or delay the movement of food and the production of digestive juices. The nerves send signals to control the actions of your gut muscles to contract and relax to push food through your intestines.

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and other components of the National Institutes of Health (NIH) conduct and support research into many diseases and conditions.

What are clinical trials, and are they right for you?

Watch a video of NIDDK Director Dr. Griffin P. Rodgers explaining the importance of participating in clinical trials.

What clinical trials are open?

Clinical trials that are currently open and are recruiting can be viewed at www.ClinicalTrials.gov .

This content is provided as a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the National Institutes of Health. NIDDK translates and disseminates research findings to increase knowledge and understanding about health and disease among patients, health professionals, and the public. Content produced by NIDDK is carefully reviewed by NIDDK scientists and other experts.

• Digestive Disorders

The Digestive System: How It Works

What's the Digestive System?

Your digestive system is a group of connected organs that work together to turn the food you eat into nutrients your body needs to function. It includes your biliary system—your gallbladder, liver, and pancreas—and your digestive tract.

What is the digestive tract?

The digestive tract (or gastrointestinal [GI] tract) is a long, twisting tube that starts at the mouth and ends at the anus. It's made up of a series of hollow organs that coordinate the movement of food and waste.

Along the way are the solid organs that make up your biliary system, which produces enzymes and hormones that aid in the breakdown of food.

Everything above the large intestine is called the upper GI tract. The large intestine and everything below it is the lower GI tract.

Digestive System Function

Digestion is the complicated process of turning the food you eat into nutrients, which your body uses for energy, growth, and cell repair. The digestion process also involves creating waste to be eliminated from your body.

Digestive system process

With the help of hormones and nerves, your digestive system processes the food you take in. The muscles within your organs push it along as your digestive system breaks nutrients—carbohydrates, fats, protein, vitamin, minerals, and liquid—into pieces small enough for your body to absorb. Then your body moves the nutrients where they're needed. The waste products that are left over become poop.

Digestive System Organs

The human digestive system includes these organs:

The mouth is the beginning of the digestive tract. In fact, digestion starts here before you even take the first bite of a meal. The smell of food triggers your salivary glands, making your mouth water. When you actually taste the food, saliva increases.

Once you start chewing, other processes come into play. More saliva is produced. It contains substances, including enzymes, that start the process of breaking down food.

Also called the pharynx , the throat is the next destination for the food you've eaten. Branching off the throat is the esophagus, which carries food to your stomach, and the trachea or windpipe, which carries air to your lungs.

Swallowing takes place in the throat, partly as a reflex and partly under your control. The tongue and soft palate—the soft part of the roof of your mouth—push food into the throat, which closes off the windpipe. From here, the food travels to the esophagus or swallowing tube.

The esophagus is a muscular tube that goes from the throat, behind the windpipe, and to the stomach. Food gets pushed through the esophagus and into the stomach by a series of muscle contractions, called peristalsis.

Just before the connection to the stomach, there's an important ring-shaped muscle called the lower esophageal sphincter (LES). This sphincter opens to let food pass into your stomach and closes to keep it there. If your LES doesn't work properly, you may have a condition called g astroesophageal reflux disease ( GERD), or reflux, which causes heartburn and regurgitation (the feeling of food coming back up).

The stomach is a sac-like organ with strong muscular walls. It serves as a mixer and grinder of food. It puts out acid and powerful enzymes that continue the process of breaking down the food and changing it to a consistency of liquid or paste.

Parts of the food that can't be liquified are released from the stomach and ushered through the intestines to be eliminated.

Small intestine

Made up of three segments—the duodenum, jejunum, and ileum—the small intestine is the workhorse of digestion. This is where most nutrients are absorbed. It's a long tube loosely coiled in your abdomen (spread out, it would be more than 20 feet long).

Peristalsis is also at work here, moving food through and mixing it with enzymes from the pancreas and bile from the liver. The duodenum is largely responsible for the continuing breakdown of food. The jejunum and ileum are mainly responsible for the absorption of nutrients into your bloodstream.

A more technical name for this part of the process is "motility" because it involves moving or emptying food particles from one part to the next. It's controlled by a large network of nerves, hormones, and muscles. Problems with any of these parts can cause a variety of health conditions.

After nutrients are absorbed through the walls of your small intestine, the waste that's leftover moves into your colon.

Colon (large intestine)

The colon, or large intestine, is a 5- to 7-feet-long muscular tube linking your small intestine to your rectum. This very specialized organ processes waste so you can eliminate it as poop.

It's made up of these parts:

Cecum, the uppermost section

Ascending (right) colon, which connects to the appendix

Transverse (across) colon

Descending (left) colon

Sigmoid colon, the S-shaped section that connects to the rectum

Peristalsis pushes waste left over from the digestion process through the colon. The colon absorbs more water, and the waste ends up in solid form as poop. When the lower colon gets full of poop, it empties into the rectum. It usually takes about 36 hours for waste to get through the colon.

The rectum is the 8-inch long section at the bottom of your colon. When it fills with poop, its walls get stretched, signaling your brain that you need to go to the bathroom. If that's not convenient at the moment, the rectum stores the poop until you can go.

The rectum also puts out mucus that helps the poop pass more easily.

The endpoint of your digestive tract is the anus, the opening where poop leaves your body. It's lined with muscles that help you control when you poop. A ring of muscle called the anal sphincter keeps the opening closed until you're ready to go.

Accessory Digestive System Organs

The organs that make up the biliary system play an essential role in helping your stomach and small intestine digest food:

• Pancreas . Among other functions, the pancreas discharges enzymes into the small intestine that work to break down protein, fat, and carbohydrates from the food we eat. It also makes insulin, a hormone that helps you digest sugars.
• Liver . The liver has many jobs. Two of its main ones in the digestive system are to make bile — a compound that helps you digest fat —a nd to process nutrients absorbed by the small intestine so your body can use them.
• Gallbladder . The gallbladder is a storage sac for excess bile. Bile made in the liver travels to the small intestine via the bile ducts. If the intestine doesn't need it, the bile travels to the gallbladder, where it awaits the signal from the intestines that food is present.

Digestive System Diseases

Because digestive system anatomy and functions are so complicated, many health conditions can affect it. They range from short-term, minor issues to long-lasting (chronic) diseases.

From time to time, most people will have digestive problems like:

• Constipation
• "Stomach flu" or gastroenteritis
• Hemorrhoids

But if you often have digestive symptoms, let your doctor know. It could be a sign of something more serious such as:

• Irritable bowel syndrome , a problem with your colon that causes uncomfortable symptoms like gas, cramps, and changes in your bowel movements
• Inflammatory bowel disease like Crohn's disease and ulcerative colitis, which causes widespread irritation in your digestive tract lining
• GERD , where acid from your stomach often goes the wrong way up your esophagus
• Celiac disease , in which your body has a serious autoimmune reaction to gluten, a protein in barley, rye, and wheat
• Diverticulitis and diverticulosis , two conditions in which pockets bulge out from the lining of your colon
• Cancers of digestive organs, such as the colon, esophagus, liver, pancreas, or stomach

Your digestive system consists of a series of linked organs, stretching from your mouth to your anus. Together, they work to turn food you eat into nutrients your body needs for energy, growth, and cell repair. Your digestive system also helps your body eliminate the waste products that are left over after this process.

Digestive System FAQs

What are the four main functions of the digestive system?

The main jobs of your digestive system are:

• Taking in and digesting food
• Absorbing nutrients from the food
• Releasing enzymes and liquid to help digestion
• Eliminating waste

How long is your large intestine?

Your large intestine is around 5 feet long. It's shorter and wider than your small intestine and takes a much direct path through your body.

How do you keep your digestive system healthy?

Some things you can do to keep your digestive system working its best include:

• Follow a well-balanced diet that limits saturated fats and processed foods. Include plenty of fruits, veggies, and whole grains, which are rich in fiber and help keep you regular.
• Exercise regularly. Moving your body helps keep food moving through your digestive system. Try taking a walk each evening after dinner.
• Drink enough fluids. Dehydration can lead to constipation.
• Chew your food well. It kick-starts the digestive process by starting to break down your food into smaller pieces and helps you make plenty of saliva.
• Stop smoking and limit alcohol. These habits contribute to digestive issues like reflux, ulcers, and heartburn.

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The Digestive System

Apr 02, 2019

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The Digestive System . Functions of the Digestive System. 1. Ingestion—placing food into the mouth 2. Propulsion—moving foods from one region of the digestive system to another Peristalsis—alternating waves of contraction and relaxation that squeezes food along the GI tract

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• hormone gastrin
• food breakdown
• blood vessels
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• pharynx esophagus stomach pancreas
• duodenum pyloric sphincter

Presentation Transcript

Functions of the Digestive System 1. Ingestion—placing food into the mouth 2. Propulsion—moving foods from one region of the digestive system to another • Peristalsis—alternating waves of contraction and relaxation that squeezes food along the GI tract • Segmentation—moving materials back and forth to aid with mixing in the small intestine

Figure 14.12a-b

Functions of the Digestive System 3. Food breakdown as mechanical digestion • Examples: • Mixing food in the mouth by the tongue • Churning food in the stomach • Segmentation in the small intestine • Mechanical digestion prepares food for further degradation by enzymes

Functions of the Digestive System 4. Food breakdown as chemical digestion • Enzymes break down food molecules into their building blocks • Each major food group uses different enzymes • Carbohydrates are broken to simple sugars • Proteins are broken to amino acids • Fats are broken to fatty acids and alcohols

Figure 14.13 (1 of 3)

Figure 14.13 (2 of 3)

Figure 14.13 (3 of 3)

Functions of the Digestive System 5. Absorption • End products of digestion are absorbed in the blood or lymph • Food must enter mucosal cells and then into blood or lymph capillaries 6. Defecation • Elimination of indigestible substances from the GI tract in the form of feces

Ingestion Food Mechanical digestion Pharynx Esophagus • Chewing (mouth) Propulsion • Churning (stomach) • Segmentation • (small intestine) • Swallowing • (oropharynx) Chemical digestion • Peristalsis • (esophagus, • stomach, • small intestine, • large intestine) Stomach Absorption Lymph vessel Small intestine Blood vessel Large intestine Mainly H2O Feces Anus Defecation Figure 14.11

Organs of the Digestive System • Two main groups of organs • Alimentary canal (gastrointestinal or GI tract)—continuous coiled hollow tube • These organs ingest, digest, absorb, defecate • Accessory digestive organs • Includes teeth, tongue, and other large digestive organs

Parotid gland Mouth (oral cavity) Sublingual gland Tongue Salivary glands Submandibular gland Pharynx Esophagus Stomach Pancreas (Spleen) Liver Gallbladder Transverse colon Duodenum Descending colon Small intestine Jejunum lleum Ascending colon Large intestine Cecum Sigmoid colon Rectum Appendix Anus Anal canal Figure 14.1

Organs of the Alimentary Canal • Mouth • Pharynx • Esophagus • Stomach • Small intestine • Large intestine • Anus

Mouth (Oral Cavity) Anatomy • Lips (labia)—protect the anterior opening • Cheeks—form the lateral walls • Hard palate—forms the anterior roof • Soft palate—forms the posterior roof • Uvula—fleshy projection of the soft palate • “Tongue-Tied” – extremely short lingual frenulum results in distorted speech. Corrected by surgically cutting the frenulum.

Mouth Physiology • Mastication (chewing) of food • Mixing masticated food with saliva • Initiation of swallowing by the tongue • Allows for the sense of taste

Pharynx Physiology • Serves as a passageway for air and food • Food is propelled to the esophagus by two muscle layers • Longitudinal inner layer • Circular outer layer • Food movement is by alternating contractions of the muscle layers (peristalsis)

Esophagus Anatomy and Physiology • Anatomy • About 10 inches long • Runs from pharynx to stomach through the diaphragm • Physiology • Conducts food by peristalsis (slow rhythmic squeezing) • Passageway for food only (respiratory system closed off in the pharynx by the epiglottis) • Choking – the epiglottis fails to close properly and food enters the trachea (talking with your mouth full…)

Bolus of food Tongue Pharynx Epiglottis up Upper esophageal sphincter Glottis (lumen) of larynx Esophagus Trachea (a) Upper esophageal sphincter contracted Figure 14.14a

Uvula Bolus Epiglottis down Larynx up Esophagus (b) Upper esophageal sphincter relaxed Figure 14.14b

Bolus (c) Upper esophageal sphincter contracted Figure 14.14c

Relaxed muscles Cardioesophageal sphincter open (d) Cardioesophageal sphincter relaxed Figure 14.14d

Stomach Anatomy • Located on the left side of the abdominal cavity • Food enters at the cardioesophageal sphincter • Food empties into the small intestine at the pyloric sphincter (valve) • Heartburn – cardioesophageal sphincter fails to close tightly and gastric juice backs up into esophagus.

Stomach Anatomy • Regions of the stomach • Cardiac region—near the heart • Fundus—expanded portion lateral to the cardiac region • Body—midportion • Pylorus—funnel-shaped terminal end

Stomach Anatomy • Rugae—internal folds of the mucosa • Stomach can stretch and hold 4 L (1 gallon) of food when full • External regions • Lesser curvature—concave medial surface • Greater curvature—convex lateral surface

Cardioesophageal sphincter Fundus Esophagus Muscularis externa Serosa • Longitudinal layer • Circular layer Body • Oblique layer Lesser curvature Rugae of mucosa Pylorus Greater curvature Duodenum Pyloric Sphincter (valve) Pyloric antrum (a) Figure 14.4a

Structure of the Stomach Mucosa • Mucosa is simple columnar epithelium • Mucous neck cells—produce a sticky alkaline mucus • Gastric glands—situated in gastric pits and secrete gastric juice • Chief cells—produce protein-digesting enzymes (pepsinogens) • Parietal cells—produce hydrochloric acid • Enteroendocrine cells—produce gastrin

Gastric pits Surface epithelium Gastric pit Pyloric sphincter Mucous neck cells Parietal cells Gastric gland Gastric glands Chief cells (c) Figure 14.4c

Pepsinogen Pepsin HCl Parietal cells Chief cells Enteroendocrine cell (d) Figure 14.4d

Stomach Physiology • Temporary storage tank for food • Site of food breakdown • Chemical breakdown of protein begins • Delivers chyme (processed food) to the small intestine

Pyloric sphincter slightly open Pyloric sphincter closed Pyloric sphincter closed Propulsion: Peristaltic waves move from the fundus to the pylorus. Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. Retropulsion: The pyloric end of the stomach pumps small amounts of chyme into the duodenum, while simultaneously forcing most of its contents backward into the stomach. 1 2 3 Figure 14.15

Food Breakdown in the Stomach • Gastric juice is regulated by neural and hormonal factors • Presence of food or rising pH causes the release of the hormone gastrin • Gastrin causes stomach glands to produce • Protein-digesting enzymes • Mucus • Hydrochloric acid

Food Breakdown in the Stomach • Hydrochloric acid makes the stomach contents very acidic • Acidic pH • Activates pepsinogen to pepsin for protein digestion • Provides a hostile environment for microorganisms • Vomiting (emesis)– reverse peristalsis caused by emetic center in the brain (medulla). • Peptic ulcers – breakdown of the mucosal lining of the stomach caused by a bacterium.

Small Intestine • The body’s major digestive organ • Site of nutrient absorption into the blood • Muscular tube extending from the pyloric sphincter to the ileocecal valve • Suspended from the posterior abdominal wall by the mesentery

Subdivisions of the Small Intestine • Duodenum • Attached to the stomach • Curves around the head of the pancreas • Jejunum • Attaches anteriorly to the duodenum • Ileum • Extends from jejunum to large intestine

Small Intestine Anatomy • Three structural modifications that increase surface area • Microvilli—tiny projections of the plasma membrane (create a brush border appearance) • Villi—fingerlike structures formed by the mucosa • Circular folds (plicae circulares)—deep folds of mucosa and submucosa

Blood vessels serving the small intestine Lumen Muscle layers Circular folds (plicae circulares) Villi (a) Small intestine Figure 14.7a

Absorptive cells Lacteal Villus Blood capillaries Lymphoid tissue Intestinal crypt Venule Muscularis mucosae Lymphatic vessel Submucosa (b) Villi Figure 14.7b

Microvilli (brush border) (c) Absorptive cells Figure 14.7c

Chemical Digestion in the Small Intestine • Chemical digestion is completed in the small intestine • Enzymes are produced by • Intestinal cells • Pancreas • Pancreatic ducts carry enzymes to the small intestine • Bile, formed by the liver, enters via the bile duct

Absorption in the Small Intestine • Water is absorbed along the length of the small intestine • End products of digestion • Most substances are absorbed by active transport through cell membranes • Lipids are absorbed by diffusion • Substances are transported to the liver by the hepatic portal vein or lymph

Large Intestine • Larger in diameter, but shorter in length, than the small intestine • Extends from the ileocecal valve to the anus • Subdivisions: • Cecum • Appendix • Colon • Rectum • Anal canal

Large Intestine Anatomy • Cecum—saclike first part of the large intestine • Appendix • Accumulation of lymphatic tissue that sometimes becomes inflamed (appendicitis) • Hangs from the cecum • Appendicitis – inflammation of the appendix caused by a bacterium.

Large Intestine Anatomy • Colon • Ascending—travels up right side of abdomen • Transverse—travels across the abdominal cavity • Descending—travels down the left side • Sigmoid—S-shaped region; enters the pelvis • Rectum and anus also are located in the pelvis

Large Intestine Anatomy • Anus—opening of the large intestine • External anal sphincter—formed by skeletal muscle and under voluntary control • Internal involuntary sphincter—formed by smooth muscle • These sphincters are normally closed except during defecation

Left colic (splenic) flexure Transverse mesocolon Right colic (hepatic) flexure Transverse colon Haustrum Descending colon Ascending colon IIeum (cut) Cut edge of mesentery IIeocecal valve Teniae coli Sigmoid colon Cecum Appendix Rectum Alan canal External anal sphincter Figure 14.8

Food Breakdown and Absorption in the Large Intestine • No digestive enzymes are produced • Resident bacteria digest remaining nutrients • Produce some vitamin K and B • Release gases • Water and vitamins K and B are absorbed • Remaining materials are eliminated via feces

Food Breakdown and Absorption in the Large Intestine • Feces contains • Undigested food residues • Mucus • Bacteria • Water

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KS3 and 4 Science Digestive system GCSE

Subject: Biology

Age range: 14-16

Resource type: Lesson (complete)

Last updated

11 September 2024

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Full lesson suitable for both KS3 and KS4 for teaching the Digestive system. There are worksheets available too.

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