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Anatomy and Physiology of Gastrointestinal Tract

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Introduction

Nutrition helps us to gain energy for proper functioning of the body. This is possible when the consumed food is digested and converted into energy. The digestive or gastrointestinal system includes several organs and glands, which are involved in this process of eating and digesting. Starting from the mouth, the gastrointestinal (GI) tract is a long muscular tube, which is 9 m in length and ends with anus. After food is consumed, the body mechanically and chemically breaks it down, transports it for absorption, and defecation (final waste removal). The digestive glands (salivary glands, pancreas, liver and gallbladder) produce or store secretions that the body carries to the digestive tract in ducts and breaks down chemically. . The digestive system is controlled by the central nervous system, peripheral nervous system and enteric nervous system.1

Structure and Function of the Gastrointestinal Tract

GI tract is a complex structure consisting of the following parts and perform many functions in the body.

Mouth

Mouth, which is also called the oral cavity, is the first part of the GI tract. The lips, cheeks and palate form the boundaries. The oral cavity contains the teeth (incisors, canines, premolar and molar) and tongue. The tongue is a strong muscular sensitive organ. It has specialized sensors known as papillae containing taste buds. There are three pairs of salivary glands, which surround the oral cavity.

  • Parotid gland: These are large, irregular shaped and located below the cheekbone.
  • Submandibular gland: There are found in the floor of the mouth in a groove along the inner surface of the lower jaw-bone.
  • Sublingual gland: They are the smallest and located at the floor of the mouth, covered by a thin tissue.

Mechanical Breakdown of Food

The presence of food in the mouth triggers the cascade of events to take place. Chewing with the help of teeth mechanically breaks down the food in the mouth. The tongue helps the food to come in contact with the teeth and also senses touch, temperature and taste. It also receives secretions from the salivary glands as food enters the mouth. The mouth plays an important role in speech, expressions, eating, swallowing etc. From the mouth, the food passes through pharynx and esophagus by the process of swallowing.

Pharynx

Pharynx, a fibro-muscular passageway is commonly called as the throat. The pharynx opens into the larynx and esophagus. It is divided into three regions according to the location:

  • Nasopharynx: Present posterior to the nasal cavity.
  • Oropharynx: Present posterior to the oral cavity.
  • Laryngopharynx or hypopharynx: Extends along with the larynx.

The pharyngeal, palatine and lingual tonsils, also called Waldeyer’s ring, are located in the pharynx. Tonsils are clusters of lymphatic tissue present under the mucous membranes that line the nose, mouth and pharynx.

Dual Passage for Food and Air

Pharynx serves as a part of both the respiratory system by receiving air from nasal cavity and the digestive system by receiving air, food and water from the oral cavity. The upper part of the pharynx (throat) lets only air pass through, and lower part permit air, food and fluid to pass. Food is forced into the pharynx by the tongue. The epiglottis (thin elastic cartilaginous structure located at the root of the tongue) prevents the food from entering the larynx and trachea and directs the food into the esophagus through peristalsis (synchronized contraction of the muscles to propel the food).

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Esophagus

It is a muscular tube approximately 25 cm in length and 2 cm in diameter. It serves as a passage between the pharynx and the stomach. It passes through an opening in the diaphragm (a sheet of muscle separating the thorax and abdomen), called the esophageal hiatus, and then empties into the stomach.  Secretions of the glands lubricate the passage.

The upper and lower parts of the esophagus have sphincters to control the movement of the food. The food passes into the esophagus through peristalsis (coordinated contraction of the muscles to push the food to facilitate digestion). The lower esophageal sphincter, also called the cardiac sphincter is a thick ring of muscle and acts as a one-way valve to stop regurgitation (reflux) of stomach.

Stomach

Stomach is a muscular sac, located on the left of the midline between esophagus and small intestine. The borders of the stomach are called lesser and greater curvature. It is closed at the top by the lower esophageal sphincter and at the bottom by the pyloric sphincter. The muscles layer of the stomach wall contains specialized muscle fibers that churn the stomach contents. The mucosal layer of the stomach contains many secretory glands. It is divided into the fundus, cardiac body and pylorus regions.

Stomach Plays Multiple Roles in the Process of Digestion

The stomach helps in temporary storage of food, mixing, secreting hydrochloric acid, intrinsic factor, pepsinogen, mucus and extracts iron from food. The major motor  function of the stomach  is,2

  • Act as a reservoir to store and mix the ingested food
  • Modulating its transfer into the duodenum

Stomach secretes gastric juice and this involves three phases.

  • Cephalic phase: Thoughts and smells of food initiate, the secretion of gastric juice.
  • Gastric phase: Presence of food in the stomach initiates this phase.
  • Intestinal phase: Presence of acidic chyme in the small intestine begins this phase.

The contractions of the stomach play an important role in the digestion. Once the food reaches the stomach, peristalsis is initiated. Chemical digestion of proteins takes place with the help of acid and enzymes. Absorption of some substances takes place. The food here is partially digested and is called chyme. Chyme is acidic in nature due to acid secretion. Later, waves of peristalsis open the pyloric sphincter and allow the passage of chyme into the duodenum.

Small Intestine

The small intestine begins at the pyloric sphincter and joins the large intestine (colon) at the ileocecal valve. It is approximately 6.5 m long and has a diameter of approximately 2.5 cm. The small intestine is compressed into numerous folds and occupies a large portion of the abdominal cavity. The small intestine is divided into the duodenum, jejunum, and ileum. The lining of the small intestine is made up of plicae circulares, villi and microvilli.

Portal for Absorption of all Nutrients into Blood

Duodenum is the site where it receives secretions from the pancreas, liver, gallbladder and the contents expelled from the stomach. The partly digested food is further broken down to simpler form to facilitate absorption into the blood stream. Jejunum is the site where majority of digestion and absorption occurs. The villi and microvilli increase the surface area of absorbtion. The small intestine finishes the process of digestion, absorbs the nutrients and passes the residue on to the large intestine. Gastric motility and duodenal motility influence each through the release of neurotransmitters and hormones.3,4 The liver, gallbladder and pancreas are accessory organs of the digestive system that are closely associated with the small intestine.

Pancreas

The pancreas is a lobular, pinkish-grey organ that lies behind the stomach. It has a head region and a tail region. Its head is attached to the duodenum and its tail extends to the spleen. Pancreas is approximately 15 cm in length with a long, slender body connecting the head and tail segments.

Dual Gland Activity

The pancreas has both exocrine and endocrine functions. Endocrine refers to production of hormones (chemical substances produced by certain glands) like insulin and glucagons, which play an important role in the regulation of glucose in the blood. The exocrine function refers to the production of pancreatic juice, which is alkaline in nature,  digestive enzymes like trypsin, chymotrypsin, pancreatic lipase and pancreatic amylase. These enzymes secreted to breakdown carbohydrates, proteins, fats and acids in the duodenum. These enzymes travel down from pancreatic duct to bile duct in an inactive form. When chyme enters the duodenum, the hormone secretin is released and this stimulates the pancreas to secrete its juices. The pancreatic juices pass through the pancreatic ducts into the duodenum to help digestion by neutralizing the acid.

Liver

The liver is a large, reddish-brown organ situated adjacent to the stomach on the right and extends into the epigastric region. It is covered by a capsule and divided into four lobes namely the right, left, caudate and quadrate lobes.

A Master Gland with Multifaceted Functions

The liver acts as a processing plant, a battery, a filter, a warehouse and distribution centre all in one. It is engaged in so many activities. Without it one cannot survive. The number of tasks and functions it carries out is amazing. The following are the functions it performs.

  • It transforms the food into energy to build cells and tissues in body, warms the blood that passes through it thereby helping to maintain the body temperature..
  • It detoxifies all the poisonous chemicals that enter the body such as alcohol, drugs and pollutants.
  • It stores vitamins, fat, sugars and minerals, and sends them around the body as and when they are needed.
  • It makes a digestive juice called bile so that fats can properly be broken down and absorbed by the body. The bile produced here passes from the hepatic ducts into the cystic duct before entering the gall bladder for storage.
  • It is engaged in so many activities that the energy that it creates carrying them out warms the blood that passes through it thereby helping to maintain the body temperature.
  • It detoxifies all the poisonous chemicals that enter the body such as alcohol, drugs and pollutants.
  • The function of immune system, digestive tract, kidney, brain and cardiovascular systems depend on a healthy and well-functioning liver. A diseased liver can potentially affect all the body’s major systems and organs.

Gall Bladder

The gallbladder is a hollow, pear shaped organ. It is positioned in a depression on the posterior surface of the liver’s right lobe. It has three regions: fundus, body and neck.

The main functions of the gall bladder are storage and concentration of bile. Bile is a thick fluid that contains enzymes to help dissolve fat in the intestines. Bile is stored in the gallbladder until it is needed. Bile is released from the gall bladder by contraction of its muscular walls in response to the hormone that it receives from the duodenum in the presence of food.

Large Intestine

Large intestine is shaped such that it extends around the small intestine like a frame. It is approximately 1.5 m in length and 7.5 cm in width. It consists of the appendix, caecum, ascending, transverse, descending and sigmoid colon, and the rectum. Caecum is an expanded pouch that receives material from the ileum and starts to compress food products into fecal material. The wall of the colon is made up of several pouches (haustra) that are held under tension by three thick bands of muscle (taenia coli). The wall of the large intestine lacks villi. The gut contraction is triggered by Interstitial cell of cajal (ICC), which serves as pacemakers and present in the gastrointestinal tract.5

Food travels through the colon. Large intestine does not produce digestive enzymes. Large intestine is responsible for accumulation of unabsorbed food material to form feces and reabsorption of water, salts and vitamins.

Rectum

The rectum is the final 15 cm of the large intestine. It expands to hold fecal matter before it passes through the anorectal canal to the anus.

Thick bands of muscle, known as sphincters, control the passage of feces. The mucosal surface here has several intestinal glands, which secrete mucous to lubricate fecal matter as it solidifies.

Anus

Anus is the terminal opening of the GI tract. It is a specialized opening bound with elastic membranes, sensitive tissues, muscles and nerves allowing the fecal matter to be eliminated. The last 2–3 cm of the GI tract is the anal canal, which continues from the rectum and opens to the outside at the anus. There is an internal anal sphincter at the upper end of the anal canal, which is under involuntary control. There is also an external anal sphincter at the lower end of the anal canal, which is under voluntary control.

Conclusion

Digestion involves the mixing of food, its movement through the digestive tract, and the mechanical and chemical breakdown of the large molecules of food into smaller molecules. This article discusses on the mechanism of digestion and absorption of food in the various organs involved in this process. Thereby, energy is gained by the absorption of these nutrients in the body.

References:

1. Rajan, R. K,  Nemchausky, B. A. “Gastrointestinal emergencies in patients with spinal cord injury”. Emergencies in chronic spinal cord injury patients. 2001; 47–65. Jackson Heights, NY: Eastern Paralyzed Veterans Association.

2. Wilson P, Perdikis G, Redmond EJ, Anselmino M, Hinder RA,. Quigley EMM. “ Prolonged ambulatory antroduodenal manometry in humans”. Am J Gastroenterol. 1994; 89: 1489.

3. Shafik A, El Sibai O, Shafik AA. “Study of the duodenal contractile activity during antral contractions”. World J Gastroenterol. 2007; 13: 2600–2603.

4. Shafik A, El Sibai O, Shafik AA, Shafik IA. “Mechanism of gastric emptying through the pyloric sphincter: a human study”. Med Sci Monit. 2007; 13: CR24–CR29.

5. Sanders K, Koh S, Ward S. "Interstitial cells of cajal as pacemakers in the gastrointestinal tract". Annu Rev Physiol. 68: 307–343.

Written by: healthplus24.com team

Date last updated: February 05, 2015