GASTROINTESTINAL TRACT

Course Name : B.Sc Zoology(H)

Semester : IV

Paper-C9T: Animal Physiology

Teacher: Manisha Das

Learning Objectives

After studying this unit, you shall be able to:

• locate the parts of the digestive system.
• identify the function of the various parts of the digestive system
• draw pictures of each part of the digestive system and place it in the correct location on a self-drawn picture of their own body

​

​

​

• The human gastrointestinal tract consists of the esophagus, stomach, and intestines, and is divided into the upper and lower gastrointestinal tracts. The GI tract includes all structures between the mouth and the anus, forming a continuous passageway that includes the main organs of digestion namely, the stomach, small intestine, and large intestine.
• Lower Gastrointestinal Tract : The lower gastrointestinal tract includes most of the small intestine and all of the large intestine.

​

• In human anatomy, the intestine is the segment of the gastrointestinal tract extending from the pyloric sphincter of the stomach to the anus and, as in other mammals, consists of two segments, the small intestine and the large intestine. In humans, the small intestine is further subdivided into the duodenum, jejunum and ileum while the large intestine is subdivided into the, cecum, ascending, transverse, descending and sigmoid colon, rectum, and anal canal.

• Small Intestine:
• The small intestine begins at the duodenum and is a tubular structure, usually between 6 and 7 m long. Its mucosal area in an adult human is about 30 m². Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. There are three major divisions:
• DUODENUM: A short structure (about 20–25 cm long) which receives chyme from the stomach, together with pancreatic juice containing digestive enzymes and bile from the gall bladder. The digestive enzymes break down proteins, and bile emulsifies fats into micelles. The duodenum contains Brunner's glands which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in the chyme.

​

• JEJUNUM: This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m long, and contains the circular folds also known as plicae circulares, and villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here. The pH in the jejunum is usually between 7 and 8 (neutral or slightly alkaline).
• The jejunum and the ileum are suspended by mesentery which gives the bowel great mobility within the abdomen. It also contains circular and longitudinal smooth muscle which helps to move food along by a process known as peristalsis.
• If the jejunum is impacted by blunt force the emesis reflex (vomiting) will be initiated.

​

​

​

• ILEUM: The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients.
• The ileum has more fat inside the mesentery than the jejunum.
• The diameter of its lumen is smaller and has thinner walls than the jejunum.
• Its circular folds are smaller and absent in the terminal part of the ileum.
• While the length of the intestinal tract contains lymphoid tissue, only the ileum has abundant Peyer's patches, unencapsulated lymphoid nodules that contain large numbers of lymphocytes and other cells of the immune system.

​

​

• Large Intestine:
• The large intestine also called the colon, consists of the cecum, rectum, and anal canal. It also includes the appendix, which is attached to the cecum. The colon is further divided into:
• Cecum (first portion of the colon) and appendix
• Ascending colon (ascending in the back wall of the abdomen)
• Right colic flexure (flexed portion of the ascending and transverse colon apparent to the liver)
• Transverse colon (passing below the diaphragm)
• Left colic flexure (flexed portion of the transverse and descending colon apparent to the spleen)
• Descending colon (descending down the left side of the abdomen)
• Sigmoid colon (a loop of the colon closest to the rectum)
• Rectum
• Anus.

​

​

Function of GI Tract

• The time taken for food or other ingested objects to transit through the gastrointestinal tract varies depending on many factors, but roughly, it takes less than an hour after a meal for 50% of stomach contents to empty into the intestines while total emptying takes around 2 hours. Subsequently, 50% emptying of the small intestine takes between 1 and 2 hours. Finally, transit through the colon takes 12 to 50 hours with wide variation between individuals.

​

• Immune function:
• Immune barrier:
• The gastrointestinal tract forms an important part of the immune system. The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court. With such a large exposure (more than three times larger than the exposed surface of the skin), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems.

​

• Immune System Homeostasis
• Intestinal Micro biota: The large intestine hosts several kinds of bacteria that can deal with molecules that the human body cannot otherwise break down. This is an example of symbiosis. Health-enhancing intestinal bacteria of the gut flora serve to prevent the overgrowth of potentially harmful bacteria in the gut
• Detoxification and Drug Metabolism:
• Enzymes such as CYP3A4, along with the antiporter activities, are also instrumental in the intestine's role of drug metabolism in the detoxification of antigens and xenobiotic.

​

​

​

​

Thank you.