Human Physiology: digestive system.

Introduction to digestive system.

The gastrointestinal tract -GI tract, GIT, digestive tract, digestion tract or alimentary canal is the tract from the mouth to the anus which includes all the organs of the digestive system in humans. Food is taken in through the mouth and the waste is expelled as feces. The mouth, esophagus, stomach and intestines are all part of the gastrointestinal tract. 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. However, the complete human digestive system is made up of the gastrointestinal tract plus the accessory organs of digestion.

Based on embryological origin of each segment the tract may also be divided into

· Fore Gut

· Mid Gut

· Hind Gut

The whole human Gastro Intestinal tract is about nine meters (30 feet) long at autopsy. It is considerably shorter in the living body because the intestines, which are tubes of smooth muscle tissue, maintain constant muscle tone in a halfway-tense state but can relax in spots to allow for local distention and peristalsis.

The gastrointestinal tract contains trillions of microbes, with some 4,000 different strains of bacteria having diverse roles in maintenance of immune health and metabolism. Some estimates claim that the number of microbes outnumber our own cells 10:1. Cells of the GI tract release hormones to help regulate the digestive process. These digestive hormones, like gastrin, secretin, cholecystokinin, and ghrelin, are mediated through either intracrine or autocrine mechanisms.

The structure and function can be described both as gross anatomy and as microscopic anatomy or histology. The tract itself is divided into upper and lower tracts and the intestines small and large parts.

Upper gastrointestinal tract

The upper gastrointestinal tract consists of the mouth, pharynx, esophagus, stomach and duodenum. The exact demarcation between the upper and lower tracts is the suspensory muscle of the duodenum. This differentiates the embryonic borders between the foregut and mid gut, and is also the division commonly used by clinicians to describe gastrointestinal bleeding as being of either upper or lower origin. Upon dissection, the duodenum may appear to be a unified organ, but it is divided into four segments based upon function, location, and internal anatomy. Which are as follows (starting at the stomach, and moving toward the jejunum)-

· Bulb

· Descending

· Horizontal

· Ascending

The suspensory muscle attaches the superior border of the ascending duodenum to the diaphragm. The suspensory muscle is an important anatomical landmark which shows the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively. This is a thin muscle which is derived from the embryonic mesoderm.

Lower gastrointestinal tract

The lower gastrointestinal tract includes most of the small intestine and all of the large intestine. In human anatomy, the intestine (Greek: éntera bowel, or gut) is the segment of the gastrointestinal tract extending from the pyloric sphincter of the stomach to the anus and 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 (320 sq ft). The combination of the circular folds, the villi, and the microvilli increases the absorptive area of the mucosa about 600 fold, making a total area of about 250 m² (2,700 sq ft) for the entire small intestine. Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream.

There are three major divisions of small intestine-

1. Duodenum- A short structure (about 20–25 cm long) which receives chyme from the stomach with pancreatic juice containing digestive enzymes and bile from the gall bladder. The digestive enzymes break down the 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.

2. Jejunum This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m (8.2 ft) 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.

3. 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.

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: