The below mentioned article provides a notes on gastrointestinal tract.
Gastrointestinal tract can be described as a specialized tube communicating with the external environment both at its upper and lower ends. There is regional specialization suited for the local functions.
The main functions of GIT are:
1. Secretion: Exocrine—enzymes; endocrine— hormones of GI tract
2. Motility
3. Absorption
4. Storage and excretion of undigested waste materials.
The functions of gastrointestinal tract are controlled by both neural and hormonal mechanisms.
Motility of GIT is mainly the function of the smooth muscle found in most part of the GIT.
The movement helps in two ways:
1. Local mixing of the food components
2. Forward propulsion of the food materials
GIT can be described as a barrier between blood and the components of food that has been eaten. Unless the nutrients, vitamins, minerals and other substances pass through the wall of the GIT into the blood, they are not useful to the body. Absorption is, therefore, the process by which the nutrients, minerals, vitamins, water and other substances pass into the blood across the wall of the GIT.
The wall has got four layers (Fig. 5.1):
1. Serous coat:
Contributed by visceral peritoneum throughout except mouth, pharynx, esophagus, rectum and anal canal, where the serous coat is replaced by fibrous tissue.
2. Muscular coat:
Contributed by smooth muscles arranged generally in two layers; an outer longitudinal and an inner circular muscle. The latter is thickened in the region of sphincters. Between the two layers of muscles, there is the myenteric or Auerbach plexus formed by sympathetic and parasympathetic fibres.
3. Submucous coat:
Formed of fibrous tissue, lymphatic and blood vessels. Between the muscular and submucous coats is another nerve plexus namely sub- mucous or Meissner’s plexus formed by sympathetic and parasympathetic nerves and also fibers from myenteric plexus.
4. Mucous coat has three important components:
a. Musuclar mucosa formed of smooth muscles
b. Lamina propria which is connective tissue support structure for the epithelium.
c. Epithelium besides lining the GIT is also modified into special glands.
The entire digestive processes namely secretion, motility and absorption are under dual control.
1. Neural regulatory mechanism:
Mainly through autonomic nervous system controlled by hypothalamus (Figs 5.2 and 5.3), various parts of limbic system and cerebral cortex.
2. Hormonal regulation:
In this, the GIT hormones as well as the hormones from other specialized endocrine glands outside the GIT, help to regulate the GIT functions.