Gastrointestinal physiology

The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and excrete waste products.

The GI tract is composed of the alimentary canal, that runs from the mouth to the anus, as well as the associated glands, chemicals, hormones, and enzymes that assist in digestion.

The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation.

The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.

[1][2] The gastrointestinal tract generates motility using smooth muscle subunits linked by gap junctions.

As the contraction is graded based upon how much Ca2+ enters the cell, the longer the duration of slow wave, the more action potentials occur.

Both amplitude and duration of the slow waves can be modified based upon the presence of neurotransmitters, hormones or other paracrine signaling.

The number of slow wave potentials per minute varies based upon the location in the digestive tract.

[4] The peristalsis and segmentation, detailed below and pendular movement are famous examples of distinct patterns of GI contraction.

The contractions occur directly behind the bolus of food that is in the system, forcing it toward the anus into the next relaxed section of smooth muscle.

This contraction pattern depends upon hormones, paracrine signals, and the autonomic nervous system for proper regulation.

This mixing allows food and digestive enzymes to maintain a uniform composition, as well as to ensure contact with the epithelium for proper absorption.

The arrangement of these proteins on the apical and basolateral sides of the epithelium determines the net movement of ions and water in the tract.

This process also requires ATP as a source of energy; however, Cl− then follows the positive charge in the H+ through an open apical channel protein.

However, once it reaches the gastric lumen it becomes activated into pepsin by the high H+ concentration, becoming an enzyme vital to digestion.

The bile salt component is an active non-enzymatic substance that facilitates fat absorption by helping it to form an emulsion with water due to its amphoteric nature.

[4] Although this may be the case in several situations, the ENS can also work in conjunction with the CNS; vagal afferents from the viscera are received by the medulla, efferents are affected by the vagus nerve.