Motility, digestion, secretion, and absorption are the four basic functions of the GI tract
The primary function of the gastrointestinal tract is to deliver nutrients, water and electrolytes contained in the food we eat to the body’s internal environment.
In this lecture, we shall outline the GI processes that transform the food we eat into nutrients for body use.
The digestive system is able to perform its job by coordinating the four basic processes and functions of motility, digestion, secretion, and absorption.
Motility involves the movement of materials in the gastrointestinal tract because of muscle contraction. The body regulates GI motility because if food moves through the tract too rapidly, there will be inadequate time for digestion and absorption of food contained within the GI lumen.
Hormones such as cholecystokinin, secretin, motilin, gastric inhibitory peptide all affect gastrointestinal motility.
Two different functional types of movement (i.e. peristaltic and segmental movements) occur along the length of the gastrointestinal tract. Peristaltic movement also called propulsive movements is most relevant in the esophagus. In the esophagus, peristaltic movements cause swallowed food to move to the stomach. Peristalsis also occurs in other segments of the GI tract though the wave may not be as strong as that of the esophagus.
Normally, the stimulus for peristalsis is distension of the wall of the gastrointestinal tract, which occurs when food collect at any point along the GI tract.
In some segments of the gut, local constrictive contractions that do not get beyond few centimeters occur and usually last for about 10 to 30 seconds after which new constrictive contractions occur at other points along the gut. These constrictive contractions also called mixing contractions helps in mixing intestinal content properly to aid proper digestion and absorption.
Digestion is the mechanical and chemical breakdown of food macromolecules into smaller absorbable units that can cross the intestinal epithelium into the body. Digestion makes food useable by the body.
The food we eat is in the form of macromolecules such as complex carbohydrates and proteins mainly, the digestive system secretes powerful enzymes that breakdown the macromolecules in food into smaller molecules which the body can absorb. The entire length of the GI tract has protective mechanisms to prevent the powerful enzymes from digesting the walls of the GI tract (autodigestion).
If the protective mechanisms against autodigestion fail, we develop raw patches (internal wounds) called peptic ulcers along the walls of the GI tract.
Absorption is the transfer of substances from the lumen of the GI tract to the extracellular fluid. Absorption can occur by either active or passive mechanism. Thus, absorption delivers digested nutrients to the body for use. The body fails to regulate the absorption of most nutrients, thus, your body absorbs as much as you can eat.
Secretion is the process that delivers materials from epithelial cells to the GI lumen. Secretion occurs in two forms: it could be the transepithelial transfer of water and ions from the ECF to the lumen of the digestive tractor the release of substances synthesized by epithelial cells of the GI tract.
Along the length of the GI tract, (secretory epithelium) glandular structures deliver secretions into the lumen particularly in the mouth, stomach and intestines.
In mouth, salivary glands secrete saliva. Among the substances secreted by different cells of the gastric glands are Hcl, mucus and bicarbonate, pepsinogen, intrinsic factor, etc. Cells of the gastric glands secrete these substances into the stomach lumen. Pancreatic and biliary secretions play important roles in digestion.
A great challenge the digestive system faces is ensuring that the amount of secreted fluid does not exceed the amount of reabsorbed fluid, to prevent dehydration. Exocrine glands and other cells of the digestive system secrete an estimated 7 Liters of fluid every day into the lumen of the GI tract. These secretions are particularly rich in digestive enzymes, electrolytes, mucus and water, which are all necessary for proper digestive function. The challenge is that the body must reabsorb the fluid volume; otherwise, it will fast dehydrate.
For instance, diarrhea can become an emergency if fluid loss significantly depletes ECF volume so that the circulatory system is unable to maintain adequate blood pressure.
The digestive system has significant immune function
Perhaps, we may think of the digestive system as a system primarily in terms of digestion, however, the digestive system faces the challenge of repelling foreign invaders.
The GI tract offers the largest interface between the internal environment of the body and the external environment.
Therefore, the digestive system faces the challenge of absorbing nutrients, and water while at the same time preventing bacteria, fungi, viruses and other pathogens from entering the body. Consequently, the transporting epithelium of the gastrointestinal tract possesses numerous physiological defense mechanisms against pathogens that may try to invade the body through the GI tract.
Among the defense mechanisms of the GI tract against pathogens, include the following:
- Mucus (which entraps and immobilizes bacteria and other pathogens),
- Digestive enzymes and stomach acid (which digest pathogens), and
- A large collection of lymphoid tissues called the gut-associated lymphoid tissue (GALT). Perhaps, GALT is the largest collection of lymphoid tissues in the entire body. Going by estimates, about 70% of all lymphocytes in the body reside in the small intestine.
Saliva contains enzymes (such as lysozyme, phospholipase etc), and immunoglobulin A which act as first line of defense against pathogens and toxic materials.
Stomach has a highly acidic environment (due to Hcl secretion) that is unfavorable to pathogenic microbes. Stomach acidity kills most ingested pathogens.
Pathogens and toxic materials that find their way into the small intestine encounter specialized immune cells of the gut-associated lymphoid tissues and sensory receptors, which trigger responses that eliminate the pathogens.