Q. Describe the Role of Stomach in the Digestion and Absorption of Food
The stomach is a J-shaped saclike chamber lying between the esophagus and small intestine.
Stomach performs three main functions that aid digestion
- The stomach’s most important function is to store ingested food until it can be emptied into the small intestine at a rate appropriate for optimal digestion and absorption. It takes hours to digest and absorb a meal that was consumed in only a matter of minutes. Because the small intestine is the primary site for this digestion an absorption, it is important that the stomach store the food and meter it into the duodenum at a rate that does not exceed the small intestine’s capacities
- The stomach secretes hydrochloric acid (HCl) and enzymes that begin protein digestion
- Through the stomach’s mixing movements, the ingested food is pulverized and mixed with gastric secretions to produce a thick liquid mixture known as chyme. The stomach contents must be converted to chyme before they can be emptied into the duodenum
Stomach Motility and its Role in Digestion and Absorption of Food
The interior of the stomach is thrown into deep folds. During a meal, the folds get smaller and flatted out as the stomach relaxes slightly with each mouthful, much like the gradual expansion of a collapsed ice bag as it is being filled. This reflex relaxation of the stomach as it receiving food is called receptive relaxation; it enhances the stomach’s ability to accommodate the extra volume of food with little rise in stomach pressure.
Part of the motility function of the stomach is to store the food. Food storage takes place in the body of the stomach. Because only feeble mixing movements occur in the body and fundus, food emptied into the stomach from the esophagus is stored in the relatively quiet body without being mixed. The fundic area usually does not store food but contains only a pocket of gas. Food is gradually fed from the body into the antrum where mixing take place.
Gastric mixing takes place in the antrum of the stomach. The strong antral peristaltic contractions are responsible for mixing the food with gastric decorations to produce chyme.
The Process of Gastric Mixing and Emptying: Tonic contractions of the pyloric sphincter normally keep it almost, but not completely, closed. The opening is large enough for water and other fluids to pass through with ease but too small for the thicker chyme to pass through except when a strong peristaltic contraction pushes it through.
Even then, of the 30 ml of chyme that the antrum can hold, usually only a few millimeters of antral contents are forced into the duodenum with reach peristaltic wave.
Before more chyme can be squeezed out, the peristaltic wave reaches the pyloric sphincter and causes it to contract more forcefully, sealing off the exit and blocking further passage into the duodenum.
The bulk of the antral chyme that was being propelled forward but failed to be pushed into the duodenum is abruptly halted at the closed sphincter and is tumbled back into the antrum, only to be propelled forward and tumbled back again as the new peristaltic wave advances. This tossing back and forth accomplishes thorough mixing of the chyme in the antrum.
Stomach Secretion and its Role in Digestion and Absorption of Food
A. The Stomach Secretes Hydrochloric Acid (HCl) and Enzymes that begin Protein Digestion
Each day the stomach secretes about 2 liters of gastric juice. The cells responsible for the gastric secretions are located in the lining of the stomach, the gastric mucosa.
Parietal cells actively secrete HCl into the lumen of the gastric pits, which in turn empty into the lumen.
Although HCl does not actually digest anything, it performs several functions that assist digestion. Specifically, HCl
- Activates the enzyme precursor pepsinogen to an active enzyme, pepsin, and provides an acid medium that is optimal for pepsin activity
- Aids in the breakdown of connective tissue and muscle fibers, thereby reducing large food particles
- Denatures protein; that is, it uncoils proteins from their tertiary structure, thus exposing more of the peptide bonds for enzymatic attack
B. Stomach Secretes Intrinsic Factor, which plays an Important Role in Absorption of Vitamin B12
Intrinsic factor, another secretory product of the parietal cells in addition to HCl, is important in the absorption of vitamin B12. This vitamin can be absorbed only hen in combination with intrinsic factor. Binding of the intrinsic factor-vitamin B12 complex with a special receptor located only in the terminal ileum, the last portion of the small intestine triggers the receptor-mediated endocytosis of the complex at this location.
Role of Stomach In Absorption
No food or water is absorbed into the blood through the stomach mucosa. However, two noteworthy non-nutrient substances are absorbed directly by the stomach (ethyl alcohol and aspirin).
Alcohol is somewhat lipid-soluble, so it can diffuse through the submucosal capillaries. Yet although alcohol can be absorbed by the gastric mucosa, it can be absorbed even more rapidly by the intestine mucosa, because the surface area for absorption in the small intestine is much greater than in the stomach.
Another category of substances absorbed by the gastric mucosa includes weak acids, most notably acetylsalicylic acid (aspirin). In the highly acidic environment of the stomach lumen, weak acids are almost totally un-ionized; that is, the H+ and associated anion of the acid are bound together.
In an un-ionized form, these weak acids are lipid-soluble, so they can be absorbed quickly by crossing the plasma membranes of the epithelial cells that line the stomach. Most other drugs are not absorbed until they reach the small intestine, so they do not begin to take effect as quickly.