In the stomach, the food is mixed with the stomach secretion, the gastric juice (solution rich in hydrochloric acid and enzymes (pepsin and renin).
Pepsin breaks down proteins into small peptides. Renin, produced in large quantities in the stomach of newborns, separates milk into liquid and solid fractions. Despite being protected by a dense mucus layer, stomach mucosa cells are continually damaged and killed by the action of gastric juice. Therefore, the mucosa is always being regenerated. It is estimated that our stomach surface will be fully reconstituted every three days.
The stomach produces about three liters of gastric juice a day. The food can remain in the stomach for up to four hours or more and mixes with the gastric juice aided by stomach muscle contractions. The cake becomes an acidified and semi-liquid mass, the chyme. Passing through a muscle sphincter (the pylorus), the chyme is gradually released into the small intestine, where the most important part of digestion occurs.
Small intestine, pancreatic juice and bile
The small intestine is divided into three regions: duodenum, jejunum and ileum. Digestion of the chymus occurs predominantly in the duodenum and the first portions of the jejunum. In the duodenum also acts the pancreatic juice, produced by the pancreas, which contain various digestive enzymes. Another secretion that acts in the duodenum is bile, produced in the liver, which, although not containing enzymes, has the important function, among others, of transforming fats into microscopic droplets.
The pancreas secretes pancreatic juice, an alkaline solution formed by salts (among them sodium bicarbonate), water and several enzymes, whose main ones are:
- trypsin and chymotrypsin, two proteases that break down proteins into peptides. These enzymes are released by the pancreas in the inactive form of trypsinogen and chymotrypsinogen, respectively;
- pancreatic lipase, which acts on the digestion of lipids (triglycerides);
- pancreatic amylase (or amylopsin) which acts on starch, turning it into maltose;
- miscellaneous peptidases, which disrupt peptide bonds in peptides formed in protein digestion, leading to the release of amino acids;
- nucleases, which digest nucleic acids.
Bile: Physical Action on Lipid Digestion
Bile is a greenish fluid produced in the liver. Does not contain digestive enzymes. It is rich in water and mineral salts of alkaline nature. It is stored in the gallbladder, where it is concentrated for later release into the small intestine.
The action of bile in the digestive process is physical. It acts as a detergent and causes fat to emulsify by reducing the surface tension between the lipid molecules. This promotes the formation of droplets, which increases the total surface exposure of lipids, thus favoring the action of lipases.