Function and Physiology of digestion

Digestion

The complicated process of digestion, which takes place in the alimentary canal, involves chemical and physical changes to the meal that get it ready for absorption. As food is broken down into tiny pieces and moved down the alimentary canal by mechanical digestion, digestive wastes are eventually expelled from the body. The primary functions of mechanical digestion include chewing or mastication, swallowing or deglutition, peristalsis, and feces. Large, indigestible food molecules that may pass past the intestinal mucosa and into the blood and lymph are broken down by chemical digestion. Saliva, gastric juice, pancreatic juice, and intestinal juice enzymes all play a role in the various chemical processes that make up chemical digestion. There are three stages to the digestive process:

• The Cephalic Phase.
• The Gastric Phase.
• The Intestinal Phase.

Cephalic Phase

Beginning with the external stimulation of feeding is the cephalic phase. The cerebral cortex, the hypothalamus's hunger region, and the amygdala are all stimulated by the smell, sight, and thinking of food. In response, the salivary glands release saliva, salivary amylase, salivary lysozyme, and salivary lingual lipase. The vagus nerve also causes the stomach glands to release gastric juice.

Chemical and physical digestion take place in the mouth through lingual lipase and salivary amylase, respectively. The bolus is now transported to the stomach by gravity and peristalsis.

Gastric Phase

The gastric phase begins when food enters the stomach. It is period in which swallowed food activated gastric activity in the stomach. The swallowed food distends the stomach, raises the pH of ts contents and stimulates stretch receptors in the stomach walls to release gastrin. The gastrin stimulates parietal cells to secrete hydrochloric acid and intrinsic factor. The chief cells secrete pepsinogen in response to gastrin. The hydrochloric acid in the gastric juice activates the pepsin from the pepsinogen which breaks up protein into peptides and amino acids. The gastrin secretion is inhibited when the pH of gastric juice drops below 2.0 and stimulated when the pH rises. The peristaltic waves not only mix the food with gastric food, but also push the food contents (chyme) into the duodenum. Thus the stomach digests the food into chyme by the action of pepsin (protein digestion) and mechanical digestion (churing) during the gastric phase of digestion. The acidic gastric environment kills the microbes present in food.

Intestinal Phase

The intestinal phase of digestion begins once food enters the small intestine. This phase occurs in the duodenum as a response to the arriving chyme. It moderate gastric activity via hormones and nervous reflexes.

As chyme enters the duodenum, it releases a hormone, intestinal gastrin, which further stimulates the gastric gland secretion. Further distention of the duodenal wall by chyme stimulate stretch receptors in the duodenal wall to stimulate the sympathetic nerves to the stomach. As a result, gastric motility is inhibited, contraction of the pyloric sphincter is increased which decreases gastric emptying.

The chyme containing partially digested protein and fat stimulate the release of two major intestinal hormones: cholecystokinin (CCK) and secretin. CCK stimulates secretion of pancreatic juice that is rich in digestive enzymes. It also causes contraction of the gall bladder to release stored bile through the common bile duct which helps in fat digestion. The secretin further stimulate the pancrease for the flow of pancreatic juice.

The intestinal enzymes along with pancreatic enzymes and bile help in the digestion of complex carbohydrates, partially digested protein and fat molecules.

Absorption

Following digestion, food is absorbed, or travels through the small intestine's mucous membrane lining and into the blood and lymph. The process through which molecules of amino acids, glucose, fatty acids, and glycerol are absorbed from the interior of the intestines into the body's circulating fluids is known as food absorption. Food absorption is just as important as food digestion. The cause is rather clear. Food that remains in the intestines cannot feed the zillions of cells that make up the rest of the body. Their survival depends on the blood's ability to carry the food that has been digested to them and absorb it. The absorptive surface as well as the effectiveness and speed of material transfer from the intestinal lumen to bodily fluids are increased by structural modifications of the digestive tube, including folds in the lining mucosa, villi, and micro villi. The gut mucosa actively transports several salts, including sodium. Water comes next via osmosis. In the intestinal villi, additional nutrients are also actively carried into the blood capillaries. Fats enter the lacteals in the intestinal villi or lymphatic vessels.