Digestion of food forms an essential process in a living organism. Digestion of food can be classified into two categories: (1) mechanical digestion and (2) chemical digestion. Whereas mechanical digestion contributes to the physical breakdown of food using mechanical forces, chemical digestion contributes via breaking down the chemical bonds present in the food. The process is manifested by muscular contraction. The intestine develops undulations via contraction of the muscle fiber that involves contraction and relaxation of muscle fibers (referred to as the peristalsis). As a consequence, the momentum generated by the muscular contraction serves to develop fluid motion in the lumen, which eventually leads to mixing and transport. Whereas the bodily secretions of the digestive enzymes participate in innate digestion, the gut flora, on the other hand, also influence digestion to an extent by means of fermentation. Small intestinal peristalsis serves as the key mediator, whether the digestion proceeds normally during physiology or abnormally leading to pathology.
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Digestion is the process of breaking down ingested food into a simpler form that can be easily absorbed by the body. It is carried out by two sets of organs, primary and assisting organs. Primary organs include the mouth, esophagus, stomach, small intestine, and large intestine, whereas, assisting organs include salivary glands, pancreas, liver and gall bladder. Primary organs function as storage, mixing and transport and the assisting organs helps in the facilitating the digestive process through secretion of the digestive juices directly in to the lumen of the primary organ
The process of digestion begins in mouth after ingestion of food (Figure 1). It is broken down into smaller particles by teeth (also known as mastication) and mixed with saliva to bind it together in the form of a bolus. It then enters the esophagus and moves towards stomach by the action of peristalsis. Stomach breaks down the food into simpler molecules with the help of enzyme in a highly acidic condition (pH 1-3). Such acidic environment is crucial for digestion as it not only facilitates the mechano-chemical breakdown but also helps in destroying pathogens that come along with the food during ingestion.
After a lapse of 1-2 hours in the stomach, the partially digested food, also known as the chyme, enters the duodenum (duodenum is first segment of the small intestine, followed by jejunum and ileum) through a small opening known as the pylorus. The pH of chyme entering from stomach to small intestine increases from pH of 1.7 to 5 at initial portion of the duodenum and then gradually increases to pH 6 in the distal duodenum to a pH of 7.4 in the ileum (Avvari, 2019a). Duodenum, which lies next to the pyloric valve, is the shortest in length of around 30 cm. It facilitates the digestion of the chyme by allowing the digestive enzymes from pancreas and bile juice from liver to directly enter into the lumen. These enzymes help in digesting carbohydrates and lipids, whereas, secretions from pancreas increase the pH level of duodenum to make it slightly alkaline. Jejunum, connected next to duodenum, is of length 2.5m. It is involved in absorption of sugar, fatty acids and amino acids from digested food with the help of villi. Villi are small finger-like structures present throughout the interior surface of the gut. It helps to the effective surface area of contact for the luminal contents so the digested product can be absorbed. Ileum is the last section of small intestine of length 3m. It also contains small finger like projections for nutrient absorption similar to jejunum. It is responsible for absorption of nutrients that were not absorbed in jejunum. Ileum is then connected to large intestine by ileocecal valve. Food at the terminal ileum is then emptied into the large intestine eventually, where the final transformation of the food takes place via absorption of water to eliminate the waste in the form of feces before excreting it from the body.
Figure 1. Schematic diagram of the human digestive system showing small intestine as a convoluted tube and its association with pancreas, liver and gall bladder.
The digestion can be classified into two types, mechanical digestion and chemical digestion.
Chemical digestion involves the chemical breakdown of bonds of ingested food to produce simpler compounds and provide an appropriate pH balance to ease the process of digestion. This is carried out with the help of several enzymes and secretions such as saliva (in mouth), hydrochloric acid and gastric juice (in stomach), digestive enzymes (in pancreas) and bile juice (in liver). Chemical digestion starts in the mouth by digestion of carbohydrates content of the food by salivary amylases. In stomach, secretion of hydrochloric acid and pepsin aids the digestion of protein content and reduces the pH levels to 1.5-3. The walls of stomach secrete mucus and bicarbonate to shield itself from such low pH environment. In small intestine, digestive enzymes from pancreas and bile juice from liver mix together with the chyme coming from stomach to aid digestion during peristalsis and to increase pH levels.