Digestion of Food

• Digestion is the process of breaking down food into smaller molecules to be absorbed by the body.
• The journey of food starting from the mouth down to the anus can be summarized by a sequence of events namely: ingestion, propulsion, digestion, absorption, assimilation, and excretion.
• Ingestion


  It is the process of taking in food into the mouth and the gastrointestinal tract.
• Saliva has the enzyme amylase. This enzyme is responsible for breaking down of starch (sugars in plants) and glycogen (sugars in animals).
• Ingestion also involves grinding of food by the teeth known as mastication.
• Cows, sheep, goats and other ruminants are capable of breaking down cellulose found in plant material. It is because they have bacteria in their gastrointestinal tract called the rumen microbes. These bacteria aid in the production of an enzyme called cellulase that serves to break down cellulose.
• Propulsion


  Food must then be moved from the mouth into the gastrointestinal tract.
• Peristalsis is the process wherein muscles of the gastrointestinal tract involuntarily contract and relax alternately to create rippling movements.
• The mixed and partially digested food is called chyme.
• The process of digestion can be divided into mechanical and chemical digestion.
• Mechanical digestion is the process of physically breaking down food into smaller pieces without chemical reactions taking place. The mastication of food done by teeth exemplifies mechanical digestion.
• Done by the small intestinal muscles, segmentation is the action of moving food back and forth the intestine.
• Chemical digestion, on the other hand, is the breakdown of larger molecules into their smaller building blocks via chemical reactions.
• The building blocks of carbohydrates are monosaccharides. Amino acids are the building blocks of proteins. As for lipids, it would be fatty acids and glycerol.
• The presence of food triggers the stomach to release the hormone gastrin. Consequently, gastrin signals the stomach to produce more pepsinogen, mucus, and hydrochloric acid.
• The high acidity of the stomach brought by hydrochloric acid activates pepsinogen to become pepsin which will digest proteins into amino acids.
• Villi are finger-like projections which cover the walls of the intestine. Villi are further divided into smaller projections called microvilli.
• Absorption in the small intestine happens when nutrients that come from the digested food are absorbed and transported into the bloodstream.
• Capillaries are tiny blood vessels that absorb water-soluble nutrients such as glucose and amino acids.
• Lacteals are lymphatic vessels which absorb fatty acids and glycerol.
• The process of building up of complex substances after absorption is called assimilation. It also involves the movement of nutrients absorbed by the blood.
• Feces is formed upon the absorption of excess water in the chyme. It contains indigestible materials like cellulosic fiber from plants, small amounts of water, mucus, and a large number of bacteria.
• Mass movements are slow and long contractions within the large areas of the large intestine.
• As feces fills the rectum, a defecation reflex then signals excretion.
• Excretion in the intestine happens as the feces are pushed through the anus and are eliminated from the body.