Digestion is the process through which food is broken down into smaller components for easy absorption into the bloodstream.
The digestion process involves several stages involving both the mechanical and chemical breakdown of food. Mechanical digestion refers to the physical breakdown of food, such as through chewing or the muscular movements of the stomach that turn and mix the food with digestive enzymes. Chemical digestion refers to the breakdown of food into simple molecules by digestive enzymes and stomach acids. For example, starch is broken down by amylase in the saliva and protein is broken down by gastric juices in the stomach.
Digestion involves four different stages and these are outlined below.
Ingestion describes the entry of food into the gastrointestinal tract through the mouth. The food is then swallowed and carried down to the stomach by the esophagus.
Inside the mouth, food is immediately acted on by mechanical digestion as it is moved around by the tongue and broken down by the teeth in a process called mastication. Chemical digestion also begins with the breakdown of starch components by amylase present in the saliva.
The ball of food that remains after the actions of the tongue, teeth and saliva is referred to as a bolus. This bolus passes down the esophagus and into the stomach and intestine where the food is mixed with gastric juices so that complex food molecules can be broken down into simple molecules. For example, carbohydrate is broken down to give glucose.
The majority of digestion occurs in the small intestine where digestive enzymes secreted by the liver and pancreas complete the digestive process so that the nutrients can be absorbed.
Absorption begins in the small intestine. Small projections called villi and microvilli present in the folds and wrinkles of the stomach provide a large surface area for the diffusion of food molecules into the blood vessels of the intestinal wall.
Waste such as undigested materials and bacteria are passed out of the rectum in the form of feces.