Digestion refers to the breakdown of food into smaller components that can be absorbed into the bloodstream. This digestion or catabolism is divided into two types – the mechanical digestion of food that occurs in the mouth when it is physically broken up into smaller pieces and the chemical digestion that occurs in the gastrointestinal tract when the food is broken down into small molecules by digestive enzymes.
In mammals, digestion involves the intake of food through the mouth where it is chewed between the teeth in a process called mastication. The saliva present in the mouth contains amylase which begins the break down of starch in the food.
The food then travels down the food pipe or the esophagus to reach the stomach where gastric juices start to breakdown the protein in the food. The gastric juice mainly comprises pepsin and hydrochloric acid. As these substances can be damaging to the stomach, mucus is also secreted to line the stomach and protect it. The acids in the stomach work by denaturing the proteins present in food. While this chemical digestion is happening, further mechanical digestion is occurring through persitalsis, as the muscles in the stomach wall contract to mix up the food and digestive enzymes.
In humans, the food stays in the stomach for around an hour or two and then moves into the small intestine where numerous enzymes are released to further break down the food components. The walls of the small intestine form wrinkles and folds that contain projections called villi and mirovilli that provide a large surface area for the diffusion of digested food molecules into the blood vessels of the intestinal wall.
The liver and pancreas both play important roles in digestion. The liver produces the bile (required for digestion), which is carried into the small intestine by the bile duct and the pancreas secretes the pancreatic juices that also aid digestion and nutrient absorption. For example, the complex carbohydrates in food such as those found in rice and bread are broken down into simple sugars such as glucose that can be readily absorbed by the body to provide energy for various cellular functions.
After passing thought the small intestine, food moves into the large intestine which absorbs the remaining nutrients and water. The resulting fecal material is stored in the rectum, from where it passes out of the body though defecation.