digestion process in human being
Digestion can be defined as the mechanical and chemical reduction of ingested nutrients first into particles, then into molecules, so that they become small enough to move through columnar epithelial cells of the intestine into the blood.
Digestive organs and their functions
(i) Mouth: Digestion of food begins in the mouth. The mouth comprises of the following:
Teeth : They tear and break down the food.
Saliva : It contains a digestive enzyme called salivary amylase, which breaks down starch into sugar.
Tongue : It is a sense organ with taste buds, which help us to differentiate between various food items. The muscular movements in the tongue move the food from the mouth into the throat, or pharynx.
(ii) Pharynx: It is a common passageway for food and air. It opens into the oesophagus (which leads to the stomach) and trachea (which leads to the lungs).
(iii) Oesophagus: The circular smooth muscles in the oesophagus contract when food is swallowed. This prevents the chewed food material from moving back into the mouth. This is followed by the contractions and relaxations of the longitudinal smooth muscles, which push the digested food forward. These movements are called peristaltic movements, which push the food into the stomach.
(iv) Stomach: The stomach stores and mixes the food received from the oesophagus with the gastric juice. The gastric glands present in the walls of the stomach secrete several substances, which together constitute the gastric juice.
The main components of gastric juice are hydrochloric acid, mucus, and pepsinogen.
Hydrochloric acid dissolves bits of food and creates an acidic medium. In this medium, pepsinogen is converted to pepsin, which is a protein-digesting enzyme.
The food from the stomach passes into the small intestine.
(v) Small intestine: It is the longest part of the alimentary canal. It is made up of three parts- duodenum, jejunum, and ileum.
The small intestine produces intestinal juice from the glands present in the wall, which helps in further digestion of food.
Digestive juices from two glands, namely the liver and pancreas mixes with the food in the small intestine.
The liver produces bile juice (which causes emulsification of fats) and the pancreas produces pancreatic juice (for digesting proteins and emulsified fats).
The small intestine is the site for complete digestion of carbohydrates, proteins, and fats.
This digested food is then absorbed through the intestinal walls. The inner lining of the small intestine has millions of tiny finger-like projections called the villi . These projections increase the surface area of the small intestine for more efficient food absorption.
Within these villi, the capillaries absorb nutrients from the digested products of proteins and carbohydrates and lead them into the blood stream.
(vi) Large intestine: The indigestible material and water enters the large intestine. It also has villi to absorb water and some vitamins from the undigested food. This absorption of water helps to compact the faeces. It also performs the function of storage of wastes before they are excreted from the body via the anus.
Digestionis the mechanical and chemical breakdown offoodinto smaller components that are more easilyabsorbedinto ablood stream, for instance. Digestion is a form ofcatabolism: a breakdown of large food molecules to smaller ones.
In thehuman digestive system, food enters the mouth and mechanical digestion of the food starts by the action ofmastication, a form of mechanical digestion, and the wetting contact ofsaliva. Saliva, a liquid secreted by thesalivary glands, containssalivary amylase, an enzyme which starts the digestion of starch in the food. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called abolus. It will then travel down theesophagusand into thestomachby the action ofperistalsis.Gastric juicein the stomach startsprotein digestion. Gastric juice mainly containshydrochloric acidandpepsin. As these two chemicals may damage the stomach wall,mucusis secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs byperistalsis, which is waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes.
After some time (typically 1-2 hours in humans, 46 hours in dogs, 3-4 hours in house cats), the resulting thick liquid is calledchyme. When thepyloric sphincter valveopens, chyme enters theduodenumwhere it mixes with digestive enzymes from thepancreas, and then passes through thesmall intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. Water and minerals are reabsorbed back into the blood in thecolon(large intestine) where the pH is slightly acidic about 5.6 ~ 6.9. Some vitamins, such asbiotinandvitamin K(K2MK7) produced by bacteria in the colon are also absorbed into the blood in the colon. Waste material is eliminated from therectumduringdefecatio