Digestion in Stomach
At the junction between oesophagus and the stomach, there is a special ring of muscles called the cardiac sphincter. When the sphincter muscles contract, the opening to the stomach closes and thus avoids the contents of the stomach from returning into the oesophagus.
Also Read: Part 1 – Digestive System of Humans
It opens when a wave of peristalsis coming down the oesophagus reaches it. The stomach is located below the diaphragm on the left side of the stomach cavity. It is a flexible muscular bag that stores food from meals for some time, making alternate feeding possible. It likewise partly digests the food.
The stomach wall is composed of 3 primary layers: an external layer of connective tissue; the middle layer of smooth muscles and the inner layer (mucosa) of connective tissue with lots of glands. The middle layer of muscles includes external longitudinal and inner circular muscles. These muscular layers assist in churning and mixing the food with the stomach secretions. The mucosa of the stomach possesses various tubular gastric glands, which are made up of three kinds of cells;
- a) mucous cells, that secrete mucous,
- b) parietal or oxyntic cells produce hydrochloric acid and
- c) zymogen cells, which secrete pepsinogen.
The secretion of all these cells is collectively called gastric juice. The secretion of the stomach juice is regulated by odour, sight, and quality of food.
Mucous is a thick secretion that covers the inside of the stomach. It protects the underlying walls. Hydrochloric acid is secreted in concentrated form. It adjusts the pH of stomach contents ranging from 2-3 for the pepsin to act on proteins. It also softens the food and eliminates lots of bacteria taken in along with the food from being digested.
Pepsin is an enzyme secreted in a non-active form called pepsinogen. Pepsinogen is converted to pepsin when exposed to the acidic medium or to some currently activated pepsin. Pepsin hydrolyses protein to yield peptones and polypeptides. The muscles of the stomach wall thoroughly mix up the food with stomach juice and ultimately convert it to a semi-solid mass called chyme. Gradually the stomach empties into the duodenum through the relaxed pyloric sphincter.
Food digestion in the small intestine
The small intestine in humans consists of the duodenum, jejunum, and ileum. Duodenum is about 20-25 cm long, which leads into the jejunum and then ileum. When chyme passes from the stomach into the duodenum, its acidity promotes the release of secretions from pancreas, liver and duodenal cells.
Pancreas is a large gland whose exocrine tissue secretes a juice that flows through pancreatic duct into the duodenum. This juice is consisting of enzymes that absorb all primary elements of food i.e. carbohydrates, fats and proteins.
Carb- absorbing enzyme is pancreatic amylase also called amylopsin, which absorbs starch into maltose. The fat absorbing enzyme is lipase, that hydrolyzes a little percentage of fats into fatty acids and glycerol. Like pepsin, trypsin is also secreted as inactive trypsinogen, which is activated by enterokinase, an enzyme secreted by the lining of the duodenum.
Trypsin splits proteins into peptones and polypeptides. Pancreatic juice also contains sodium bicarbonate, which partially neutralizes the chyme coming from the stomach. This is necessary because enzymes of the pancreas do not work well in acid conditions.
Liver secretes bile, which might be briefly kept in the gall bladder and released into the duodenum through the bile duct. The bile is green, watery fluid. It includes no enzymes, but its green colour is because of the bile pigments, which are formed from the breakdown of haemoglobin in the liver. The bile also includes bile salts, which act on fats, and emulsifies them. It means that they break them up into little globules, which are then easily digested by water-soluble lipase.
If bile pigments are prevented from leaving the gastrointestinal system, they might collect in the blood, causing a condition referred to as jaundice. Cholesterol, produced by the liver, may speed up in the gall bladder to produce gall stones, which might block the release of bile.
Jejunum is the second part of the small intestine extending from the duodenum to the ileum. It is about 2.4 meters in length comprising about two-fifth of the small intestine. The lower three-fifths of the small intestine from the jejunum is the ileum. The food, which escapes undigested from the duodenum, is totally absorbed in the jejunum and ileum by a group of enzymes included in the gastric juice.
Absorption of food
As we know the small intestine includes the duodenum, jejunum, and ileum. Nearly all absorption of the items of digestion happens in the ileum. The internal surface area of ileum has lots of folds, which show velvety look due to the existence of various finger-like outgrowths called villi.
Each villus is richly supplied with blood capillaries and a vessel called lacteal of lymphatic system with a covering of epithelial cells. Electron microscopic lens exposes that these cells have countless, closely packed cylindrical processes, microvilli. The overall area of absorption becomes extremely large due to the enfolding, villi and microvilli.
Sugars and amino acids are absorbed by diffusion or active transport into the blood capillaries through the microvilli. Some of the fatty acids and glycerol are also taken into the bloodstream. However, a large proportion of fatty acids and glycerol get in the epithelial cells of villi, where they recombine into fats. These fats then go into the lacteals.
Proteins present in lymph vessels combine with fat molecules to form lipoprotein droplets. These pass into the bloodstream via the thoracic lymphatic duct. The lipoproteins are subsequently hydrolysed by blood plasma enzyme and enter body cells, where they might be utilized in respiration or kept as fat in the liver, muscle or under the skin.
The intestinal tract contents are pushed along the alimentary canal by typical peristaltic activity. At the end of ileum, there is an ileocolic sphincter that opens and closes time to time to allow a small amount of residue from the ileum to go into the large intestine.
Large intestinal intestine
The large intestinal tract is made up of a caecum, colon and rectum. Caecum is a blind sac that projects from the large intestine in between ileum and colon. From the blind end of the caecum there emerges a finger-like process called appendix. The appendix sometimes gets inflamed due to entrapping and then purification of food causing appendicitis, which has to be removed surgically in numerous instances.
The product that passes from the small intestine to the big intestine includes a large quantity of water, dissolved salts, and undigested material. Water and salts are taken in into blood, while undigested material is rejected as faeces. The faeces contain a large number of bacteria, plant fibers, sloughed of mucosal cells, mucous, cholesterol, bile pigments and water. Large intestinal tract also harbors a big population of useful bacteria that manufacture some vitamins especially vitamin K,which are absorbed in the blood.
Rectum is the last part of the large intestinal tract, where faeces are temporarily saved and turned down through the rectum, at intervals. The anus is surrounded by 2 sphincters, the internal is of smooth and external of striped muscles. Under typical conditions, as the anus is filled up with faeces, it generates a defecation reflex. This reflex can be consciously inhibited in individuals other than infants. Slowly the kids learn to bring this reflex under control.