The digestive system of human is composed of salivary glands, liver, gall bladder, and pancreas which produce different substances that contribute to digestion. The alimentary canal then does not have a uniform diameter and, in some sections, folds numerous times on it to increase the surface useful for digestion and absorption.
The path of food is from the mouth, it passes into the pharynx and from here into the esophagus; then accumulates in the stomach and finally pours into the intestine. Digestion begins in the mouth due to the mechanical shredding action of the teeth and the biochemical action of saliva, secreted by three pairs of salivary glands. Saliva contains the enzyme amylase, which begins the demolition of starch (a polysaccharide) into maltose (a disaccharide sugar). While it is shredded, the food is also fluidized by the aqueous component of the saliva, until it becomes dough called a bolus. The bolus is swallowed through the pharynx by means of coordinated acts and reflexes of the tongue.
The pharynx is the canal that connects the mouth with the esophagus and the trachea. At the base of the pharynx, there is a membrane, the epiglottis, which during swallowing, lowers and closes the entrance to the trachea to prevent the bolus from passing into the respiratory system.
The esophagus is a muscular canal about 25 cm long, which runs along the thorax, parallel to the trachea. Its function is to convey the bolus into the stomach by means of peristaltic movements (coordinated contractions of the circular and transverse muscle bundles surrounding the esophagus). The passage of the bolus from the esophagus to the stomach is regulated by a muscle ring, the cardia.
The stomach is a muscle bag, capable of accommodating 2 to 4 liters of liquid and solid substances, in which food remains for 2 to 5 hours, depending on its composition. The mucous membrane of the stomach is provided with numerous glands that secrete gastric juice, composed of pepsinogen (the inactive form of the pepsin enzyme), hydrochloric acid, and mucus.
The stomach performs four functions:
1. Stores food and regulates its flow to the intestine.
2. It breaks down the food mechanically, by means of numerous contractions.
3. Transforms pepsinogen into pepsin, which splits proteins into shorter chains of amino acids.
4. Creates a very acidic environment (pH 1 to 3), which, in addition to promoting the action of pepsin, has an antibacterial action.
The mucus produced by the gastric glands covers the internal wall of the stomach to protect it from the aggression of hydrochloric acid and pepsin (which could act on the proteins of the gastric cells themselves). In the stomach, the bolus is transformed into mush, the chyme, which gradually passes through the muscular ring of the pylorus, which enters the intestine.
The intestine is divided into two parts, the small intestine and the large intestine, distinguished by the shape and function they perform. In the small intestine, digestive processes are completed, and most of the absorption of nutrients occurs. The secretions of two annexed organs of the digestive tract pour into the liver and pancreas.
About 6 m long in an adult man, the small intestine wraps itself several times. It is divided into three portions i.e. the duodenum (where the ducts that come from the liver and pancreas), the jejunum, and the ileum. The intestinal wall is home to numerous glands that produce enteric juice. It is composed of several enzymes like protease, which acts on proteins completing their cleavage into amino acids, lactase, maltase, and saccharase. They split carbohydrates into monosaccharides, lipase, which acts on lipids.
Other glands produce mucus, which protects the walls of the intestine.
The internal wall of the small intestine folds in on itself and is covered with villi, tiny overtones, which in the duodenum reach a few thousand per 2 cm. In turns, the villi are covered with microvilli, cells whose wall facing the lumen. Villi and microvilli contribute to increasing the absorption surface of the small intestine. Each villus is internally traversed by blood capillaries and a lymphatic vessel, which absorb nutrients like water and mineral salts. They rapidly absorb the carbohydrates in the form of monosaccharides (glucose, galactose) and proteins. The protein split into amino acids and lipids in the form of fatty acids.
The large intestine is made up of three parts i.e. colon (ascending, transverse, and descending) rectum and cecum. The appendix protrudes from the blind pouch, a small worm-like diverticulum that plays no role in the digestion.
Digestion residues, mainly water, and undigested substances reach the colon. The water is absorbed, and the waste substances reach the rectum in a semi-solid form (feces), ready to be expelled through the anus.
The liver is the most voluminous gland in the human body, located in the upper right part of the abdomen. It has an ovoid shape and a reddish-brown color; it consists of two large lobes (right and left) and two smaller parts. It intervenes in digestion by secreting bile.
The liver also performs multiple other functions, including stores glycogen.
- Transforms excess amino acids compared to the needs of protein synthesis into sugars and fats. It transforms fats into sugars and converts ammonia, which forms as a nitrogen waste substance into urea; demolishes toxic or harmful substances.
Bile is a greenish mixture containing water, cholesterol, and bile salts. Bile salts are not enzymes, they emulsify fats, that is, they disperse them in minute droplets, thus preparing them for the action of lipase. The liver produces about 1 liter of bile per day, which it stores in its small bag, the gallbladder. During the meal, bile is discharged into the duodenum through the bile duct.
The pancreas is a large, elongated gland located transversely in the upper part of the abdomen. The activity of the pancreas is twofold i.e. as an endocrine gland, it produces the hormones insulin and glucagon, which regulate glucose metabolism, the exocrine part secretes the pancreatic juice. In pancreatic juice, there are several digestive enzymes, amylase, lipase, and protease, which are respectively responsible for the demolition of sugars, lipids, and proteins. The pancreatic juice has a high concentration of sodium bicarbonate, which neutralizes the acidity of the chyme coming from the stomach and helps to maintain an alkaline environment inside the intestine.