The digestive system: what is it, how is it made, what is it for and how does it work?

Our digestive system has the task of introducing, digesting and absorbing the nutrients contained in food by eliminating the residues that cannot be used for one’s metabolism, in the form of feces. It has a simple structure, similar to a long and tortuous tube, but at the same time extremely complex and functional for the various tasks that it must perform in the different sections. Let’s briefly see what they are.

Mouth
Here the food is ingested and in some cases mechanical fragmentation begins by means of the teeth and chemical fragmentation by means of the secretions of the oral glands, among which the most common are the salivary ones. Not all animals have these structures attached to the oral cavity that have evolved in terrestrial tetrapods mainly to moisten food and facilitate its ingestion, a feature often useless in the aquatic environment. In mammals saliva also contains certain enzymes such as lysozyme which performs an antibacterial function and ptyhaline amylase, which begins the digestion of starches, especially cooked ones.

Pharynx
The pharynx is the second portion of the alimentary canal and is an organ in common with the respiratory system. It receives the food bolus from the oral cavity and conveys it, by swallowing, into the esophagus. At the same time it is able to convey the inspired air towards the larynx. With swallowing, the larynx rises and the epiglottis (a small valve), closes the entrance to the trachea, moving together with the initial part of the same upward and forward providing space for the passage of the food bolus in the esophagus. With this semi-voluntary action, peristalsis begins, which will spread through the esophagus, thus transporting the bolus to the stomach. The peristaltic movements of the digestive tract are so powerful that they can get food into the stomach even by swallowing upside down.

Esophagus
The esophagus is a canal about 24 cm long that crosses the diaphragm and reaches the stomach. The wall of the esophagus, like that of the rest of the alimentary canal, is made up of two layers of smooth muscle (longitudinal and circular). The contraction of the innermost circular layer causes the closure of the alimentary canal, while the longitudinal muscular layer causes the shortening of the esophagus. The alternating contractions of the circular and longitudinal muscles are responsible for the peristaltic movements, which push the bolus downwards towards the cardioesophageal valve.

Stomach
It is the first hollow organ of the digestive system identifiable in the abdominal cavity, it has a temperature of 37 degrees centigrade and a pH of 3.5. The epithelium of the stomach mainly secretes proteolytic enzymes such as pepsin and chymosin, H2O and HCl (hydrochloric acid, which has the function of activating the (inactive) pepsinogen enzyme in pepsin (active), which hydrolyzes proteins into amino acids). The esophagus, after passing through the esophageal vent (lat: aditus esofageum), merges with the stomach at the level of the (incontinent) valve of the cardias. In the stomach, the main attack on food substances occurs and their lysis (especially as regards proteins), by means of lytic enzymes, including pepsin, which is used to make proteins simpler substances, and acid hydrochloric, which makes the contents of the stomach acid; these substances, together with water, make up gastric juices. The walls of the stomach are also covered by a layer of mucus, produced by the muciparous cells of the gastric dimples, which has the task of protecting the gastric wall from the corrosive action of hydrochloric acid. Thanks to the peristaltic movements, the bolus, transformed into chyme, moves towards the pylorus which will take it to the small intestine.

Intestine
It is a tubular hollow organ, which varies its diameter during its tract, divided into: small intestine and large intestine.
The small intestine is 3-4 m long; it is divided into duodenum, jejunum and ileum. Digestion is completed in the small intestine thanks to the action of bile (which emulsifies fats), pancreatic juice (composed of the enzymes amylase trypsin lipase) and enteric juice (contains the enzymes that conclude digestion, transforming the various nutrients in amino acids, glucose, fatty acids and glycerin). The small intestine has an inner membrane that makes it rough, the intestinal villi that are rough thanks to the blood vessels that have a thin wall just to better absorb substances. Small and large intestine are divided thanks to the ileociecal valve.

Large intestine

The large intestine (also called the ” large intestine “) is on average 170 cm long, with a diameter of about 7 cm. The connection point between the small and large intestine are the ileocecal valve, which connects the ileum (last part of the small intestine) and the blind part of the large intestine (first part of the large intestine). Anatomically the large intestine is divided into three sections which are respectively called:

  • blind;
  • colon;
  • rectum.

The colon is further divided into four parts:

  • ascending colon;
  • transverse colon;
  • descending colon;
  • sigmoid or iliopelvic colon or sigmoid colon.

The rectum is followed by the anus, which allows the stool to escape.

Differences in morphology between small and large intestine (summary)

The large intestine physically differs from the small intestine, in that the former turns out to be much larger. The wall of the large intestine is lined with simple columnar epithelium; instead of having the evaginations of the small intestine (intestinal villi), the large intestine has invaginations (the intestinal glands). Although both the small and large intestines have goblet cells, these are more abundant in the second.

Differences in functions

  • Inside the duodenum (the first portion of the small intestine) most of the digestive processes (chemical digestion) are completed. In fact, the bile coming from the pancreas through the bile duct serves to emulsify (reduce to small droplets) the fats and make them digestible. In addition, pancreatic juice is used to digest carbohydrates, proteins and lipids – in partnership with bile – thanks to the cocktail of enzymes produced by its exocrine portion, such as proteases, lipases and pancreatic amylase. In addition, the same enterocytes actively participate in chemical digestion thanks to some enzymes present within them that have to do above all with the digestion of carbohydrates and proteins. They contain exopeptidases (for proteins), lactases, sucrases and maltases for carbohydrates,
  • The mesenteric small intestine (i.e., jejunum and ileum), on the other hand, is particularly the theater for the absorption of nutrients. The process is favored by the vast absorption surface with villi, microvilli and circular folds.
  • The task of the large intestine is to absorb water and electrolytes, equal to about 1.5 liters per day, which pass through the ileocecal valve. The colon absorbs vitamins that are created by colon bacteria, such as vitamin B, vitamin K, cobalamin, thiamine and riboflavin. It also compacts the stool and keeps the fecal matter in the rectum until it can be discharged through the anus during defecation.

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