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Functions of the digestive system

In the digestive tract the food is digested to the form that can easily be absorbed by cells, and then the food is absorbed. Mechanical processing of food begins in the oral cavity and is based on processing the portions of food. Thanks to this, during the next stage the enzymes that participate in the chemical digestion have easier access to them. Chemical digestionchemical digestionChemical digestion is an enzymatic (chemical) process of breaking down the ingredients of food, during which complex organic compounds are broken down into simple compounds that can be absorbed by the body. Digestive enzymes are secreted by secreting cells:

• salivary glands,

• wall of the stomach,

• pancreas,

• small intestine walls.

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Grafika przedstawia działanie enzymów. Wskazano białko, enzym, aminokwas, skrobie, cukry proste, tłuszcze, kwasy tłuszczowe i glicerol

Digested food in form of chemical compounds that can be used by the cells (e.g. amino acids, monosaccharides, glycerol and fatty acids) is absorbed by the intestinal cells into blood vessels and lymph vessels. Through them, food reaches the liver, and then all the tissues in the body. Undigested food is expelled out of the organism in the form of faeces. Working, the digestive system cooperates with other systems of our body.

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Schemat blokowy przedstawiający cztery przykłady interakcji układu trawiennego z innymi układami człowieka. Punktem centralnym schematu jest stwierdzenie: taking and chewing the food, które ma wpływ na 1. Recognizing the type and quality of food by means of receptors of taste and smell belonging to the nervous system. 2. Chewing billet implemented by the muscles of the face and lower jaw being a part of the musculoskeletal system. 3. Taking food as an unconditional reflex (sucking and conscious decision directed by the nervous system). 4. Neutralization of some pathogens found in the diet by immune cells.

Chemical digestion

Chemical digestion takes place because of digestive enzymes. These are the substances that work inside the digestive system, and which, with the help of water, allow to break down complex chemical compounds into simpler elements.

In the oral cavity the process of breaking down polysaccharides (starch, glycogen) into monosaccharides is initiated. This is possible thanks to the fact that saliva includes an enzyme called salivary amylasesalivary amylasesalivary amylase. It works on food while it passes through the oesophagus. It works only for a short time, so the saccharides are not completely broken down.

Stomach wall cells secrete gastric juice, which, besides hydrochloric acid includes also an enzyme that breaks down proteins - pepsin. PepsinpepsinPepsin cuts long strings of proteins into smaller fragments. Both hydrochloric acid and pepsin can be dangerous for the stomach, whose tissues are composed of proteins. Because of this, the hydrochloric acid is secreted only when the stomach receives food that irritates the stomach walls, whereas digestive enzymes work only in the presence of the hydrochloric acid.

Proteins, saccharides and fats are digested in the duodenum. Liver secretes bilebilebile which breaks down fats into small drops that are easily accessible for the enzymes that digest fats.

Pancreas secretes a number of enzymes, such as pancreatic amylasepancreatic amylasepancreatic amylase (that digests saccharides), lipaselipaselipase (that digests fats), nucleasenucleasenuclease (that digest nucleic acids), trypsintrypsintrypsin (that digests proteins). Trypsin cuts long strings of proteins into shorter ones, peptides, and then other digestive enzymes break them down into even smaller particles. The final product of the digestion of proteins are amino acids.

Absorbtion, use and storing of nutrients

Food remains in the duodenum long enough so that most of the compounds with large particles are digested to the point when they become monosaccharides, amino acids, glycerol and fatty acids.

Liquid contents of the digestive system are transported to the next of the small intestine which is surrounded by a very dense net of blood vessels and lymph vessels. That is where the products of digestion are absorbed through the walls of the small intestine into the blood and the lymph.

Blood vessels of the intestinal villi absorb glucose and amino acids, which then are transported to liver and then to body cells as energy source. Excess glucose is converted by the liver and the muscles into glycogen, spare sugar, and stored there. Mineral salts and vitamins are dissolved in water and transported into blood vessels. The result of the digestion of fats, fatty acids and glycerol, pass from the intestine into lymph vessels and then into blood, with which they flow to all the cells of the body. There, they will be used to build the cell membrane and as energy. Excess of fat is stored in adipocytes. Together with fatty acids, lymph received vitamins that are dissolved in fats.

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Ilustracja przedstawia schematycznie drogę strawionego pokarmu do komórek ciała. Na tle trzech szarych kół narządy człowieka. Od lewej kosmki jelitowe w przekroju. Tu do żył i naczyń limfatycznych wchłaniane są składniki pokarmowe. Od kosmka błękitna żyłka biegnie do wątroby, podpisana jako żyła wrotna. Z wątroby żyłami krew dociera do serca. Różowe strzałki nad łukiem aorty wskazują, że składniki pokarmowe wchłonięte w jelitach trafiają do komórek ciała.

Expulsion of undigested food

Digestive enzymes cannot digest certain ingredients of the food. One of such ingredients is dietary fiber, an ingredient of vegetables. It is moved to the large intestine. The large intestine is inhabited by many types of bacteria, which feed of undigested food and break them down in fermentation processes. Bacteria are about 80% of the mass of the faeces that is expelled from the body through rectum with a constrictor. Water, mineral salts and vitamins are absorbed from the faecal mass in the large intestine.

Summary

• The digestive tract is where the food is processed and where chemical digestion and absorption take place.

• Preliminary digestion of polysaccharides take place in the oral cavity.

• The digestion of proteins is initiated in the acidic environment of the stomach.

• Intense digestion of sugars, proteins and fats takes place in the small intestine.

• Liver secretes bile which destroys large particles of fat into small drops.

• The products of digestion of sugars, proteins and fats are absorbed in the small intestine.

• The large intestine is where the water is absorbed and where faecal mass is thickened.

• Undigested food is expelled from the body in form of faeces.

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Keywords

digestion, digestive enzymes, intestinal villi, absorbtion

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