1. Physiology of absorption in gastrointestinal tract

2. Determine of notion “absorption”
Absorption is a complex of processes, which are provide transport of substances from digestive tract into internal surroundings of organism (blood, lymph, intercellular substances)

3. Main types of transport of nutritive substances in internal surroundings of organism
1. Passive;
2. Active.

4. 1. Passive transport include diffusion and osmosis
1. Passive transport include diffusion and osmosis. This transport do not need presents of energy. In this case substances transport through the mucus shell by help of concentrative gradient. This way of transport have water, water disolved vitamins (C, B6, B2).
2. Active transport include pinocytosis and active transport by help of protein and energy. Active transport need energy of ATP. This way characteristic of amino acids, monosaccharids, vitamin B12, ions of calcium, enzymes. Pinocytosis – by help of pynocytic bulb, where secreted enzymes for proteins hydrolysis. Products of hydrolysis adsorbed by cell.

5. The total quantity of fluid that must be absorbed each day is equal to the ingested fluid (about 1,5 liters) plus that secreted in the various gastrointestinal secretions (about seven liters). This comes to a total of approximately 8 to 9 liters. All but 1,5 liters of this is absorbed in the small intestine, leaving only 1,5 liters to pass through the ileocecal valve into the colon each day.

6. Absorption in the mouth cavity and stomach
In the mouth cavity absorbed water, water soluble medicines (validol, nitroglycerin). In our oral cavity, under the tongue present a big quantity of vessels. That is why all water soluble substances absorbed. They go to the bloodstream, and have immediately action on our receptors. They do not go through the liver, and do not desintoxicated, that is why may be toxic effect of some substances. In stomach absorbed alcohol and small quantity of other substances.

7. stomach
The stomach is a poor absorptive area of the gastrointestinal tract because it lacks the typical villus type of absorptive membrane and also because the junctions between the epithelial cells are tight junctions. Only a few highly lipid-soluble substances, such as alcohol and some drugs like aspirin, can be absorbed in small quantities.

8. Absorption in intestines
Small intestines has a big length – 3-8 meters; large quantity of substances absorbed in a middle part of small intestines; common surface of intestines – 200 m2. The structure bases of absorption in small intestines are villi. In large intestines absorbed water and completed absorption of a small quantity of hydrolitic substances of carbohydrates, proteins and lipids. By help of this way may absorbed components of nutritive elements (glucose, vitamins, water etc).

9. The Absorptive Surface of the Intestinal Mucosa – The Villi
The Absorptive Surface of the Intestinal Mucosa – The Villi. The absorptive surface of the intestinal mucosa, showing many folds called valvulae conniventes (or folds of Kerckring), which increase the surface area of the absorptive mucosa about threefold. These folds extend circularly most of the way around the intestine and are especially well developed in the duodenum and jejunum, where they often protrude as much as 8 mm into the lumen.

10. The intestinal epithelial cells are characterized by a brush border, consisting of about 600 microvilli 1 μm in length and 0,1 μm in diameter protruding from each cell. This increases the surface area exposed to the intestinal materials another 20-fold. Thus, the combination of the folds of Kerckring, the villi, and the microvilli increases the absorptive area of the mucosa about 600-fold, making a tremendous total area of about 250 square meters for the entire small intestine – about the surface area of a tennis court.

11. ABSORPTION IN ORAL CAVITY, ESOPHAGUS AND STOMACH
In the mouth cavity absorbed water, water soluble medicines (for example, validol, nitroglycerin, adelphan, furosemid, corinfar and others). In our oral cavity, under the tongue present a big quantity of vessels. That is why all water soluble substances absorbed in this place. They go to the bloodstream, and have immediately action on our receptors. They do not go through the liver, and do not desintoxicated, that is why may be toxic effect of some substances, for example products of food, drugs.
In esophagus do not absorbed nutritive substances as a rule.
In stomach absorbed alcohol, water and small quantity of other substances.

12. ABSORPTION IN THE SMALL INTESTINE
Normally, absorption from the small intestine each day consists of several hundred grams of carbohydrates, 100 or more grams of fat, 50 to 100 grams of amino acids, 50 to 100 grams of ions, and 7 to 8 liters of water. However, the absorptive capacity of the small intestine is far greater than this as much as several kilograms of carbohydrates per day, 500 to 1000 grams of fat per day, 500 to 700 grams of amino acids per day, and 20 or more liters of water per day. In addition, the large intestine can absorb still more water and ions, though almost no nutrients.

13. ABSORPTION IN THE LARGE INTESTINE
Approximately 1500 ml of chyme pass through the ileocecal valve into the large intestine each day. Most of the water and electrolytes in this are absorbed in the colon, usually leaving less than 100 ml of fluid to be excreted in the feces. Also, essentially all the ions are also absorbed, leaving only about 1 mEq each of sodium and chloride ions to be lost in the feces.
Most of the absorption in the large intestine occurs in the proximal half of the colon, giving this portion the name absorbing colon, whereas the distal colon functions principally for storage and is therefore called the storage colon.

14. Methods of absorptions’ investigation
1. Angiostoma.
2. X-ray investigation.
3. Biochemical method of investigation.

15. Regulation of absorption
Absorption regulated by nervous system, for example by cortex; by vegetative nervous system; endocrine glands, for example adrenocorticotropin, aldosteron, glucocorticoids, insulin, thyroxin, local factors of food, for example pepper, mustard.

16. Peculiarities of absorption of organic and inorganic substances
Water and mineral salts
Products of proteins hydrolyses
Products of carbohydrates hydrolyses
Products of fats hydrolyses

17. Water and mineral salts
Main quantity of water absorbed in small intestines, small quantity – in stomach and large intestines. Water absorbed by help of osmotic gradient. Water absorbed together with amino acids, carbohydrates, salts; vitamins absorbed with water. Main role of water transport through the membrane have sodium and chloride. Sodium transport in two stages: passive (sodium go through the apical part of enterocytes membrane from chyme) and active (sodium go by help of Na+,K+-ATPase on baso-lateral part of enterocytes membrane in blood). Chloride and HCO3- go together with sodium. Ca2+, Mg2+ absorbed actively.

18. Products of proteins hydrolyses
Proteins absorbed in free amino acids, dipeptides, threepeptides appearance. Their absorption into cell through the apical part of membrane connects with secondary active sodium transport. Most actively absorbed arginine, methionine, leucine; L-form of aminoacids. Then they transport through the basolateral part of membrane in blood by help of concentrative gradient.

19. Products of carbohydrates hydrolyses
Carbohydrates absorbed in monosaccharides appearance. Most actively absorbed glucose and galactose. Their absorption into cell through the apical part of membrane connects with secondary active sodium transport. Then glucose transport through the baso-lateral part of membrane in blood by help of concentrative gradient. Mannose and fructose transport by help of diffusion.

20. That the transport of most monosaccharides through the intestinal membrane is an active process is demonstrated by several important experimental observations:
1. Transport of most of them, especially glucose and galactose, can be blocked by metabolic inhibitors, such as iodoacetic acid, cyanides, and phlorhizin.
2. The transport is selective, specifically transporting certain monosaccharides without transporting others. The order of preference for transporting different monosaccharides and their relative rates of transport in comparison with glucose are:
3. There is a maximum rate of transport for each type of monosaccharide. The most rapidly transported monosaccharide is galactose, with glucose running a close second. Fructose, which is also one of the three important monosaccharides for nutrition, is absorbed less than half as rapidly as either galactose or glucose; also, its mechanism of absorption is different, as will be explained below.
4. There is competition between certain sugars for the respective carrier system. For instance, if large amounts of galactose are being transported, the amount of glucose that can be transported simultaneously is considerably reduced.

21. Products of fats hydrolyses
In small intestines developed micellas – connection of fat acids, cholesterol, monoglycerides, phospholipids with bile acids salts. Bile salts acids act transport role of micella to apical part of membrane. Then by help of diffusion fat acids, cholesterol, monoglycerides, phospholipids penetrate into enterocyte, where synthesis new triglycerides. Inside enterocyte develop chilomicrones. It consists from triglycerides, phospholipids, cholesterol and other lipids. They penetrate in lymph vessels by help of pinocytosis.