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A SHORT INFORMATION OF THE KIDNEYS' PHYSIOLOGY

The kidneys perform a number of many-sided functions directed at the maintenance of the organism's homeostasis.

The main structural-and-functional unit of the kidney is nephron. There are about one million of them in each kidney. Each of them consists of Shumlyansky-Bowmen's capsule, inside of which is Malpighian glomerulus consisting of capillaries. Water and all substances of blood plasma, except macromolecular proteins, are filtrated into the capsule cavity. So formed liquid is called a primary urine and up to 180 liters of it are generated daily. A filtration pressure is of particular significance for the amount of filtration. The pressure in renal capillaries makes up about 70 mm Hg. The oncotic pressure of blood plasma proteins and hydrostatic pressure withstand it, together they make up 50 mm Hg. Thus, a gradient of percolation (filtration) pressure is equal to 20 mm Hg, its reduction leads to a decrease of the primary urine formation.

A convoluted tubule of the 1-st order (proximal) originates from Shumlansky-Bowmen's capsule. A reabsorption of 85% isotonic water, reabsorption of sodium, potassium, phosphates, amino acids and glucose occur in it, as well as an excretion of paraaminohippuric acid. At the boundary of cortical and medullary substance of the kidney a convoluted tubule becomes narrow and straightened forming in the medullary substance Henle's loop having ascending and descending portions. A particular feature of the thin descending portion of loop is its high permeability for water. The urine becomes hypertonic as a result of water removal due to osmotic gradient, and concentration of sodium and potassium ions increases. A functional feature of the distal portion of Henle's loop (wide, ascending) is its low permeability for water, reabsorption of sodium chloride and chlorine, secretion of potassium, ammonia, hydrogen ions. Owing to a low reabsorption of water and significant of sodium a hypoosmotic urine may be formed. In the cortical layer of the kidney a tubule again acquires its convoluted form (a convoluted tubule of II order, distal). A further reabsorption of sodium and secretion of potassium occurs in it. A greater part of potassium is secreted irrespective of the amount of reabsorption of sodium ions, and potassium comes into the tubule in exchange for sodium by means of sodium-potassium pump. A reabsorption of magnesium, phosphates and greater part of calcium also ends there. A distal convoluted tubule falls into a collecting tube. Formation of final hypertonic urine, excretion of potassium, reabsorption of sodium and calcium, equalization of acid-alkaline balance of urine at the expense of HC0₃, NH₃, C0₂ transfers, diffusion of urea and water occur in it.

Owing to the above-described mechanisms and some other properties the kidneys perform a number of the most important functions:

1) osmoregulation — excretion of hypotonic urine in excess of water, and, on the contrary, an "economy" of water due to the excretion of hypertonic urine — this function is regulated by antidiuretic hormone;

2) maintenance of ionic composition of blood — specific receptors sensitive to those or other ions, and hormones regulating the excretion and reabsorption of ions (for example, aldosterone accelerates sodium reabsorption 20 times and K⁺ excretion 5 times, insulin reduces potassium excretion) are revealed;

3) maintenance of acid-alkaline balance depends on secretion of ions of hydrogen and carbonic acid by the kidneys;

4) excretory function — excretion of harmful, for the organism, products of metabolism: urea, uric acid, creatine, creatinine, hormones, enzymes, etc.;

5) incretory function — lies in the synthesis (by juxtaglomerular systen. of the kidneys when a blood flow is reduced in them) of renin able to activate renin-angiotensin system and thereby influence the level of arterial pressure;

6) metabolic function — the kidneys take part in metabolism of proteins, fats and carbohydrates. In the glomeruli, peptides are filtered splitted by the cells of proximal portion of tubules down to amino acids transported back into the blood. There is also a system of synthesis of glucose and phospholipids able to utilize fatty acids;

7) regulation of erythropoiesis — through erythrogenin stimulating
production of erythropoietins in hypoxia. There is some information of
synthesis of substances inhibiting erythropoiesis;

8) blood coagulability regulation — by means of urokinase production the
kidneys participate in the process of fibrinolysis. The blood outflowing from
the kidneys contains more heparin and antithrombin.

Hormones play a principle role in the regulation of uropoiesis. A completely denervated kidney may function normally. The main hormones in the regulation of renal function are:

— antidiuretic hormone (ADH) — it stimulates formation of hyaluronidase, cells of collecting tubes that leads to a depolimerization of hyaluronic structures of the intercellular substance and increase of distances between the cells, under the action of osmotic gradient water goes out from a tubule. ADH secretion increases upon elevation of osmotic blood concentration, pain stimulation, morphine and nicotine effect;

— decorticosterone increases reabsorption of sodium and water;

— aldosterone is significantly more active than the former by these functions, besides, it increases a renal blood flow, elevates osmotic pressure in the medullary substance of the kidneys, facilitating sodium reabsorption;

— natriuretic hormone increases an excretion of sodium and water;

— cortisone and hydrocortisone increase sodium reabsorption and elevate potassium secretion;

— somatotropic hormone increases glomerular filtration and diuresis;

— thyroxin has a similar effect;

— adrenalin and norepinefrine increase the excretion of salts and water, inhibit ADH removal;

— insulin contributes to a retention of water in the intercellular space;

— thyrocalcitonin and parathormone regulate calcium excretion.

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