Physiology of kidneys

The main functions of the kidneys are excretory and gidroliticheskie. The first provides excretion with urine all want to remove metabolic products and non-toxic and indifferent) substances. The second regulates the constancy of volume of extracellular fluid.
Urine formation is a reflection of the many functions of the kidneys, aimed to maintain the constancy of the internal environment. Into the cavity of the ball diffuses of the water plasma together with dissolved crystalline inorganic and organic substances with a certain amount of molecules through filtration pressure, representing the difference between the hydrostatic pressure in the capillaries of the glomeruli, on the one hand, and the sum of the oncotic pressure of plasma and glomerular pressure capsule. Liquid glomeruli composition is equivalent to plasma ultrafiltrate. The osmotic pressure of the liquid glomeruli, its conductivity, pH, and the content in Wei glucose, sodium phosphate, creatinine, urea and uric acid are in line with the indicators of plasma. For 1 min in the glomerulus of man is formed on the average 125 ml of the filtrate, and the urine is allocated for 1 minute only 1 ml of the tubular Epithelium has the ability differently to the transport of different substances; some returned from the tubular lumen into the blood (re-absorption), other are extracted from the blood into the cavity of the tubule (active excretion). In the proximal tubules fully reabsorbed glucose, about 4/5 prefiltermessage sodium and chlorides; there is a reduction of glomerular filtrate on 7/8. The osmotic pressure of urine in this Department tubules is equal to the pressure of the blood. Substances which should be derived from an organism (in particular, urea), also partly reabsorbiruetsa in the proximal tubules, but as the wall tubules relatively resistant to penetration through it urea, the latter concentrated in large quantities (60 times).
In the distal tubules and collecting pipes, the formation of the final urine. As there is an active absorption (reabsorption against electrochemical gradient) and it is extremely volatile, it is called optional reabsorption. Concentrating mechanism of the kidneys is according to the principle protivootecna-rotary system: two parallel tribes and the loop Genle urine moving in opposite directions (Fig. 6). Rising knee waterproof, and the cells it is able to absorb sodium, directing it in the interstitium, creates a gradient concentrating mechanism. The downward elbow loop permeable to water and sodium, therefore, between the urine in descending knee Genle hinges and interstitial fluid establishes the equilibrium. The accumulation of sodium in the interstitium it diffuses into the loop, and the water goes out. Between ascending and descending knee Genle hinges set transverse gradient in 200 milliosmoles (Asmol, or 1000 milliosmoles,- the amount of a substance that being dissolved in 1 liter of water, develops the osmotic pressure of 22.4 ATM.), whereas the length of the loop integrated gradient equal to 2000 milliosmoles, which provides concentration of the urine in 7 times.

Fig. 6. The scheme protivootecna-rotary system Genle hinges. Vertical arrows the direction of current flow of urine in the knees. Horizontal arrows: I - the active transport of sodium cells ascending portion of the loop that creates a transverse gradient; 2 - water outlet from the collective tube in the process of the optional reabsorption.

Natriuretic kidney function provides a constant concentration of sodium in the blood. For days in the glomerular filtrate kidney receives about 600 g of sodium, and is excreted with urine only a few grams. Kidney shall osmoregulation through the ability of the tubular epithelium to reabsorbiruetsa Na from the tubular lumen into the blood. In proximal part of the nephron reabsorbed 85% Na, which is the main driving force reabsorption of water. The reabsorption Na tubular epithelium is active due to ferment systems. The main role in this process belongs to the dehydrogenases of succinic acid. The activity of this process is proved different content of Na in the bone and plasma (respectively 60 mEq/l against 140 mEq/l). Distal reabsorption Na highly variable. The content of sodium in the liquid tubules can reach only 8 mEq/l, which provides a significant gradient. The main role in the regulation of the selection Na belongs to the hormone of the adrenal glands (see) - aldosterone, which is increased reabsorption Na. The lack of aldosterone leads to the termination of the distal reabsorption of sodium and hyponatremia.
The kidney is the main regulator of the water balance of the body. About 99% of the water is reabsorbed into the tubules, with about 85% - isoosmotic in the proximal tubules. However, for the formation of the final urine far greater importance is distal reabsorption of water. Recent increases under the influence of antidiuretic hormone posterior pituitary, when the cells of the distal nephron allocate hyaluronidase, which carries out the depolymerization of hyaluronic complexes intercellular substance. Consequently, the intracellular membrane becomes permeable to water, which (due to osmotic gradient) leaves tubules. Thus is antidepres (the so-called antidiuretic reflex).
The main stimulator of antidiuretic hormone is the osmotic pressure of the extracellular fluid, the increase of which causes irritation of osmoreceptors and the antidiuretic hormone in the blood, the tubular reabsorption of water increases the osmotic pressure is levelled and urine output decreases. Decrease volume of circulating blood irritating to Volum-receptors located at the confluence of the pulmonary veins to the left atrium, which also increases the secretion of hormones and leads to the decrease of diuresis. The increase in volume of circulating blood, reduces the secretion of antidiuretic hormone and increases urine output.
The kidneys are also angiotonic function (see the Renin-angiotensin system).