Urine formation

The formation of the final urine by the kidney consists of several basic processes:

• ultrafiltration of arterial blood in the renal glomeruli;
• reabsorption of substances in the tubules, secretion of a number of substances into the lumen of the tubules;
• the synthesis of new substances by the kidney, which enter both the lumen of the tubule and into the blood;
• the activity of the countercurrent system, as a result of which the final urine is concentrated or divorced.

Ultrafiltration

Ultrafiltration from the blood plasma into the Bowman capsule occurs in the capillaries of the renal glomeruli. GFR is an important indicator in the process of urine formation. Its value in a separate nephron depends on two factors: the effective pressure of ultrafiltration and the coefficient of ultrafiltration.

The driving force of ultrafiltration is the effective filtration pressure, which is the difference between the hydrostatic pressure in the capillaries and the sum of the oncotic pressure of proteins in the capillaries and the pressure in the glomerulus capsule:

P _effect = P _hydra - (P _onk + R _caps )

Where P _effect - an effective filtration pressure, P _hyd - the hydrostatic pressure in the capillaries, P _ONC - oncotic pressure in capillaries proteins, P _capsules - pressure in the glomerular capsule.

The hydrostatic pressure on the afferent and efferent end of the capillaries is 45 mm Hg. It remains constant along the entire filtering length of the capillary loop. He contrasted the oncotic pressure of plasma proteins, which increases toward the efferent end of the capillary from 20 mm Hg. Up to 35 mm Hg, and the pressure in the Bowman capsule is 10 mm Hg. As a result, the effective filtration pressure is 15 mm Hg at the afferent end of the capillary. (45- [20 + 10]), and on the efferent - 0 (45- [35 + 10]), which in terms of the entire length of the capillary is approximately 10 mm Hg.

As stated earlier, the wall of the glomerular capillaries is a filter that does not allow the passage of cellular elements, large-molecule compounds and colloidal particles, while water and low-molecular substances pass through it freely. The condition of the glomerular filter characterizes the coefficient of ultrafiltration. Vasoactive hormones (vasopressin, angiotensin II, prostaglandins, acetylcholine) change the coefficient of ultrafiltration, which consequently affects GFR.

Under physiological conditions, the aggregate of all renal glomeruli forms 180 liters of filtrate per day, i.е. 125 ml of filtrate per minute.

Reabsorption of substances in tubules and their secretion

The reverse absorption of filtered substances occurs predominantly in the proximal part of the nephron, where all the physiologically valuable substances that enter the nephron and about 2/3 of the filtered sodium, chlorine and water ions are absorbed. The peculiarity of reabsorption in the proximal tubule is that all substances are absorbed with an osmotically equivalent volume of water and the liquid in the tubule remains practically iso-osmotic blood plasma, while the volume of primary urine decreases by more than 80% toward the end of the proximal tubule.

The work of the distal nephron makes up the composition of the urine due to both the processes of reabsorption and secretion. In this segment, sodium is reabsorbed without an equivalent volume of water and potassium ions are secreted. From the cells of the tubules, hydrogen ions and ammonium ions enter the nephron lumen. Transport of electrolytes controls antidiuretic hormone, aldosterone, kinin and prostaglandins.

Counterflow system

The activity of the countercurrent system is represented by the synchronous operation of several kidney structures - descending and ascending thin segments of the Henle loop, cortical and brain segments of collecting tubes and direct vessels that penetrate the entire thickness of the medulla of the kidneys.

Basic principles of the countercurrent system of the kidneys:

• at all stages water moves only passively along the osmotic gradient;
• the distal straight canaliculus of Henle's loop is impermeable to water;
• In the direct tubule of the Henle loop, active transport of Na ⁺, K ⁺, CI occurs ;
• The thin descending knee of Henle's loop is impermeable to ions and permeable to water;
• there is a urea circulation in the internal medulla of the kidney;
• antidiuretic hormone provides permeability of collecting tubes for water.

Depending on the state of the body's water balance, the kidneys can produce hypotonic, very dilute or osmotically concentrated urine. In this process, all sections of tubules and vessels of the medulla of the kidney function as a countercurrent rotary multiplying system. The essence of the activity of this system is as follows. The ultrafiltrate entering the proximal tubule is quantitatively reduced to 3 / 4-2 / 3 of its original volume due to the reabsorption of water in this compartment and the substances dissolved in it. The remaining liquid in the tubule is osmolarity different from the blood plasma, although it has a different chemical composition. Then, the fluid from the proximal tubule passes into the thin descending segment of the Henle loop and moves further to the apex of the renal papilla where the Henle loop bends 180 ° and the contents through the ascending thin segment pass into the distal straight tubule located parallel to the descending thin segment.

The thin downward segment of the loop is permeable to water, but relatively impervious to salts. As a result, water passes from the lumen of the segment into the surrounding interstitial tissue along the osmotic gradient, as a result of which the osmotic concentration in the lumen of the tubule gradually increases.

After the fluid enters the distal straight channel of the Henle loop, which, on the contrary, is impermeable to water and from which active transport of osmotically active chlorine and sodium occurs in the surrounding interstitium, the contents of this compartment lose osmotic concentration and become hypo-osmolar, which determined its name - "diluting segment of the nephron. " In the surrounding interstitium, the opposite process occurs - the accumulation of the osmotic gradient due to Na ⁺, K ⁺ and C1. As a result, the transverse osmotic gradient between the contents of the distal direct tubule of the Henle loop and the surrounding interstitium will be 200 mOsm / l.

In the inner zone of the medulla, an additional increase in osmotic concentration provides a urea circulation, which passes passively through the epithelium of the tubules. The accumulation of urea in the brain substance depends on the different permeability for the urea of the cortical collection tubes and the collecting tubes of the medulla. For urea, impermeable cortical collection tubes, distal straight tubule and distal convoluted tubule. Collective tubes of the medulla are highly permeable to urea.

As the filtered liquid passes from the Henle loop through the distal convoluted tubules and cortical collecting tubes, the urea concentration in the tubules increases due to the reabsorption of water without urea. After the fluid enters the collecting tubes of the inner medulla, where the urea permeability is high, it moves to the interstitium, and then is transported back to the tubules located in the inner medulla. The increase in osmolality in the brain substance is due to urea.

As a result of these processes, the osmotic concentration increases from the cortical substance (300 mOsm / l) to the renal papilla, reaching 1200 mOsm / l both in the lumen of the initial part of the thin ascending knee of the Henle loop and in the surrounding interstitial tissue. Thus, the cortico-medullary osmotic gradient produced by the countercurrent multiplying system is 900 mOsm / l.

An additional contribution to the formation and maintenance of the longitudinal osmotic gradient is made by direct vessels that repeat the course of the Henle loop. The interstitial osmotic gradient is maintained by the effective removal of water through ascending direct vessels, which have a larger diameter than the descending direct vessels, and are almost twice as numerous as the latter. A unique feature of straight vessels is their permeability to macromolecules, resulting in a large amount of albumin in the brain substance. Proteins create an interstitial osmotic pressure that enhances the reabsorption of water.

Final concentration of urine occurs in the area of collecting tubes, which change their permeability for water, depending on the concentration of the secreted ADH. With a high concentration of ADH, the permeability to water of the membrane of the cells of the collecting tubes increases. Osmotic forces cause the movement of water from the cell (through the basement membrane) into the hyperosmotic interstitium, which ensures equalization of osmotic concentrations and the creation of a high osmotic concentration of the final urine. In the absence of ADH products, the collecting tube is practically impermeable to water and the osmotic concentration of the final urine remains equal to the concentration of interstitium in the region of the cortical substance of the kidney, i.e. Isoosmotic or hypoosmolar urine is excreted.

Thus, the maximum level of urine dilution depends on the ability of the kidneys to reduce the osmolality of the tubular fluid due to active transport of both potassium, sodium and chlorine ions in the ascending section of the Henle loop and the active transport of electrolytes in the distal convoluted tubule. As a result, the osmolality of the tubular fluid at the beginning of the collection tube becomes smaller than the blood plasma and is 100 mOsm / l. In the absence of ADH in the presence of additional transport from the tubules of sodium chloride in the collecting tube, osmolality in this department of the nephron can be reduced to 50 mOsm / l. The formation of concentrated urine depends on the presence of high osmolality interstitial medulla and ADH production.