Urine formation

The formation of final urine by the kidney consists of several main 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 the blood;
• the activity of the countercurrent system, as a result of which the final urine is concentrated or diluted.

Ultrafiltration

Ultrafiltration from blood plasma into Bowman's capsule occurs in the capillaries of the renal glomeruli. SCF is an important indicator in the process of urine formation. Its value in a single 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 glomerular capsule:

P _effect = P _hydr - (P _onc + P _caps )

Where P _effect is the effective filtration pressure, P _hydr is the hydrostatic pressure in the capillaries, P _onc is the oncotic pressure of proteins in the capillaries, P _caps is the pressure in the glomerular capsule.

The hydrostatic pressure at the afferent and efferent ends of the capillaries is 45 mm Hg. It remains constant along the entire filtration length of the capillary loop. It is opposed by the oncotic pressure of plasma proteins, which increases toward the efferent end of the capillary from 20 mm Hg to 35 mm Hg, and the pressure in Bowman's capsule, which is 10 mm Hg. As a result, the effective filtration pressure is 15 mm Hg (45 - [20 + 10]) at the afferent end of the capillary, and 0 (45 - [35 + 10]) at the efferent end, which, when converted to 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 cellular elements, large-molecular compounds and colloidal particles to pass through, while water and low-molecular substances pass through it freely. The state of the glomerular filter is characterized by the ultrafiltration coefficient. Vasoactive hormones (vasopressin, angiotensin II, prostaglandins, acetylcholine) change the ultrafiltration coefficient, which accordingly affects the SCF.

Under physiological conditions, the totality of all renal glomeruli produces 180 liters of filtrate per day, i.e. 125 ml of filtrate per minute.

Reabsorption of substances in the tubules and their secretion

Reabsorption of filtered substances occurs mainly in the proximal part of the nephron, where all physiologically valuable substances that have entered 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 fluid in the tubule remains practically isoosmotic to the blood plasma, while the volume of primary urine by the end of the proximal tubule decreases by more than 80%.

The work of the distal nephron forms the composition of urine due to both reabsorption and secretion processes. In this segment, sodium is reabsorbed without an equivalent volume of water and potassium ions are secreted. Hydrogen and ammonium ions enter the lumen of the nephron from the tubular cells. Electrolyte transport is controlled by antidiuretic hormone, aldosterone, kinins and prostaglandins.

Counter-current system

The activity of the countercurrent system is represented by the synchronous work of several structures of the kidney - the descending and ascending thin segments of the loop of Henle, the cortical and medullary segments of the collecting ducts and the straight vessels that penetrate the entire thickness of the renal medulla.

The basic principles of the countercurrent system of the kidneys:

• at all stages, water moves only passively along the osmotic gradient;
• the distal straight tubule of the loop of Henle is impermeable to water;
• in the straight tubule of the loop of Henle, active transport of Na ⁺, K ⁺, Cl occurs;
• the thin descending limb of the loop of Henle is impermeable to ions and permeable to water;
• there is a urea cycle in the inner medulla of the kidney;
• Antidiuretic hormone ensures the permeability of the collecting ducts to water.

Depending on the state of the body's water balance, the kidneys can excrete hypotonic, very dilute or osmotically concentrated urine. All sections of the tubules and vessels of the renal medulla participate in this process, functioning as a countercurrent rotary multiplying system. The essence of this system's activity is as follows. The ultrafiltrate that entered the proximal tubule is quantitatively reduced to 3/4-2/3 of its original volume due to the reabsorption of water and substances dissolved in it in this section. The fluid remaining in the tubule does not differ in osmolarity from blood plasma, although it has a different chemical composition. Then the fluid from the proximal tubule passes into the thin descending segment of the loop of Henle and moves further to the apex of the renal papilla, where the loop of Henle bends by 180° and the contents pass through the ascending thin segment into the distal straight tubule, located parallel to the descending thin segment.

The thin descending segment of the loop is permeable to water but relatively impermeable 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 tubule of the loop of Henle, which, on the contrary, is impermeable to water and from which the active transport of osmotically active chlorine and sodium into the surrounding interstitium occurs, the contents of this section lose osmotic concentration and become hypoosmolal, which determined its name - "the diluting segment of the nephron". In the surrounding interstitium, the opposite process occurs - the accumulation of an osmotic gradient due to Na ⁺, K ⁺ and Cl. As a result, the transverse osmotic gradient between the contents of the distal straight tubule of the loop of Henle and the surrounding interstitium will be 200 mOsm/l.

In the inner zone of the medulla, an additional increase in osmotic concentration is provided by the circulation of urea, which enters passively through the epithelium of the tubules. The accumulation of urea in the medulla depends on the different permeability of the cortical collecting ducts and the collecting ducts of the medulla to urea. The cortical collecting ducts, the distal straight tubule, and the distal convoluted tubule are impermeable to urea. The collecting ducts of the medulla are highly permeable to urea.

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

As a result of the listed processes, the osmotic concentration increases from the cortex (300 mOsm/l) to the renal papilla, reaching 1200 mOsm/l both in the lumen of the initial part of the thin ascending limb of the loop of Henle and in the surrounding interstitial tissue. Thus, the corticomedullary osmotic gradient created 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 the vasa recta, which follow the course of the loop of Henle. The interstitial osmotic gradient is maintained by the effective removal of water through the ascending vasa recta, which have a larger diameter than the descending vasa recta and are almost twice as numerous. A unique feature of the vasa recta is their permeability to macromolecules, resulting in a large amount of albumin in the medulla. Proteins create interstitial osmotic pressure, which enhances water reabsorption.

The final concentration of urine occurs in the collecting ducts, which change their permeability to water depending on the concentration of secreted ADH. At high concentrations of ADH, the permeability of the membrane of the collecting duct cells to water increases. Osmotic forces cause water to move 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 production, the collecting duct is practically impermeable to water and the osmotic concentration of the final urine remains equal to the concentration of the interstitium in the renal cortex, 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 the active transport of potassium, sodium, and chloride ions in the ascending limb of the loop of Henle, 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 collecting duct becomes lower than that of blood plasma and is 100 mOsm/L. In the absence of ADH, with additional transport of sodium chloride from the tubules in the collecting duct, the osmolality in this part of the nephron can decrease to 50 mOsm/L. The formation of concentrated urine depends on the presence of high osmolality of the medulla interstitium and ADH production.