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Renal function in WEE regulation

The main organs regulating a balance of water and electrolytes in the organism are the kidneys. The patients with disturbed renal function die not from the increased content of nitrous waste products, but from water-electrolyte balance disorders.

The most important mechanisms owing to which the kidneys perform their functions are filtration, reabsorption and secretion.

The anatomic-and-functional unit of the kidneys is nephron that consists of Shumlansky's capsule, proximal convoluted tubule, Henle's loop and distal convoluted tubule turning into a collecting tubule. 1200 ml of blood passes through a glomerular system per lmin. Filtration and formation of the so-called primary urine occurs at the level of Shumlyansky's capsule.

Filtration is due to the difference between arterial pressure 75 mm Hg and Qsmotic pressure — 35 mm Hg.

When the pressure drops a filtration reduces, and when the total pressure is 60 mm Hg the filtration of urine discontinues. The primary urine, by its composition and osmotic pressure, corresponds to plasma, but without proteins, since, normally, protein does not pass through a capillary barrier. But in forced diuresis the cells of Shumlyansky's capsule are somewhat moved apart and in this state are able to pass through the proteins.

Osmotic pressure of the primary urine is equal to 380 mosm/1.

The primary urine is formed in the amount of 120 ml/min and the process of reverse absorption starts at once in the proximal portion of convoluted tubule. This occurs at the expense of sodium return to the blood that is passively followed by the water — a stage of passive isoosmotic compulsory reabsorption.

At this stage, 85% of electrolytes and water are absorbed, 15% of isoosmotic fluid (15-18 ml/min) come up to Henle's loop. At the level of ascending portion of Henle's loop a "sodium pump" operates, ejecting actively sodium without osmotically equivalent amount of water into the surrounding interstice. As long as, the epithelium of descending portion of Henle's loop is permeable for water, it is passively reabsorbed from the lumen of the loop into interstice. Osmolarity of fluid in the lumen of tubule increases achieving in the region of its bend 1500 mosm/1, as well as in the surrounding interstitial space. With its rise upward the ascending portion of Henle's loop the fluid loses sodium and its osmolarity drops achieving 200 mosm/1 in the cortical layer of the kidney.

Further concentration of urine depends on the influence of ADH of neurohypophysis upon the permeability of the wall of collecting tube that passes through a medullary layer of the kidney with increased osmolarity.

If ADH is not secreted the wall of collecting tube is impermeable for water and fluid in the lumen of tube achieves Bellini's duct not changing its osmolarity and volume, and significant amount of urine with a low specific gravity is excreted. But if ADH is secreted, hyaluronidase is activated under its effect and increases a permeability of the walls of collecting tube for water, the latter goes from the tubular lumen in the surrounding interstice with high osmolarity, and from there — into the blood. A volume of fluid in the collecting tube decreases, but its osmolarity increases, and a little urine with high specific gravity is excreted (Fig. 46).

A process of urine concentration to the extent of its movement through Henle's loop and collecting tube resembles the process of heating a liquid passing through the system that is called in heat technology as bend-multiplying system.

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