Theory of the formation of hypertonic and hypotonic urine

The formation of hypertonic and hypotonic urine, i.e. its dilution and concentration, is an extraordinary property of the kidneys. This contributes to the maintenance of osmolality, which ranges from 50-1200 mosm/l . By regulating the formation of urine, excess solutes and excess water are excreted.

Terms of origin
For the normal function of urine production it is needed:
 * countercurrent multiplication system;
 * production of ADH;
 * circulatioh of urea in the medulla of the kidneys (maintains an acidic environment).

We have to remember that solutes also passively pass through the cell membrane with water. Thus, in the proximal tubule, water passively follows the solutes - this liquid, which is absorbed is isotonic.

Processes in the loop of Henle
Two very important and looping processes take place in the loop of Henle:
 * diluting the tubular fluid (on the basis of the absorption of solutes in the thick segment, no water passes there and also on the condition that there is no ADH secretion), hypotonic urine is produced;
 * concentrating of the tubular fluid, which is caused by the release of ADH to create an osmotic gradient, on the basis of which hypertonic urine is produced.

Hypotonic urine
The descending limb of the loop of Henle is permeable to water. However, the tubular fluid contains more sodium cations, chloride anions and less urea. So we can say that the osmolarity in the tubular fluid depends on the concentration of NaCl. In the interstitium it is mainly urea.

The thin segment of the ascending limb has a different function. It is impermeable to water but permeable to NaCl and urea. They diffuse passively in the direction of their concentration gradients. Due to these conditions, the volume does not change, but what changes is its composition.

The thick segment of the ascending limb of Henle's loop is also impermeable to water. The absorption of solutes is of great importance here. Thanks to this important property, the urine is diluted and becomes hypotonic (approx. 150 mosm/l).

In the distal tubule and collecting duct, in the absence of ADH (whose production is sometimes even stopped), solutes continue to be absorbed and the urine becomes more diluted. It approaches values of 50 mosm/l.

We must not forget that at the end of the collecting duct the urine is concentrated with a small amount of urea and the concentration thus increases to a final 600 mosm/l.

Hypertonic urine
The thick segment of the ascending limb, which is impermeable to water, plays an important role in the formation of hypertonic urine. Solutes are absorbed here, which leads to the formation of concentrated urine. However, the absolute basis for the formation of hypertonic urine is the countercurrent multiplication system, the basic principle of which is the increase in the concentration of NaCl and urea in the renal medulla. In the presence of antidiuretic hormone (ADH), water is mainly reabsorbed from the collecting duct and the fluid thus logically becomes hypertonic, i.e. concentrated.

However, ADH in its function also enables the transfer of urea into the interstitium (urea circulation in the marrow). It thus increases the osmolarity up to the mentioned 1200 mosm/l. .

Related Articles

 * Kidneys
 * Examination of urinary sediment
 * Tubular processes
 * Osmolarity

Source

 * Notes from the lectures of prof. MD Otomar Kittnar, CSc., MBA from the Institute of Physiology 1 LF