Control of urine formation
The control of urine formation is mainly done by two different hormones: adiuretin and aldosterone. Adiuretin, also called antidiuretic hormone, is produced in the hypothalamus and enters the bloodstream via the posterior lobe of the pituitary gland. Adiuretin binds to V2 receptors in the distal tubule and in the collection tube and increases the incorporation of aquaporin 2 (AQP2) into the membrane.
These are water channels, so that more water can be extracted from the urine and enter the bloodstream. This results in an increase in blood volume and concentration of the urine. Via V1 receptors on smooth vascular muscle cells, adiuretin causes their tension.
Due to the contraction of the vascular muscle cells and the increased blood volume, adiuretin has a blood pressure-increasing effect. Aldosterone belongs to the renin-angiotensin-aldosterone system (RAAS) and is produced in the adrenal cortex. Aldosterone acts by stimulating a receptor inside the cells of the distal tubule.
Aldosterone causes an increase in the expression of various proteins: luminal sodium and potassium channels (in the direction of the “urinary vessel”) and sodium/potassium pumps directed towards the blood vessel. These proteins lead to increased sodium being withdrawn from the urine. This creates a gradient which leads to the passive recovery of water.
Potassium, on the other hand, is excreted in greater quantities. The basal secretion of aldosterone fluctuates in the course of the day. It can be additionally regulated by several factors.
In cases of low blood volume (hypovolaemia), sodium deficiency (hyponatremia), excess potassium (hyperkalaemia) or when renal blood flow decreases, aldosterone is excreted more into the bloodstream. Aldosterone and adiuretin therefore result in less water in the urine; consequently, the volume of urine decreases while the concentration increases. On the other hand, the blood volume and thus secondarily the blood pressure increases. In addition, aldosterone and adiuretine influence drinking behaviour by promoting a feeling of thirst, which in turn leads to increased fluid intake.
Task in electrolyte and mineral balance
The kidney serves to maintain the balance of mineral salts and electrolytes. Electrolytes play a central role in many cell processes, so their concentrations must be strictly regulated. The kidney has various mechanisms to ensure an optimal balance.
The kidney tends to reabsorb the electrolytes in the primary urine. Through various transport systems and channels, the electrolytes are absorbed from the filtrate and can be returned to the blood system. In this way the kidney protects the organism from unnecessary electrolyte loss.
For example, if the concentration of an electrolyte is too high, the kidney can increase the excretion of this electrolyte by selectively lowering the absorption of this ion. If the kidney function is disturbed, the electrolyte concentration in the blood plasma can fall outside the normal value, i.e. either increase or decrease. Some drugs, such as diuretics, can block the kidney’s transport systems and therefore also lead to electrolyte disorders.