7.4. Regulation of water and salt metabolism and uropoiesis

Urine volume and ion content in it is regulated by the combined action of hormones and renal structural features. The daily urine volume is influenced by hormones aldosterone and vasopressin.

Aldosterone is a steroid hormone of the adrenal cortex. It belongs to mineralocorticoids. Aldosteron provides increased sodium reabsorption in the distal renal tubule by active transport. It is actively secreted by a significant decreasing of sodium concentration in blood plasma. Almost complete removal of sodium from the urine can occur in the case of very low concentrations of sodium in the blood plasma under the action of aldosterone. Increased sodium reabsorption leads to water retention in the body. Hypersecretion of aldosterone (hyperaldosteronism) results in sodium and water retention and leads to edema and hypertension, even heart failure. Insufficiency of aldosterone leads to a significant loss of sodium, chloride and water. The volume of blood plasma is decreased. In the kidneys, the processes of secretion of H⁺ and NH₄⁺ are disturbed. It can lead to acidosis.

Vasopressin (antidiuretic hormone, ADH) is a peptide hormone synthesized in the hypothalamus and secreted from the neurohypophysis. It has a membrane mechanism of action. This mechanism is realized through the adenylatecyclase system in target cells. Vasopressin secretion increases with increasing osmotic pressure of blood plasma, for example, with an increase in salt intake or dehydration. Vasopressin causes peripheral vasoconstriction (arterioles), resulting in increased blood pressure. In the kidney, vasopressin increases the rate of water reabsorption. As a result, the relative concentrations of Na, C1, P and total N are increased. Vasopressin secretion increases with increasing osmotic pressure of blood plasma, for example, with an increase in salt intake or dehydration. It is believed that the action of vasopressin is associated with phosphorylation of proteins of apical membrane of the kidney, thus increasing its permeability. In the pituitary gland lesions, in the case of abnormal vasopressin secretion, diabetes insipidus is observed – a sharp increase in urine volume (4-5 liters) with a low specific gravity.

Parathhormone is parathyroid hormone of peptide nature (membrane mechanism of action via cAMP); it also affects the excretion of salts from the body. In the kidney, it increases the tubular reabsorption of Ca²⁺ and Mg²⁺, increases the excretion of K⁺, phosphate, HCO₃^ and decreases the excretion of H⁺ and NH₄⁺. This is mainly due to reduced tubular reabsorption of phosphate. At the same time calcium concentration in plasma is increased. Hyposecretion of parathyroid hormone leads to the opposite phenomenon – an increase of phosphate in the blood plasma and to decrease the content of Ca²⁺ in the plasma.

Estradiol is a female sex hormone. It stimulates the synthesis of 1,25-dihydroxy vitamin D₃; it increases the reabsorption of calcium and phosphorus in the kidney tubules.

Natriuretic factor (NFE, atrial natriuretic peptide (ANP), atriopeptin) is a peptide, which is formed in the cells of the atrium in the hypothalamus. This is hormone-like substance. Its targets are the cells of the distal renal tubules. It operates through guanilatcyclase messenger system, i.e. cGMP is its intracellular mediator. The result of the influence of NFE on tubular cells is a reduction of reabsorption of Na, i.e. natriuria develops.

The retention of a certain amount of water in the body is caused by adrenal hormones – aldosterone and cortisone. In this case, we have the retention of Na ions in the body and as a result – water retention. Hormone thyroxine leads to a decrease of the body weight due to enhanced excretion of water, mainly through the skin.

These mechanisms are controlled by the CNS. In the regulation of water metabolism diencephalon and gray tuber of the brain are involved. Excitation of the cerebral cortex leads to changes in the kidneys as a result of a direct transmission of the impulses along nerve pathways, or by bringing some of the endocrine glands, particularly the pituitary gland.

Disturbances of water balance in various pathological conditions may lead either to water retention in the body, or to a partial dehydration of tissues. If water retention in the tissues is of a chronic nature, it usually leads to development of various forms of edema (inflammatory, salt, and hunger).

Abnormal tissue dehydration is usually caused by increased excreation of water through the kidney (up to 15-20 liters of urine a day). This increased urination, accompanied by strong thirst, observed in diabetes insipidus. Patients, suffering from diabetes insipidus on the basis of lack of vasopressin, lose their ability to concentrate the primary urine; the urine becomes very dilute and has a low specific gravity. However, the restriction of drinking in this disease can lead to tissue dehydration incompatible with the life.