The reorganization of the functions of the pregnant body for the further normal development of the fetus leads to changes in the activity of both extrarenal (hormonal and hemodynamic) mechanisms of regulation of volumic-osmatic homeostasis and mechanisms of kidney activity.
In the first trimester of pregnancy, compared with the indices in non-pregnant women, there is an increase in diuresis and sodium nares by increasing the glomerular filtration rate and lowering the tubular reabsorption of water and sodium. The increase in glomerular filtration occurs due to the increase in renal plasma and blood circulation as a result of a decrease in the overall resistance of renal vessels.
The level of aldosterone in blood plasma in healthy women in the first trimester of pregnancy increases. Given that aldosterone increases tubular reabsorption of sodium, we are faced with a paradoxical reaction: against the background of an increase in the level of aldosterone, sodium excretion increases. This fact is explained by a significant increase in the content of the antagonist aldosterone - natriuretic hormone - in the urine of the women examined. Natriuretic hormone, a hormone of peptide nature, is synthesized by right atrial cardiomyocytes in response to an increase in blood volume and an increase in pressure in the cavity of this atrium. It has the ability to suppress the reabsorption of sodium in the proximal tubule of the kidneys, which leads to an increase in its excretion in the urine.
In response to the loss of sodium, the osmolality of the plasma of the pregnant woman decreases. To prevent further increase in blood hypo-osmolality, the corresponding regulatory systems limit the production of vasopressin.
The combination of all these factors leads to an increase in excretion of water and sodium in the urine in the first trimester of pregnancy. As a result, the volume of extracellular fluid decreases somewhat, mainly due to a decrease in the volume of circulating blood.
In the second trimester of pregnancy, compared with the first trimester, there is a decrease in diuresis and natriuresis due to some decrease in glomerular filtration of water and sodium, and a moderate increase in their reabsorption in the tubules of the kidneys. The renal blood flow remains elevated and the decrease in the glomerular filtration rate is due to a decrease in the resistance of the renal vessels that divert blood from the glomeruli of the kidneys.
The increase in tubular reabsorption of sodium in the second trimester of pregnancy is due to a change in the balance of aldosterone and natriuretic hormone in favor of aldosterone. Thus, the level of aldosterone in the blood plasma during this period comparatively varies. In the urine, the content of natriuretic hormone significantly decreases compared to that in the first trimester, and reaches the level of not pregnant women.
An increase in the content of vasopressin in the blood plasma leads to an increase in tubular water reabsorption.
Due to the reduction of glomerular filtration and, mainly, the growth of tubular reabsorption, the fluid is retained in the body and distributed between sectors of the extracellular space (both intravascular and intercellular). Hypervolaemia and hyperhydration allow filling the increased capacity of the vascular bed, providing an increase in BCC and improving the supply of fetus oxygen and nutrients.
A significant increase in the volume of circulating blood leads to an increased venous return of blood to the heart. Probably, this could cause a massive release of nougat. But for a pregnant woman at this gestational age, a certain inertness of the volumer regulating system is characteristic, associated with the adaptation of the volumoreceptors of the right atrium to a growing BCC.
In the third trimester of physiological pregnancy, there is a decrease in urinary excretion and sodium excretion due to a further moderate decrease in glomerular filtration and a slight increase in tubular reabsorption of water and sodium. Moderate decrease in renal plasma and blood circulation, which leads to a decrease in the glomerular filtration rate. The increase in the intensity of tubular water reabsorption is caused by an increase in the level of vasopressin in the third trimester of pregnancy. A reabsorption of sodium increases due to a further increase in plasma aldosterone levels with a constant concentration of natriuretic hormone (under conditions of spontaneous diuresis).
In response to volume stimulation in pregnant women in the third trimester, as opposed to not pregnant women, there is only a tendency to increase diuresis and natriuresis, despite the significant increase (in response to stimulation) of the concentration of natriuretic hormone in the blood plasma. This is due to a decrease in the sensitivity of the kidneys of pregnant women in the third trimester to this hormone.
Thus, the peculiarities of water-salt metabolism and volumoregulation in physiological pregnancy are, firstly, the accumulation of a large amount of fluid in the vascular bed (to increase the volume of circulating blood and ensure the vital activity of the fetus) and, secondly, the inertness of the volumoregulation system, which allows to preserve this increased volume in the vascular bed.
Changes in volumoregulation are closely related to hemodynamic changes and are aimed at maintaining sufficient perfusion of organs and tissues.