Obstetrics and Gynecology - A.M. Hromova 2000

Changes in the Woman's Body During Pregnancy
Water-Salt Metabolism

The restructuring of the pregnant woman's bodily Functions to ensure the normal further Development of the fetus leads to changes in both extrarenal (hormonal and hemodynamic) regulatory mechanisms of volume-osmotic Homeostasis and renal activity mechanisms.

In the first trimester of Pregnancy, compared to non-pregnant women, there is an increase in diuresis and natriuresis due to an elevated Glomerular Filtration rate and a reduction in tubular reabsorption of Water and sodium. The increase in glomerular filtration is driven by a rise in renal plasma flow and renal Blood flow resulting from a decrease in total renal vascular resistance.

Plasma aldosterone levels in healthy women increase During the first trimester of pregnancy. Given that aldosterone promotes tubular reabsorption of sodium, we encounter a paradoxical reaction: sodium excretion increases despite rising aldosterone levels. This phenomenon is explained by a significant increase in the concentration of the aldosterone antagonist—natriuretic peptide—in the urine of the examined women. Natriuretic peptide is a peptide hormone synthesized by the cardiomyocytes of the right atrium in response to increased blood volume and elevated pressure within the atrial cavity. It has The ability to inhibit sodium reabsorption in the proximal renal tubules, thereby leading to increased urinary excretion of sodium.

In response to sodium loss, the plasma osmolarity of the pregnant woman decreases. To prevent any further increase in blood hypo-osmolarity, the respective regulatory systems limit vasopressin production.

The combination of all these factors leads to increased urinary elimination of water and sodium during the first trimester of pregnancy. As a result, the extracellular fluid volume decreases slightly, primarily due to a reduction in circulating blood volume.

In the second trimester of pregnancy, compared to the first trimester, There is a decrease in diuresis and natriuresis due to a slight reduction in the glomerular filtration of water and sodium, along with a moderate increase in their tubular reabsorption. Renal blood flow remains elevated, and the decrease in glomerular filtration rate is caused by a reduction in the resistance of the efferent renal arterioles.

The increased tubular reabsorption of sodium in the second trimester of pregnancy is caused by a shift in the balance between aldosterone and natriuretic peptide in favor of aldosterone. Specifically, plasma aldosterone levels remain relatively unchanged during this period. In the urine, the concentration of natriuretic peptide drops noticeably compared to the first trimester, reaching the levels observed in non-pregnant women.

An elevated concentration of vasopressin in Blood Plasma leads to an increase in the tubular reabsorption of water.

Due to decreased glomerular filtration and, most importantly, increased tubular reabsorption, fluid is retained in the body and distributed among the extracellular fluid compartments (both intravascular and interstitial). Hypervolemia and hyperhydration help fill the expanded vascular bed capacity, ensure an increased blood volume, and improve the supply of oxygen and nutrients to the fetus.

A significant increase in circulating blood volume leads to enhanced venous return to The Heart. Logically, this could trigger a massive release of natriuretic peptide; however, at this stage of gestation, the pregnant woman's volume-regulatory system exhibits a certain degree of inertia associated with the adaptation of right atrial volume receptors to the expanding blood volume.

In the third trimester of physiological pregnancy, there is a reduction in urine output and sodium excretion resulting from a further moderate decrease in glomerular filtration and a slight increase in the tubular reabsorption of water and sodium. Renal plasma and blood flow decrease moderately, leading to a reduced glomerular filtration rate. The increased intensity of tubular reabsorption of water is driven by elevated vasopressin levels in the third trimester. Meanwhile, sodium reabsorption increases due to a further rise in plasma aldosterone concentration, while the level of natriuretic peptide remains unchanged (under conditions of spontaneous diuresis).

In response to volume stimulation, pregnant women in the third trimester—unlike non-pregnant women—show only a tendency toward increased diuresis and natriuresis, despite a substantial rise (triggered by the stimulation) in plasma natriuretic peptide concentration. This is explained by a decreased renal sensitivity to this hormone in women during the third trimester.

Thus, the Characteristic Features of water-salt METABOLISM and Volume Regulation in a physiological pregnancy are, firstly, the accumulation of a significant amount of fluid within the vascular bed (to increase circulating blood volume and support fetal vital activity) and, secondly, the inertia of the volume-regulatory system, which helps maintain this elevated volume within the vasculature.

Changes in volume regulation are closely linked to hemodynamic shifts and are aimed at maintaining adequate perfusion of Organs and Tissues.