Control of sodium excretion regulation of plasma volume

Sodium is the major extracellular cation. Because of its osmotic effects, changes in sodium content in the body have an important influence on extracellular fluid volume, including plasma volume. For example, excess sodium leads to the retention of water and an increase in plasma volume. Increased plasma volume then causes an increase in blood pressure. Conversely, sodium deficit leads to water loss and decreased plasma volume. A decrease in plasma volume then causes a decrease in blood pressure. Therefore, homeostatic mechanisms involved in the regulation of plasma volume and blood pressure involve regulation of sodium content, or sodium balance, in the body.

Sodium balance is achieved when salt intake is equal to salt output. The intake of salt in the average American diet (10 to 15 g/day) far exceeds what is required physiologically. Only about 0.5 g/day of salt is lost in sweat and feces. The remaining ingested salt must be excreted in the urine. The amount of sodium excreted by the renal system is determined by:

• Amount of sodium filtered at the glomerulus

• Amount of sodium reabsorbed from the tubules

Sodium is freely filtered at the glomerulus. Therefore, any factor that affects GFR will also affect sodium filtration. As discussed previously, GFR is directly related to RBF. In turn, RBF is determined by blood pressure and the resistance of the afferent arteriole (RBF = DP/R). For example, an increase in blood pressure or a decrease in resistance of the afferent arteriole will increase RBF, GFR, and, consequently, filtration of sodium. The amount of sodium reabsorbed from the tubules is physiologically regulated, primarily by aldosterone and, to a lesser extent, by ANP. Aldosterone promotes reabsorption and ANP inhibits it. The alterations in sodium filtration and sodium reabsorption in response to decreased plasma volume are illustrated in Figure 19.6.

A decrease in plasma volume leads to decreased MAP, which is detected by baroreceptors in the carotid sinuses and the arch of the aorta. By way of the vasomotor center, the baroreceptor reflex results in an overall increase in sympathetic nervous activity. This includes stimulation of the heart and vascular smooth muscle, which causes an increase in cardiac output and total peripheral resistance. These changes are responsible for the short-term regulation of blood pressure, which temporarily increases MAP toward normal.

Changes in sodium filtration and sodium reabsorption, which lead to a change in sodium excretion, are responsible for the long-term regulation of blood pressure. These changes are brought about by increased activity of the renal sympathetic nerves. Sympathetic stimulation of a1 adrenergic receptors on renal vascular smooth muscle leads to an increase in the resistance of the afferent arteriole. This causes a decrease in RBF, in glomerular filtration iAtrial filling. !

iPlasma volume iANP

IRBF

_îSympathetic discharge

î R afferent arterioles i

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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