I

* t Aldosterone

îNa+ reabsorption iPGC

1GFR

|Na+ filtration

4Na+ excretion -Î Na+ conserved î H2O conserved Î Plasma volume

LONG-TERM REGULATION OF MAP

Figure 19.6 Renal handling of sodium.

pressure, in GFR, and a decrease in sodium filtration. If less sodium is filtered, then less sodium is lost in the urine.

Sympathetic stimulation of Pj adrenergic receptors on the granular cells of the juxtaglomerular apparatus promotes secretion of renin and, consequently, formation of angiotensin II. Angiotensin II then causes:

• Widespread vasoconstriction

• Increased resistance of the afferent arteriole

• Increased secretion of aldosterone

Widespread vasoconstriction supplements the increase in TPR induced by the sympathetic nervous system. Angiotensin II also causes vasoconstriction of the afferent arteriole in particular, which enhances the decrease in RBF and sodium filtration. Finally, angiotensin II promotes secretion of aldosterone from the adrenal cortex. Aldosterone then acts on the distal tubule and collecting duct to increase sodium reabsorption.

Sodium reabsorption is also influenced by ANP. The original decrease in plasma volume leads to a decrease in atrial filling and a decrease in the release of ANP from the myocardium. Atrial natriuretic peptide, which acts on vascular smooth muscle, granular cells of the kidney, and the adrenal cortex, normally causes the following:

• Vasodilation

• Decreased renin release

• Decreased aldosterone secretion

Therefore, inhibition of ANP release leads to vasoconstriction and increased MAP. Furthermore, less ANP promotes the release of renin and secretion of aldosterone, which further enhance sodium reabsorption.

Taken together, the homeostatic responses elicited by the initial decrease in plasma volume serve to decrease sodium filtration, increase sodium reabsorption, and, consequently, decrease sodium excretion in the urine. This conservation of sodium leads to conservation of water and an expansion of plasma volume toward normal.

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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