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Figure

The formation of angiotensin II in the bloodstream involves several organs and has multiple actions that conserve sodium and water.

the afferent and efferent arterioles. Although afferent arteriolar constriction decreases GFR, efferent arteriolar constriction minimizes the decrease, thus contributing to autoregulation of GFR. Angiotensin II has a major effect on the kidneys through the adrenal cortical hormone aldosterone, which stimulates sodium reabsorption in the distal convoluted tubule. By stimulating aldosterone secretion, angiotensin II helps to reduce the amount of sodium excreted in the urine.

The hormone atrial natriuretic peptide (ANP) also affects sodium excretion. ANP secretion increases when the atria of the heart stretch due to increased blood volume. ANP stimulates sodium excretion through a number of mechanisms, including increasing GFR.

Tubular Reabsorption

If the composition of the glomerular filtrate entering the renal tubule is compared with that of the urine leaving the tubule, it is obvious that the fluid changes as it passes through the tubule (see table 20.1). For example, glucose is present in the filtrate but absent in the urine. In contrast, urea and uric acid are considerably more concentrated in urine than they are in the glomerular filtrate. Such changes in fluid composition are largely the result of tubular reabsorption (tu'bu-lar re-ab-sorp'shun),

Afferent arteriole

Glomerular capsule

Glomerulus

Glomerular filtrate

Tubular reabsorption

Renal tubule

Figure

Afferent arteriole

Glomerular capsule

Glomerulus

Glomerular filtrate

Tubular reabsorption

Renal tubule

Efferent arteriole

Blood flow

Efferent arteriole

Blood flow

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