OBJECTIVE 4: TO UNDERSTAND THE ROLE OF THE SYMPATHETIC
NERVOUS SYSTEM IN THE CONTROL OF EXTRACELLULAR FLUID VOLUME.
Efferent sympathetic nerve activity (ESNA) to the kidney is a major signaling pathway by which the kidney is apprised of vascular volume changes (Fig. 8-9). Volume receptors in the atria (Fig. 8-2) alter ESNA in response to changes in venous volume. Baroreceptors in the carotid sinus and aortic arch alter ESNA in response to changes in arterial pressure. The atrial stretch receptors have a greater effect on ESNA directed to the kidney than do the arterial baroreceptors. ESNA will have the following effects:
1. Acting through
receptors, ESNA causes vasoconstriction of the arterioles. This tends to reduce RBF and GFR. Because of the buffering by the autoregulatory mechanisms, the fractional change in GFR is usually much smaller than the change in RBF. This results in an increase in the filtration fraction. The fall in pressure in peritubular capillaries coupled with the effect of the change in filtration fraction on peritubular
b tends to increase salt and water reabsorption in the proximal tubule (See Fig. 8-7).
Fig. 8-9. The role of efferent sympathetic nerve activity (ESNA) in the response to volume changes.
2. Acting through
receptors on the granular cells, ESNA will increase the rate of renin release which leads to increased formation of AII.
3. Sympathetic nerve fibers also innervate the proximal and distal tubules and the thick ascending limb of Henle's loop. ESNA directly stimulates tubular reabsorption of salt and water (
4. Circulating catecholamines have the same effect as ESNA.
5. Low levels of ESNA probably increase salt and water reabsorption without affecting GFR. Denervation increases salt and water excretion.
14. Describe how efferent sympathetic nervous activity is increased in response to a loss of blood?
15. How does activation of
receptors in the kidney affect salt and water excretion? Activation of 
receptors?
16. How does ESNA affect salt and water
reabsorption in the proximal tubule? In the collecting tubule?
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