SNS - Renal Effects
- Fibers of the Sympathetic Nervous System innervate the kidneys and along with circulating catecholamines enhance tubular sodium and water resorption while reducing Glomerular Filtration Rate (GFR) and total Renal Blood Flow (RBF). Renal sympathetic afferents are activated in response to large short-term drops in the systemic arterial pressure and serve to reduce blood flow to the kidneys while boosting the resorption of salt and water, together helping redirect blood to more critical organs and stem loss of extracellular volume. Although these effects may be important for responding to sudden and large reductions in arterial pressure, as may occur in a major hemorrhage, their significance for long-term modulation of renal function is likely little.
|Effects on Glomerular Filtration and Renal Blood Flow|
- The renal afferent and efferent arterioles are heavily innervated by sympathetic fibers. Significant activation of these afferent fibers or large concentrations of circulating catecholamines results in afferent and efferent arteriolar vasoconstriction, yielding a significant decrease in GFR and RBF. Not only does this allow blood volume to be redirected to more critical organs, such as the brain, the reduced GFR decreases the capacity of the kidney to filter and thus potentially excrete critical ECF volume.
|Effects on Tubular Resorption|
- Activation of sympathetic fibers and circulating catecholamines act to increase tubular resorption of sodium and water by both direct and indirect routes. Sympathetic afferents and circulating catecholamines appear to directly enhance resorption of sodium in the proximal tubule. Because water passively follows sodium resorption in this segment, water resorption is also proportionally enhanced. Sympathetic fibers also synapse on juxtaglomerular cells and appear to stimulate release of renin. This leads to activation of the Renin-Angiotensin-Aldosterone System and thus indirectly yields increased sodium and water resorption. Taken together, these sympathetic effects result in increased extracellular fluid volume serving to maintain arterial pressure in scenarios of sudden massive blood loss.