no, with sympathetic stimulation, you constrict your afferent arteriole, which decreases the hyrostatic pressure downstream, thus decreasing gfr.
Ok so here is how it goes.
The General Rule: If the flow through the glomerular capillaries is fast the fluid spends very little time in the capillaries and you tend to have a small percentage of the fluid filtered (FF). If the flow is slow through the glomerular capillaries the fluid spends more time in the capillaries and you tend to have a higher percentage of fluid filtered (FF). Remember this is a tendency but it doesn't always work that way.
FA may have a specific answer here but it is not necessarily correct and different textbooks state different things, that's why Step 1 cannot test simply afferent constriction because there is no single correct answer.
Depending on the degree of constriction you can have; a slight increase in FF, no change in FF or a decrease in FF.
When the afferent arteriole is constricted the GF pressure
decreases leading to a
decrease in GFR and if you constrict anywhere in a series circuit, you
decrease flow (RPF).
Now in your scenario remember that,
BOTH the afferent and efferent arterioles have sympathetic receptors (even though the afferent is dominant) so when stimulated we constrict both the afferent and the efferent raising renal resistance RPF goes
down and GFR goes
down BUT we
increase FF following/based on the
General Rule (The afferent arteriolar constriction does not follow the General Rule as mentioned before).
In the Sympathetic system the body is trying to conserve fluid so because we are constricting both the afferent and the efferent and the FF is
increased, the pressure downstream
decreases and so does the hydrostatic pressure that would promote reabsorption. In addition since we
increase FF, the proteins in the peritubular capillaries
increases and this
increases the forces of reabsorption. Therefore since peritubular capillary pressure goes down and the protein concentration
increases we have a net
increase in the overall force promoting reabsorption.
Ultimately, we tend to recover a very high percentage of what we filtered and very little gets beyond the proximal tubule which is what the body wants during sympathetic stimulation.
