The previous answer isn't quite correct, because, well, blood flow is complicated and the body has autoregulatory mechanisms. As Alex mentioned, vasoconstriction doesn't occur in capillaries because they lack a muscle layer. It does occur in the small arterioles before and after the glomerulus (the afferent and efferent arterioles, respectively).
Bernoulli's principle mainly applies to larger arteries and arterioles, and not capillaries. Capillaries are so tiny that the red blood cell is a significant portion of the diameter, so the assumption of no viscosity is no longer valid. See this discussion for an explanation.
To address your question, here's a basic picture of the glomerulus, from Gray's Anatomy.
The afferent arteriole is in red, and the efferent arteriole in blue. The glomerular filtration rate (GFR) depends on the pressure difference between the two, which affects the hydrostatic pressure at the glomerulus. Also, note that vasodilation has the opposite effect of vasoconstriction on GFR. (For a nice image and explanation, look at Figure 6 here) So:
Constriction of the afferent arteriole decreases the blood flow into the glomerulus and thus the glomerular hydrostatic pressure, which leads to a decrease in GFR.
In contrast, constriction of the efferent arteriole decreases blood flow out of the glomerulus, and this increases the glomerular hydrostatic pressure and leads to an increase in GFR.
Both the afferent and efferent arterioles are regulated through hormones (and drugs that inhibit the hormones). Prostaglandins dilate the afferent arterioles, and NSAIDs inhibit this action. Angiotensin II preferentially constricts the efferent arteriole, and this action is restricted by an ACE inhibitor. (Source: First Aid for the USMLE Step 1)
No comments:
Post a Comment