Question

3. In the kidney there are two separate capillary beds. Each has its own set of Starling forces for filtration and reabsorption. Solve the following sets of data. 4 points The first (glomerulus) capillary bed has an HPv of 58 mmHg at the afferent arterial end, the blood has an v of 28, and the efferent arterial end of the capillary has an HPv of 35. The HPt in the Bowman's capsule is 3 mmHg and the OPt is 2 mmHg. A) What is the pressure of filtration at the afferent arteriole end?. B) What is the pressure of filtration at the efferent arteriole end? . C) What is the net pressure? Does this match the function of the glomerulus capillary of filtering about 20% of the plasma into the Bowman's capsule and into the nephron? Explain

Answer 1

• The afferent and efferent arterioles are innervated by sympathetic neurons; however, sympathetic tone is minimal when the volume of extracellular fluid is normal (see Chapter 6). Sympathetic nerves release norepinephrine and dopamine, and circulating epinephrine (which is a catecholamine like norepinephrine and dopamine) is secreted by the adrenal medulla. Norepinephrine and epinephrine cause vasoconstriction by binding to α1-adrenoceptors, which are located mainly on the afferent arterioles. Activation of α1-adrenoceptors decreases GFR and RBF. Dehydration or strong emotional stimuli, such as fear and pain, activate sympathetic nerves and reduce GFR and RBF.

Renalase, a catecholamine-metabolizing hormone produced by kidneys, facilitates the degradation of catecholamines.

The Afferent Arteriole and Efferent Arteriole Exert Separate Effects on PGC

Constriction of either the afferent or efferent arterioles can alter the glomerular hydrostatic pressure, PGC, that drives ultrafiltration. Constriction of the afferent arterioles has two effects: it increases the vascular resistance which reduces renal blood flow (RBF), and it decreases the pressure downstream from the constriction, which reduces the GFR. However, the slower blood flow means that glomerular filtration has more time to approach filtration equilibrium, so constriction of the afferent arterioles ought to raise slightly the filtration fraction (=GFR/RBF). Dilation of the afferent arterioles has the opposite effects. Constriction of the efferent arterioles also increases the vascular resistance so it reduces RBF. The pressure within the glomerular capillaries may increase, however, because the flow is slowed by efferent arteriolar constriction. Recall that the Bernoulli principle states that movement of a fluid reduces the pressure measured normal to the direction of flow. The net result of efferent arteriolar constriction is an increased filtration fraction. The GFR most likely decreases because the RBF decreases, but the decrease in the GFR will not be proportionate because of the increase in glomerular capillary pressure. Thus constriction of the afferent and efferent arterioles exerts separate effects on RBF and the GFR, as detailed