Juxtaglomerular Apparatus
Describe or diagram a negative feedback loop similar to figure 23.12 to show how the macula densa could compensate for a drop in systemic blood pressure.
Two important points must be noted about renal autoregulation. First, it does not completely prevent changes in the GFR. Like any other homeostatic mecha-
13macula = spot, patch + densa = dense
14mes = in the middle + angi = vessel
Figure 23.11 The Juxtaglomerular Apparatus.
Figure 23.11 The Juxtaglomerular Apparatus.
nism, it maintains a dynamic equilibrium; the GFR fluctuates within narrow limits. Changes in blood pressure do affect the GFR and urine output. Second, renal autoregulation cannot compensate for extreme blood pressure variations. Over a MAP range of 90 to 180 mmHg, the GFR remains quite stable. Below 70 mmHg, however, glomerular filtration and urine output cease. This can happen in hypovolemic shock (p. 765).
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