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Renal Blood Flow. Although the kidneys represent about 0.5% of total body weight, their blood flow is disproportionately large at 20% to 25% of the cardiac output. The ability to autoregulate keeps renal blood flow relatively constant across a range of systemic mean arterial pressures. Approximately 90% of the renal blood flow is distributed to the renal cortex, with less than 10% of renal blood flow going to the medulla. The generous delivery of blood to the cortex supports flow-dependent functions such as glomerular filtration and tubular reabsorption processes of the cortex. Low blood flow also makes the medulla more susceptible to ischemia than the cortex.

  1. Renal Cortex Blood Flow: Glomerular and Peritubular Capillaries (Figs. 16-1 and 16-4)

  2. Autoregulation of Renal Blood Flow

    1. Renal blood flow and GFR are kept relatively constant within a range of mean arterial pressure between approximately 60 and 160 mm Hg (Fig. 16-5).

    2. In the setting of decreased effective circulating volume, renal blood flow may be decreased despite adequate perfusion pressure (activation of the sympathetic nervous system shunts cardiac output away from the kidneys and adequate systemic blood pressure does not necessarily indicate adequate renal perfusion in the presence of hypovolemia).

  3. Juxtaglomerular Apparatus

    1. In response to decreased renal blood flow, juxtaglomerular cells release renin into the circulation (Fig. 16-6). Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II by angiotensin-converting enzyme.

    2. Effects of angiotensin II include thirst, vasoconstriction, and salt and water reabsorption by the kidneys to maintain circulating volume and increase renal blood flow.