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OBJECTIVES

OBJECTIVES

  • Define renal blood flow, renal plasma flow, glomerular filtration rate, and filtration fraction, and give normal values.

  • State the formula relating flow, pressure, and resistance in any vascular bed.

  • Identify the successive vessels through which blood flows after leaving the renal artery.

  • State the relative resistances of the afferent arterioles and efferent arterioles.

  • Describe how changes in afferent and efferent arteriolar resistances affect renal blood flow.

  • Describe the three layers of the glomerular filtration barrier and define podocyte, foot process, and slit diaphragm.

  • Describe how molecular size and electrical charge determine the filterability of plasma solutes; state how protein binding of a low-molecular-weight substance influences its filterability.

  • State the formula for the determinants of glomerular filtration rate, and state, in qualitative terms, why the net filtration pressure is positive.

  • State the reason glomerular filtration rate is so large relative to filtration across other capillaries in the body.

  • Describe how arterial pressure, afferent arteriolar resistance, and efferent arteriolar resistance influence glomerular capillary pressure.

  • Describe how changes in renal plasma flow influence average glomerular capillary oncotic pressure.

  • Define autoregulation of renal blood flow and glomerular filtration rate.

RENAL BLOOD FLOW

imageBlood flow through the kidneys (RBF) is huge relative to their metabolic need, amounting to about 1 L/min, or 20% of the resting cardiac output. This is through tissue that constitutes less than 0.5% of the body mass! Considering that the volume of each kidney is less than 150 cc, this means that each kidney is perfused with over three times its total volume every minute. We generally think of blood flow as a means to deliver oxygen and nutrients, and remove carbon dioxide and waste. While blood flow certainly serves these functions in the kidneys, it does much more. The high blood flow and the kidney’s unique vascular anatomy play a crucial role in renal function.

All renal blood is delivered to the cortex where it flows through glomerular capillaries. Most of this blood eventually enters the post-glomerular peritubular capillaries within the cortex and subsequently returns to the general circulation. However, 5% to 10% of the cortical blood flow, after leaving the glomeruli, enter capillaries that leave the cortex to descend into the medulla before returning to the general circulation.

Blood enters each kidney at the hilum via a renal artery. After several divisions into smaller arteries blood reaches arcuate arteries that course across the tops of the pyramids in the cortex near the corticomedullary boundary. From these, interlobular arteries (also called cortical radial arteries) project upward toward the kidney surface. These arteries give off numerous arterioles, each of which leads to an individual Bowman’s capsule and the glomerulus within (see Figure 2–1). These arteries and all glomeruli are found only in the cortex, never in the medulla. The arterioles leading to glomeruli are called afferent arterioles and have important functional characteristics discussed later. In most ...

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