Key Points

• Renal blood flow (RBF) is 1.0-1.2 L/min or about 20% of the cardiac output. At a normal hematocrit, the renal plasma flow (RPF) is ~670 mL/min.

• Glomerular filtration refers to the ultrafiltration of a protein-free fluid from the blood plasma into Bowman's space, the beginning of the nephron.

• Reabsorption by the nephron restores most of the filtered water and solutes to the blood, but in a carefully regulated manner.

• The afferent and efferent arterioles are the primary resistance vessels in the renal circulation. Their independent regulation determines RBF and the glomerular filtration rate (GFR).

• The GFR is normally about 130 mL/min, or more than 180 L/day. The filtration fraction (FF)

is the ratio of GFR to RPF and is approximately 0.2 (20%).

• The forces favoring glomerular filtration are the high hydrostatic pressure in the glomerular capillaries and the small or absent colloid osmotic pressure (COP) in Bowman's space. The forces opposing filtration are the higher COP of the glomerular capillary plasma and the hydrostatic pressure in Bowman's space.

• Increased afferent arteriolar resistance decreases both RBF and GFR. A small increase of efferent arteriole resistance may augment GFR, but larger increases diminish RBF and GFR.

• The filtration barrier comprises the endothelial cells, the basement membrane, and the epithelial cells of Bowman's capsule. The ultrafiltration coefficient (Kf) relates GFR to the net

Essential Medical Physiology, Third Edition

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