The Glomerulus – Tubular Reabsorption and Secretion Quiz

See Filtration Fraction. Option B is correct because the filtration fraction is the proportion of the plasma that enters the kidney that is actually filtered into Bowman's space, which is calculated as glomerular filtration rate (GFR) divided by renal plasma flow (RPF). GFR is the volume of plasma filtered by all the glomeruli per minute, and RPF is the volume of plasma, not whole blood, that flows through the kidneys per minute. Typical values are a GFR of about 120 mL/min and an RPF of about 600 mL/min, giving a filtration fraction of 120 divided by 600, or about 0.2, which means roughly 20% of the plasma is filtered. The remaining 80% passes on into the efferent arteriole and the peritubular capillaries (the capillaries wrapped around the tubules). The filtration fraction rises when the efferent arteriole constricts, which is the kidney's usual response to volume depletion through angiotensin II. The unfiltered plasma then enters the peritubular capillaries with a higher oncotic pressure (the pull exerted by plasma proteins), which helps to drive reabsorption in the proximal tubule. A simple way to remember it: the filtration fraction is the share of the plasma that is filtered, so GFR goes on top.

• Definition: The proportion of the plasma entering the kidney that is filtered into Bowman's space.
• Formula: Glomerular filtration rate (GFR) divided by renal plasma flow (RPF).
• Normal value: About 0.2, so roughly 20% of the plasma is filtered.
• Efferent constriction: Raises it, which angiotensin II does in volume depletion.