The Glomerulus – Tubular Reabsorption and Secretion Quiz

See Filtration Fraction. Filtration fraction is glomerular filtration rate divided by renal plasma flow, B. Glomerular filtration rate, or GFR, is the volume of fluid filtered into the nephron per unit time. Renal plasma flow is the volume of plasma delivered to the kidney over the same interval, excluding blood cells. The ratio therefore asks what fraction of incoming plasma becomes filtrate, not what fraction leaves as urine. Most filtered fluid is subsequently reabsorbed, so urine output is far smaller than filtration. For example, if GFR is 100 mL/min and renal plasma flow is 500 mL/min, the filtration fraction is 0.2, or 20%; these are illustrative values.

The ratio in D uses whole blood rather than plasma and puts flow over filtration, reversing the required relationship. Proximal reabsorption (A) occurs after fluid has crossed the glomerulus and answers a different question. Urine flow divided by GFR (C) compares final excretion with initial filtration, after tubular processing. The medullary blood-flow proportion (E) concerns where blood travels within the kidney, not how much plasma enters the filtrate.