You are a physiologist on a space flight to a distant planet. You discover intelligent humanoid aliens inhabiting the planet, and they willingly submit to your tests. You discover that the alien's kidney handles the sugar alcohol mannitol just like our kidneys handle inulin. You also test the alien for its renal handling of glucose. The data from that experiment are: Plasma level (constant over
24 hrs): 6 g/dL of mannitol and 2 g/dL of glucose; 24 hr. urine sample: volume of 2 L, containing 144 g mannitol and 52 g glucose.
A. What is the alien's glomerular filtration rate?
B. What is the alien's clearance rate for glucose?
C. How does the alien kidney handle glucose?
Human kidneys filter inulin, a polysaccharide from dahlia roots, but do not reabsorb or secrete it. Thus, 100% of the
inulin in a urine sample is filtered inulin. For this reason, the inulin excretion rate is the same as the glomerular
filtration rate.
A. The alien's GFR= excretion rate of mannitol/plasma concentration of mannitol
= 144 g/day × 1 dL/6 g × 0.1 L/1 dL = 2.4 L/day.
B. Using 2.4 L/day as GFR, the filtered load for glucose = plasma concentration of glucose ×GFR
= 2 g/dL × 2.4 L/day × 1 dL/0.1 L = 48 g glucose.
C. There were 52 g of glucose produced in 24 hours. Since the clearance of glucose is greater than the filtered load, some glucose must have been secreted. Therefore the alien's kidney filters and secretes glucose.
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