Question

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 kidneys handle substance X just like human kidneys handle inulin. You also test the alien for its renal handling of substance Y. The data from that experiment are: Plasma substance X concentration: 5 g/100ml; Plasma substance Y concentration: 2 g/100ml; 24 hour urine: 2 liters, containing 150 g substance X and 100 g substance Y. 1) What is the alien's glomerular filtration rate (GFR)? 2) What is the alien's urine clearance rate for substance Y? 3) Assuming substance Y is 100% filtered in the alien’s kidney glomerulus, is there a net reabsorption or net secretion of substance Y in its renal tubules?.

Answer 1

1) Since suxstance X is handled in the similar way human kidneys handle inulin, we can say substance X neither gets absorbed nor gets secreted and hence can be used to estimate GFR.

Given,

plasma concentration of substance X (PX) = 5g / 100mL = 0.05 g / mL

urine concentration of substance X (UX) = 150g / 2L = 0.075 g / mL

urine excreted per minute (V) = (2 L / 24 hr) / 60 = 1.4 mL per minute

Now, the equation for GFR is given by,

GFR = (UX X V) / PX = (0.075 X 1.4) / 0.05 = 2.1 mL / min

2) Given,

urine concentration of substance Y (UY) = 100g / 2L = 0.05 g / mL

urine excreted per minute (V) = 1.4 mL per minute

Now, the equation for urine clearance rate UC is given by,

UC = UY X V = 0.05 X 1.4 = 0.07g / min = 70mg / min

3) Given,

plasma concentration of substance Y (PY) = 2g / 100 mL = 0.02g / mL

UY = 0.05g / mL ; V = 1.4mL / min

Therefore,

clearance rate of substance Y (CY) = (UY X V) / PY = (0.05 X 1.4) / 0.02 = 3.5 mL / min

SInce CY GFR, we can conclude that substance Y is excreted by filtration and secretion.

Hence, substance Y has net secretion in the renal tubules.