Question

For each mechanism A–E related to renal handling of plasma solutes, select the most appropriate option from the following list of nephron functions. 

1. Active tubular secretion. 

2. Passive tubular secretion. 

3. Active tubular reabsorption. 

4. Passive tubular reabsorption. 

5. Glomerular filtration. 

A. The mechanism whereby healthy people avoid having glucose in the urine (glycosuria). 

B. The mechanism whereby urea has a lower clearance than creatinine. 

C. The mechanism whereby around 99 per cent of the filtered water is not lost in the urine. 

D. The mechanism whereby the distal convoluted tubules eliminate excess hydrogen ions from the body. 

E. The only mechanism whereby the ideal substance for measuring the glomerular filtration rate is eliminated from the body by the kidneys.

Answer 1

A. Option 3 Active tubular reabsorption. Reabsorption of glucose in the proximal convoluted tubules by a carrier mechanism relies on active reabsorption of sodium ions. 

B. Option 4 Passive tubular reabsorption. Urea, like creatinine, is freely filtered, but, whereas virtually all the creatinine remains in the tubular lumen, about half the filtered urea passively diffuses back into the renal capillaries. 

C. Option 4 Passive tubular reabsorption. Water follows absorbed solutes, particularly sodium and chloride ions, passively by osmosis, as they are reabsorbed into the capillaries. 

D. Option 1 Active tubular secretion. An active pump exchanges absorbed sodium for secreted potassium and hydrogen ions. 

E. Option 5 Glomerular filtration. The ideal substance for measuring glomerular filtration rate is freely filtered and neither absorbed from nor secreted into the tubules in any way; inulin and creatinine are close to the ideal.