Question

For each urinary solute A–E, select the most appropriate option from the following list of concentrations to be found in the urine of healthy people. 

1. Always greater than in plasma. 

2. Always less than in plasma. 

3. Always the same as in plasma. 

4. Can be less than or greater than in plasma. 

A. Sodium. 

B. Hydrogen ions. 

C. Potassium ions. 

D. Urea. 

E. Para-aminohippurate (PAH). 

Answer 1

A. Option 4 Can be less than or greater than in plasma. The plasma sodium level is around 140 mmol per litre; with a low sodium intake of 50 mmol per day and a urinary volume of one litre, the sodium concentration would be 50 mmol per litre; with a fairly high sodium intake of 250 mmol per day and the same urinary volume, the concentration would be 250 mmol per litre. 

B. Option 4 Can be less than or greater than in plasma. Plasma pH is around 7.4; urinary pH can vary from less than 5 to around 8. 

C. Option 1 Always greater than in plasma. The intakes and outputs of potassium are of a similar order to those given for sodium, so to maintain balance, the urinary concentrations are also similar to those of sodium; however the plasma potassium concentration is only about 4 mmol per litre. 

D. Option 1 Always greater than in plasma. About 99 per cent of filtered water is reabsorbed, but only about half of the filtered urea. 

E. Option 1 Always greater than in plasma. Not only is all the filtered PAH excreted (compare urea above), but, provided the tubular maximum is not exceeded, all the PAH reaching the kidney is also excreted.