Question

For each description A–E, related to glomerular filtration, select the most appropriate option from the following list of pressures. 

1. 0 mmHg. 

2. 5 mmHg. 

3. 10 mmHg. 

4. 15 mmHg. 

5. 20 mmHg. 

6. 30 mmHg. 

7. 40 mmHg. 

8. 50 mmHg. 

9. 100 mmHg.

 A. The pressure drop along the glomerular capillaries when the pressure at the end of the afferent arterioles is 60 mmHg and that at the start of the efferent arterioles is 40 mmHg. 

B. The net filtration pressure from capillary lumen to Bowman’s capsule when capillary pressure is 60 mmHg, the plasma protein oncotic pressure is 25 mmHg and the capsular pressure is 15 mmHg. 

C. The net filtration pressure when capillary pressure is 35 mmHg, oncotic pressure 25 mmHg and capsular pressure 5 mmHg. 

D. The capillary pressure which would exactly balance a plasma protein oncotic pressure of 20 mmHg. 

E. A capsular pressure at which flow distally along the nephron would not occur. 

Answer 1

A. Option 5 20 mmHg. This is similar to the pressure drop along systemic capillaries when pressure at the arteriolar end is 35 mmHg and that at the venous end is 15 mmHg. 

B. Option 5 20 mmHg. This would represent the situation at the proximal ends of the glomerular capillaries; further along the capillaries, their hydrostatic pressure falls, while the oncotic pressure rises as protein-poor fluid is filtered; both factors reduce the filtration pressure. 

C. Option 2 5 mmHg. This would represent the situation in a patient with serious hypotension (due, for example, to haemorrhage), just before filtration pressure dropped to zero when no further urine could be formed (anuria). 

D. Option 5 20 mmHg. With this capillary pressure, filtration would not occur; without any flow, capsular pressure would fall to near zero. 

E. Option 1 0 mmHg. Like capillaries, nephrons need a pressure drop along them to permit flow distally.