Question

Estimate how much the brain would swell due to osmosis if the concentration of the cerebrospinal fluid fell to 280mM. In other words, find what the new volume of the brain would be once equilibrium was reestablished. Assume that (i) the cerebrospinal fluid is constantly replenished, so its solute concentration won’t change; (ii) only water is passing across the cell membranes into brain cells (not solutes); and (iii) the volume of the brain is mostly water.

Answer 1

Answer:

The correct answer is 6.67 %.

Explanation:

The average osmotic pressure of the brain is 300 mM

Given the concentration of solution is 280 mM

Difference is 300 - 280 = 20 mM

This difference is sufficient to stimulate osmosis and make water move from CSF to the brain until the concentration becomes similar.  

Also, it is known that the volume of brain is about 1200 ml on an average,  

Volume of fluid moved because of osmosis = 20 mM / 300 mM * 1200 ml = 80 ml

Percent swelling of brain = 80 ml / 1200 ml * 100 = 6.67 %