After blood is added what is the expected osmolarity and ton

After blood is added, what is the expected osmolarity and tonicity of the solution containing both .1M NaCl and .2M Urea, respectively?

a. hypoosmotic, hypotonic

b. hyperosmotic, hypertonic

c. isoosmotic, hypotonic

d. isoosmotic, isotonic

e. hyperosmotic, hypotonic

Solution

Answer: e (hyperosmotic, hypotonic).

Osmolarity refers to nonpermeable solutes (Example: NaCl) and tonocity refers to the effect of nonpenetrable solutes on cell size or volume.

In this case as the solution has 200 mosm(milli osmolar) NaCl (0.1M) and 200 mosm Urea (0.2 M). So when you add 100 (or even 200, 300 etc mOsm of urea) it is equal to adding pure water into the Na solution, therfore making the solution hypotonic (or reduced osmotic effect on a cell).

But the solution is hyperosmolar by itself compared to blood ( which is 300mOsm). So the answer is the given solution is hyperosmolar and hypotonic.