The osmolarity of body fluids is maintained at ________.

a. 100 mOsm

b. 300 mOsm

c. 1000 mOsm

d. it is not constantly maintained

Osmolarity refers to the number of osmoles of solute in one liter of solution. The maintenance of fluid balance is critical for cell survival. The cell membrane is permeable to water and therefore water loss and uptake should be balanced for cells to survive. If osmolarity changes then the cells will either lose or gain water leading to shrinking or swelling of cells respectively.

Body fluids have a normal physiologic osmolarity of approximately 300 mOsm. A value of 100 mOsm suggests that the solution has a low osmolarity suggesting a lower amount of solute in the solution. Hence, option a is incorrect.

An osmolarity of 1000 mOsm suggests the concentration of solute is very high and therefore it exhibits hyperosmolarity. Hence, option c is incorrect.

It is essential to maintain fluid balance for cell survival. If the fluid balance is disturbed the body achieves homeostasis by either stimulating the release of the antidiuretic hormone (ADH) when osmolarity is increased or suppressing the release of ADH when osmolarity is decreased. Hence, option d is incorrect.

The physiological osmolarity of body fluids is 300 mOsm when the net movement of water is zero. This means that the amount of water excreted by the kidneys is equal to the volume of fluid taken in. Hence, option b is correct.