Question

the best indicator of the bloods osmolality is the concentration of ______ .
a calcium ion (ca²⁺)
b bicarbonate ion
c sodium ion (na⁺)
d potassium ion

Explanation:

Blood osmolality reflects the concentration of solutes. Sodium ion ($\text{Na}^+$) is the most abundant extracellular cation and its concentration is the primary determinant of blood osmolality. Calcium ion ($\text{Ca}^{2+}$) has roles in muscle contraction and signaling, bicarbonate ion in acid - base balance, and potassium ion ($\text{K}^+$) is mainly intracellular. So sodium ion is the best indicator.

Answer:

C. sodium ion ($\text{Na}^+$)