This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
Which ion has the greatest concentration of saliva under resting conditions? 
Select and explain. 
A. Na+, B. K+, C. Cl-, D. HCO3-
D: HCO3-
Answer: D: HCO3- 
Explanation: Under resting conditions, the concentrations of sodium and chloride ions in the saliva are only about 15 mEq/L each, about one-seventh to one-tenth their concentrations in plasma. Conversely, the concentration of potassium ions is about 30 mEq/L, seven times as great as in plasma, and the concentration of bicarbonate ions is 50 to 70 mEq/L, about two to three times that of plasma.