What molecule is in all salivary secretions and present at concentrations of 3-10 mM?

What molecule is in all salivary secretions and present at concentrations of 3-10 mM?

• A. Calcium
• B. Urea
• C. Glucose
• D. Sodium chloride

Answer: (B)

Small, freely diffusible molecules from the blood appear in saliva, and urea is a classic example. Urea is a small, uncharged molecule, so it passes readily from blood into the salivary gland fluid. Because of this easy diffusion, urea is found in all salivary secretions, and its concentration in saliva tends to fall within the millimolar range, roughly 3–10 mM, reflecting the systemic level of urea rather than the immediate dietary state. Other options don’t fit as consistently. Calcium in saliva can be present, but its amount is often affected by binding to proteins and phosphates and by flow rate, so it isn’t reliably in a uniform 3–10 mM across all saliva samples. Glucose is usually present only in trace amounts unless blood glucose is elevated. Sodium chloride is a major electrolyte, but its levels in saliva vary with flow rate and glandular modification, so it isn’t consistently found at that specific millimolar range in all secretions.

Small, freely diffusible molecules from the blood appear in saliva, and urea is a classic example. Urea is a small, uncharged molecule, so it passes readily from blood into the salivary gland fluid. Because of this easy diffusion, urea is found in all salivary secretions, and its concentration in saliva tends to fall within the millimolar range, roughly 3–10 mM, reflecting the systemic level of urea rather than the immediate dietary state.

Other options don’t fit as consistently. Calcium in saliva can be present, but its amount is often affected by binding to proteins and phosphates and by flow rate, so it isn’t reliably in a uniform 3–10 mM across all saliva samples. Glucose is usually present only in trace amounts unless blood glucose is elevated. Sodium chloride is a major electrolyte, but its levels in saliva vary with flow rate and glandular modification, so it isn’t consistently found at that specific millimolar range in all secretions.