Total salt content in blood is about

Analyze the question: The question asks for the approximate percentage of total salt (mainly sodium chloride) in blood. This is linked to osmoregulation and the idea of an "isotonic" or "physiological" saline solution.

Explain the concept: Blood plasma contains dissolved salts, mainly sodium chloride (NaCl), along with smaller amounts of potassium, calcium, and bicarbonate ions. The salt concentration is kept steady by the kidneys, which is part of homeostasis.

A key fact: normal (physiological) saline is 0.9% NaCl. It is used in hospital drips because it is isotonic to blood, meaning it has the same osmotic concentration as blood plasma. If the blood's salt content were much higher or lower, red blood cells would shrink (crenate) or swell and burst (haemolysis).

Solution:

• Option A (1.85 to 1.9%): too high.
• Option B (11.85 to 11.9%): far too high, which would dehydrate cells by osmosis.
• Option C (2.85 to 2.9%): too high.
• Option D (0.85 to 0.9%): matches the known value of about 0.9%, the isotonic saline concentration.

Answer: The total salt content in blood is about 0.85 to 0.9%, which is option D.

Exam tip: Remember "0.9% saline = isotonic with blood." Whenever you see a question about salt concentration in blood, look for the option around 0.9%.