Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologymediummcq1 pt

Why are intravenous saline solutions formulated to be approximately isotonic with blood rather than using distilled water?

A.Isotonic saline matches blood osmolarity, protecting red cells from osmotic destruction
B.Salt ions nourish cells directly, supplying sodium for membrane repair
C.Distilled water would chemically dissolve the phospholipids of blood cell membranes
D.Cells respond to dilute media by pumping salt inward until ATP runs out

Explanation

Core Concept

An approximately isotonic saline solution produces little net osmotic water movement across red blood cell membranes, helping preserve cell volume. Distilled water is strongly hypotonic relative to plasma and can drive water into erythrocytes until they swell and lyse. Clinical 0.9% saline is commonly treated as isotonic, but it should not be described as exactly identical to plasma in every osmotic property.

Correct Answer

AIsotonic saline matches blood osmolarity, protecting red cells from osmotic destruction

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