Problem 9 - Biology: The Dynamic Science

Question

When capillaries exchange substances: a. red blood cells move through the capillary lumens in double file. b. blood flow resistance is lower than it is in arteries and veins. c. water, ions, glucose, and erythrocytes pass freely between blood and tissues. d. diffusion along a concentration gradient and bulk flow are operating. e. diffusion is greatest closest to the arterioles.

Step-by-Step Solution

Answer

Statements (b), (d) and (e) are correct while (a) and (c) are incorrect. Red blood cells pass in single file as opposed to double, and erythrocytes can't freely pass between blood and tissues due to their size.

Step 1: Red Blood Cell Movement

Analyze statement (a) - 'Red blood cells move through the capillary lumens in double file.' This is incorrect, as Red blood cells usually pass through capillaries in single file rather than double.

Step 2: Blood Flow Resistance

Analyze statement (b) - 'Blood flow resistance is lower than it is in arteries and veins.' This is a correct statement. The extensive branches of capillaries reduce the overall blood velocity and resistance, hence easing the flow of blood compared to arteries and veins.

Step 3: Substance Permeability

Evaluate statement (c) - 'Water, ions, glucose, and erythrocytes pass freely between blood and tissues.' This is partially correct. While water, ions, and glucose can freely pass between blood and tissues, erythrocytes (red blood cells) cannot. They are simply too large to diffuse through the capillary wall.

Step 4: Exchange Mechanisms

Examine statement (d) - 'Diffusion along a concentration gradient and bulk flow are operating.' This is a correct statement. The exchange of substances in capillaries occurs through these two main mechanisms: diffusion along a concentration gradient and bulk flow.

Step 5: Diffusion and Arterioles

Examine statement (e) - 'Diffusion is greatest closest to the arterioles.' This statement is correct. As the blood flow is slowest near the arterioles, it allows for efficient diffusion of materials.