Answer:
The correct answer would be Capillaries have thin walls and narrow diameters so that blood cells come close to the thin walls, across which gases can diffuse to surrounding tissue.
Capillaries are the fine branched blood vessels.
They are the smallest blood vessels with a diameter of approximately 5-10 micrometers (μm).
They are very thin walled. They are around one-celled thick.
They form the site for exchange of materials between blood and cells.
Smaller molecules such as water, gases, ions are exchanged through simple diffusion whereas larger molecules such as glucose, amino acids, albumin et cetera are passed through passive or active transport.
