Concept:Capillaries are adapted for efficient exchange of materials between blood and body cells.
Explanation:Blood flow through capillaries is slow because they are very numerous and their walls are only one cell thick.
This structure gives capillaries a very large total surface area for exchange.
Slow movement of blood means sufficient time for oxygen, nutrients, and waste products to diffuse between the blood and the body cells.
Fast blood flow would reduce the time available for this essential exchange.
So, the slow flow is directly linked to the need for a large surface area and efficient exchange.
Answer:Their large numbers and one cell-thick walls enable them to provide a large surface area through which materials can be exchanged between the blood and the body cells.