The reason why the flow of blood through the capillaries is very slow is

Step 1: Analyze the question The question asks why blood flows slowly through the capillaries. Slow flow has a purpose, which is to give enough time for exchange of materials.

Step 2: Explain the concept Capillaries are the smallest blood vessels, connecting arterioles to venules. Their key features are:

• Walls one cell thick, so the diffusion distance is very short.
• Very large numbers, which together give a huge total cross-sectional area. When blood moves from the arteries into this huge network, it spreads out and slows down, like a river flowing into a wide delta.
• Narrow lumen, which increases resistance to flow.

The slow flow gives time for oxygen, nutrients, carbon dioxide and wastes to be exchanged between the blood and the body cells.

Step 3: Evaluate the options

• A. Wrong. Capillary walls are very thin (one cell thick), not thick.
• B. Weak. Pressure does drop in the capillaries, but this is a result of their structure, not the "reason" in the sense of purpose. It is also not the best answer.
• C. Wrong. Dizziness has nothing to do with capillary blood flow.
• D. Best answer. The large numbers and thin walls give a large surface area for exchange, and the large total cross-sectional area is what slows the blood, allowing efficient exchange.

Note: Option D is worded more as a description of capillary structure and function than as a direct "reason for slow flow.

Step 4: Answer The correct option is D.

Exam tip: Remember "thin, many, slow, exchange." Capillary structure matches its function: exchange of materials. In JAMB, eliminate options that contradict basic facts, such as "thick walls" for capillaries.