In which of the following blood vessels does exchange of materials occur

Analyze the question: It asks which blood vessel allows exchange of materials (oxygen, nutrients, carbon dioxide, wastes) between blood and body tissues.

Explain the concept: Blood vessels are built differently depending on their job:

• Arteries carry blood away from the heart under high pressure. Their walls are thick, muscular and elastic, so materials cannot pass through them.
• Arterioles are small branches of arteries. They control how much blood flows into the capillaries, but their walls are still too thick for exchange.
• Capillaries are the tiny vessels connecting arterioles to venules. Their walls are only one cell thick (a single layer of endothelium), and they are very narrow and extremely numerous, so they form a large surface area.
• Venules are small vessels that collect blood from capillaries and join to form veins.
• Veins carry blood back to the heart. They have thinner walls than arteries and contain valves, but they are not adapted for exchange.

Step-by-step solution:

1. Exchange needs a very thin barrier so substances can diffuse easily.
2. Only capillaries have walls one cell thick.
3. At the capillary beds in tissues, oxygen and nutrients diffuse out of the blood into the cells, while carbon dioxide and waste products diffuse from the cells into the blood.
4. Therefore, exchange of materials occurs in the capillaries.

Correct answer: C. capillaries

Exam tip: Remember "Capillaries = Change (exchange)". Whenever you see the words "exchange of materials" or "thin, one-cell-thick wall" in a Biology question, think capillaries. Note also that the sequence is: artery → arteriole → capillary → venule → vein.