Concept:Capillaries are the smallest blood vessels with walls only one cell thick, allowing efficient exchange of substances.
Explanation:Arteries branch into smaller vessels called arterioles, which further divide into tiny vessels known as capillaries.
Capillary walls consist of a single layer of endothelial cells, making them just one cell thick.
This extremely thin wall permits easy diffusion of oxygen, nutrients, and waste products between blood and tissue cells.
Arterioles are smaller than arteries but still possess muscular walls that are thicker than one cell layer.
Arteries and veins have three distinct layers: tunica intima, tunica media, and tunica externa, so their walls are much thicker.
Capillaries are the most numerous blood vessels and form networks called capillary beds, providing a large surface area for exchange.
The diameter of a capillary is about
5–
10 μm, so red blood cells often travel through them in single file.
Thus, the vessels with one-cell-thick walls that connect arterioles to venules are capillaries.
Answer:The correct option is B. capillaries.