Blood vessel identification by wall structure in longitudinal section

Steps:

• Observe the vessel wall thickness and composition in the electron micrograph.
• Note the presence of a single endothelial layer without smooth muscle.
• Check for a narrow lumen and thin overall structure.
• Match features to capillary characteristics.

Why B is correct:

• Capillaries consist of a single layer of endothelial cells surrounded by a basement membrane, forming thin-walled tubes ideal for exchange, as defined by vascular histology.

Why the others are wrong:

• A. Arteries feature thick walls with elastic fibers and smooth muscle for high-pressure flow.
• C. Veins have collapsible walls with valves and larger lumens to handle low-pressure return.
• D. Venules include pericytes and slightly thicker walls than capillaries for post-exchange collection.

Final answer: B