Question

the structure of a capillary wall differs from that of a vein or artery because multiple choice the capillary wall has only two tunics. capillaries have more smooth muscle. capillary walls have only endothelium and a basement membrane. capillaries can constrict or dilate. capillaries are impermeable to all substances.

Explanation:

Arteries and veins have multiple layers including tunics, smooth - muscle layers etc. Capillaries are the smallest blood vessels and their walls are extremely thin, consisting only of endothelium and a basement membrane which allows for efficient exchange of substances. Capillaries don't have multiple tunics like arteries/veins, have less smooth muscle, are permeable for substance exchange and the ability to constrict/dilate is not what differentiates their structure from arteries/veins in terms of basic composition.

Answer:

capillary walls have only endothelium and a basement membrane.