AP BIOLOGY

Detailed explanation-1: -Capillaries have very thin walls and they have no muscles, even though are formed by the branching of arterioles.

Detailed explanation-2: -The capillaries do not have this smooth muscle in their own wall, and so any change in their width is passive. Any signaling molecules they release (such as endothelin for constriction and nitric oxide for dilation) act on the smooth muscle cells in the walls of nearby, larger vessels, e.g. arterioles.

Detailed explanation-3: -The capillaries have only a thin lining of endothelium with no muscular walls. The main function of capillaries is the exchange of gases and nutrients from the blood to the cells and back to the blood.

Detailed explanation-4: -Smooth muscle cells are the most abundant cell type in blood vessel walls. They occur in all vessels except capillaries and pericytic venules.

Detailed explanation-5: -The arteries branch into smaller and smaller vessels, eventually becoming very small vessels called arterioles. Arteries and arterioles have muscular walls that can adjust their diameter to increase or decrease blood flow to a particular part of the body.