Which vessels are not controlled by sympathetic neurons?

Capillaries are the vessels not directly controlled by sympathetic neurons. In the circulatory system, while sympathetic neurons generally regulate the constriction of larger blood vessels like arterioles and veins, capillaries operate differently. They lack smooth muscle in their walls and are primarily involved in the exchange of gases, nutrients, and waste between blood and tissues. This exchange process is facilitated by differences in hydrostatic and osmotic pressure rather than neural control.

Since capillary function is primarily passive and relies on the physical processes of diffusion and filtration, the sympathetic nervous system does not exert direct control over them. On the other hand, arterioles and veins have smooth muscle layers that respond to sympathetic stimulation, leading to vasoconstriction or dilation, which is essential for regulating blood pressure and flow. Venules, similar to veins, also have some sympathetic control, even though they are not as predominantly influenced as arterioles.