Final answer:
Capillaries cannot stop bleeding by constricting due to their thin walls and lack of musculature, which are necessary for the exchange of substances rather than for constrictive responses.
Step-by-step explanation:
The blood vessels that cannot stop bleeding by constricting are capillaries. Unlike arteries, arterioles, and veins, capillaries are extremely thin and lack the muscle layers necessary for constriction that could halt bleeding. They are the smallest blood vessels in the body and are predominantly involved in the exchange of gases, nutrients, and waste products between the blood and the surrounding tissues. This lack of musculature makes them unable to constrict effectively to stop bleeding.
When an injury occurs, the larger vessels like arteries and veins can constrict to reduce blood flow, a process aided by their thicker, muscular walls. However, capillaries are too thin to provide such a response. They are composed of a single layer of endothelial cells that is extremely permeable to allow exchange at the cellular level. This characteristic is vital for their role in the body's homeostasis but is also why they are not capable of vasoconstriction sufficiently to stop bleeding.