Question

Blood vessels without muscular walls are
A) Capillaries
B) Arteries
C) Veins
D) Arterioles

Answer 1

Hint: Some blood vessels are the smallest blood vessels in the body. They are made up of very thin walls of just a single layer of flat squamous epithelial cells.

Complete answer:
There are three main types of blood vessels: the arteries, which carry the blood away from the heart. It carries the oxygenated blood and cells require oxygen. The capillaries are the blood vessels where the exchange of water and chemicals takes place between the blood and the tissues; and talking about the veins, veins carry blood from the capillaries back towards the heart.
Hence, veins carry the deoxygenated blood carrying carbon dioxide. Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart. As capillaries are responsible for rapid exchange of material it does not require muscular walls, as muscular walls will not help in rapid exchange of minerals.
However, arteries and veins are responsible for transportation of blood, which has certain pressure. Therefore, arteries and veins consist of muscular walls, so as the vessels do not rupture with the pressure of blood. And as arterioles deliver blood to capillaries they can also not be formed of thin walls or else there may be chances of blood vessel rupture.

Hence, we will consider option (a) as correct.

Note: Capillaries are so thin and tiny blood vessels because even a small single red blood cell can barely fit through them. Capillaries also help to connect your arteries and veins.