Why are veins thinner than capillaries?

Capillaries are the smallest blood vessels in the body, with diameters ranging from 5 to 10 micrometers. Veins, on the other hand, are much larger, with diameters ranging from 1 to 2 millimeters.

The reason for this difference in size is due to the different functions of capillaries and veins. Capillaries are responsible for the exchange of oxygen and carbon dioxide between the blood and the tissues. This exchange occurs through the capillary walls, which are very thin and permeable. Veins, on the other hand, are responsible for transporting blood back to the heart. They do not need to be as thin and permeable as capillaries, since they are not involved in the exchange of gases.

In addition, the pressure inside capillaries is much lower than the pressure inside veins. This is because the capillaries are located at the end of the arterial system, where the blood pressure is lowest. The pressure inside veins is higher because the blood is being pushed back to the heart.

The thinner walls and lower pressure inside capillaries allow them to be very flexible and to fit into small spaces. This is important because capillaries are found in all tissues of the body, including the brain, heart, and lungs. Veins, on the other hand, are not as flexible and are not found in as many tissues as capillaries.