Your heart and blood vessels make up your overall blood circulatory system. Your blood circulatory system is made up of four subsystems.
Arterial circulation is the part of your circulatory system that involves arteries, like the aorta and pulmonary arteries. Arteries are blood vessels that carry blood away from your heart. (The exception is the coronary arteries, which supply your heart muscle with oxygen-rich blood.)
Healthy arteries are strong and elastic (stretchy). They become narrow between heartbeats, and they help keep your blood pressure consistent. This helps blood move through your body.
Arteries branch into smaller blood vessels called arterioles (ar-TEER-e-ols). Arteries and arterioles have strong, flexible walls that allow them to adjust the amount and rate of blood flowing to parts of your body.
Venous circulation is the part of your circulatory system that involves veins, like the vena cavae and pulmonary veins. Veins are blood vessels that carry blood to your heart.
Veins have thinner walls than arteries. Veins can widen as the amount of blood passing through them increases.
Capillary circulation is the part of your circulatory system where oxygen, nutrients, and waste pass between your blood and parts of your body.
Capillaries are very small blood vessels. They connect the arterial and venous circulatory subsystems.
The importance of capillaries lies in their very thin walls. Oxygen and nutrients in your blood can pass through the walls of the capillaries to the parts of your body that need them to work normally.
Capillaries' thin walls also allow waste products like carbon dioxide to pass from your body's organs and tissues into the blood, where it's taken away to your lungs.
Pulmonary circulation is the movement of blood from the heart to the lungs and back to the heart again. Pulmonary circulation includes both arterial and venous circulation.
Oxygen-poor blood is pumped to the lungs from the heart (arterial circulation). Oxygen-rich blood moves from the lungs to the heart through the pulmonary veins (venous circulation).
Pulmonary circulation also includes capillary circulation. Oxygen you breathe in from the air passes through your lungs into your blood through the many capillaries in the lungs. Oxygen-rich blood moves through your pulmonary veins to the left side of your heart and out of the aorta to the rest of your body.
Capillaries in the lungs also remove carbon dioxide from your blood so that your lungs can breathe the carbon dioxide out into the air.