Physiology of the Cardiovascular System

The cardiovascular system, also called the circulatory system, transports blood around the body to deliver oxygen and nutrients to tissues and to carry away carbon dioxide and other wastes. It consists of the heart acting as a pump, and a network of blood vessels: arteries that carry blood away from the heart, veins that return it, and tiny capillaries where the exchange of substances with the tissues takes place.

The heart is a muscular organ divided into four chambers, two atria that receive blood and two ventricles that pump it out. Heart valves between the chambers and at the exits ensure that blood flows in one direction only. Blood returning from the body enters the right side, is sent to the lungs to take up oxygen, returns to the left side, and is then driven out through the aorta, the largest artery, to the rest of the body. This double loop keeps oxygen-poor and oxygen-rich blood separate.

Blood itself carries red cells rich in oxygen-binding pigment, white cells of the immune system, platelets for clotting, and a fluid called plasma. Human blood is classified into types according to molecules on the red cells, including the ABO groups and the rhesus factor, which must be matched for safe transfusion. Hormones such as adrenaline can raise the heart rate and the force of its contraction in response to stress. Cardiovascular diseases, including narrowing of the arteries and high blood pressure, are a major cause of illness and arise when this transport system is impaired.

Frequently asked questions

What is the role of capillaries?

Capillaries are the smallest blood vessels, where oxygen, nutrients and wastes are exchanged between the blood and the surrounding tissue cells through their thin walls.

Why does the heart have valves?

Valves keep blood flowing in one direction through the heart, preventing it from leaking backward between chambers or back into the heart after being pumped out.

What determines a person's blood type?

Blood type is set by molecules on the surface of red blood cells, chiefly the ABO antigens and the rhesus factor, which must be compatible for safe transfusion.