The human heart and double circulatory system, including calculating cardiac output

Substances are transported around the body in the blood, which is pumped through vessels by an organ called the heart. The heart is made of cardiac muscle, which continually contracts and relaxes to pump blood. The heart has four chambers with valves, which keep oxygenated and deoxygenated blood separate. Cardiac output can be calculated using: cardiac output = stroke volume × heart rate. We have a double circulatory system; blood is pumped to the lungs for gas exchange then around the rest of the body.