Circulation

The heart is a pump made of cardiac muscle. It has four chambers: two atria at the top, which receive blood, and two ventricles below, which pump it out. Valves stop blood flowing backwards, and their snapping shut is the “lub-dub” of your heartbeat.

Heart diagrams face you, as if you are looking at the person, so the heart’s right side is on the left of the picture. Diagrams colour blood low in oxygen blue to tell it apart, but real blood is always red.

One full trip: body → vena cava → right atrium → right ventricle → pulmonary artery → lungs (pick up oxygen) → pulmonary vein → left atrium → left ventricle → aorta → body. Blood passes through the heart twice on every trip: a double circulation. The right side pumps blood to the lungs; the left side pumps it round the whole body, so the left ventricle has the thickest wall.

Vessel	Carries blood	What it is like
Artery	Away from the heart	Thick, muscular, stretchy walls for blood at high pressure. Your pulse is felt in arteries
Vein	Back to the heart	Thinner walls and a wide space inside, with valves; blood at low pressure
Capillary	Between them, through every tissue	Walls one cell thick, so oxygen and glucose get out and carbon dioxide gets in

Part of blood	Job
Plasma	Pale yellow liquid that carries dissolved glucose, carbon dioxide and other substances
Red blood cells	Carry oxygen, using haemoglobin. They have no nucleus, leaving more room for it
White blood cells	Fight microbes that cause infections
Platelets	Help the blood clot when you cut yourself

Pulse and exercise. Your pulse is the surge of blood through an artery with each heartbeat; feel it at your wrist or your neck. Count for 15 seconds and multiply by 4 to get beats per minute. Exercise raises the heart rate, sending more oxygen and glucose to the muscles and taking more carbon dioxide away. The time it takes to get back to the resting rate is the recovery time. Fitter people recover faster and have lower resting heart rates.

Stretch The heart muscle gets its own blood from the coronary arteries. If fatty deposits block them, part of the heart runs short of oxygen: a heart attack. Smoking, a poor diet and too little exercise all raise the risk.