The heart and blood vessels
Double circulation
The heart pumps blood around the body through a : one circuit carries deoxygenated blood to the lungs to pick up oxygen, and a second, separate circuit carries oxygenated blood from the heart to the rest of the body.
Having two separate circuits keeps oxygenated and deoxygenated blood from mixing, and lets the heart pump blood to the body at a higher pressure than it sends to the nearby lungs — delivering oxygen to tissues more efficiently.
Key terms
Structure of the heart
The heart has four chambers: the and on one side, and the left atrium and on the other. Valves between chambers stop blood flowing backwards.
The left ventricle's wall is much thicker than the right ventricle's, because it must contract with enough force to pump blood all the way around the whole body, while the right ventricle only pumps blood the short distance to the lungs.
Key terms
The heart's pacemaker
A group of cells in the right atrium, called the , produces small electrical impulses that control the resting heart rate by making the heart muscle contract in a regular rhythm. If someone's natural pacemaker doesn't work properly, an artificial electrical pacemaker device can be implanted to correct an irregular heartbeat.
Key terms
Arteries, veins and capillaries
- carry blood away from the heart, usually at high pressure. They have thick, muscular, elastic walls and a narrow lumen (channel) to withstand this pressure.
- carry blood towards the heart, at low pressure. They have thinner walls, a wider lumen, and to stop blood flowing backwards.
- link arteries to veins. Their walls are just one cell thick, allowing substances to diffuse between blood and body tissues, and they are very narrow, which slows blood flow to give more time for exchange.