The heart: structure, blood flow and blood vessels

1. Deoxygenated blood from the body enters the right atrium through the vena cava.
2. It passes into the right ventricle, which pumps it through the pulmonary artery to the lungs.
3. In the lungs it picks up oxygen and releases carbon dioxide.
4. Oxygenated blood returns through the pulmonary vein to the left atrium.
5. It passes into the left ventricle, which pumps it through the aorta to the rest of the body.

The diagram is drawn as if you're looking at someone facing you, so the heart's left side appears on the right of the page.

Blood loses pressure as it passes through the tiny capillaries in the lungs. Returning it to the heart before sending it to the body means it can be pumped out again at high pressure, delivering oxygen and glucose to the tissues quickly. This suits animals, like mammals, that need a lot of energy from respiration.

A memory trick: Arteries go Away from the heart. The pulmonary artery is the only artery carrying deoxygenated blood, and the pulmonary vein the only vein carrying oxygenated blood.

Cardiac output is the volume of blood the heart pumps each minute:

With a stroke volume of 70 ml and a heart rate of 70 beats per minute, cardiac output is $70 \times 70 = 4900$ ml per minute, about 4.9 litres. During exercise both stroke volume and heart rate increase, so more oxygen reaches the muscles. (Cardiac output is named in some exam board specifications, so check yours.)

In coronary heart disease, layers of fatty material build up inside the coronary arteries, narrowing them and reducing the flow of oxygen to the heart muscle. Treatments include:

• Stents: tubes inserted to keep the artery open.
• Statins: drugs that reduce blood cholesterol, slowing the build-up of fatty material.
• Replacement valves (mechanical or biological) for faulty valves.
• Transplants or artificial hearts when the heart fails.

Each treatment has benefits and risks, such as surgery risks, side effects or the need for a donor.

• Saying arteries carry oxygenated blood and veins deoxygenated (the pulmonary vessels are the exceptions; define them by direction).
• Mixing up left and right on the diagram.
• Saying the left ventricle is thicker because it pumps "more blood". Both ventricles pump the same volume; the left pumps it further, at higher pressure.

1. Which blood vessel carries blood from the heart to the lungs?
2. Why does the left ventricle have a thicker wall than the right?
3. Why do veins have valves?
4. Calculate cardiac output for a stroke volume of 80 ml and a heart rate of 75 beats per minute.

Answers: 1. The pulmonary artery. 2. It pumps blood round the whole body, which needs higher pressure. 3. Blood is at low pressure in veins, and valves stop it flowing backwards. 4. $80 \times 75 = 6000$ ml (6 litres) per minute.