The heart and blood circulation

Every cell of the body needs a steady supply of food and oxygen, and its waste must be carried away. This transport is the job of the circulatory system, which has three parts: the blood, the blood vessels and the heart.

Blood

Blood is a red liquid that flows through the whole body. It has four parts:

• Plasma: the pale yellow liquid part, mostly water. It carries digested food, salts, hormones and waste.
• Red blood cells (RBCs): they contain a red pigment called haemoglobin, which picks up oxygen in the lungs and delivers it to the cells. Haemoglobin gives blood its colour.
• White blood cells (WBCs): the soldiers of the body. They fight germs that enter it.
• Platelets: they help the blood to clot at a cut, so that bleeding stops.

Blood vessels

• Arteries carry blood away from the heart to all parts of the body. The blood in them is under high pressure, so their walls are thick and elastic. They mostly carry blood rich in oxygen.
• Veins bring blood back to the heart. Their walls are thinner, and they have valves that stop the blood from flowing backwards. They mostly carry blood rich in carbon dioxide.
• Capillaries are extremely fine vessels that join arteries to veins. Through their thin walls, food and oxygen pass to the cells and waste passes back into the blood.

The one exception: the artery from the heart to the lungs carries blood poor in oxygen, and the veins from the lungs to the heart carry blood rich in oxygen.

The heart

The heart is a muscular pump about the size of your fist, placed in the chest slightly to the left. It beats without rest throughout life. It has four chambers. The two upper chambers are the atria (one is an atrium); the two lower ones are the ventricles. A wall separates the right side from the left, so that oxygen-rich and oxygen-poor blood do not mix. Valves between the chambers let blood flow in one direction only.

The journey of blood

1. Blood poor in oxygen returns from the body to the right atrium and passes to the right ventricle.
2. The right ventricle pumps it to the lungs, where it gives up carbon dioxide and takes in oxygen.
3. Oxygen-rich blood returns to the left atrium and passes to the left ventricle.
4. The left ventricle, the strongest chamber, pumps it to the whole body.

Because blood passes through the heart twice in one full round, this is called double circulation. The English physician William Harvey first described the circulation of blood.

Heartbeat and pulse

Each squeeze and relaxation of the heart is one heartbeat. A doctor listens to it with a stethoscope. Each beat sends a wave of blood along the arteries, which can be felt as the pulse at the wrist. A resting adult has a pulse of about 70 to 80 beats a minute; children have a faster one. It rises during exercise.

Caring for the heart

Regular exercise, a diet with plenty of fruits and vegetables and little fried food, enough sleep and staying away from tobacco keep the heart strong.