The heart, blood and blood vessels · GCSE Biology

The heart and double circulation

The heart is an organ that pumps blood around the body in a double circulatory system.
The right ventricle pumps deoxygenated blood to the lungs, where it picks up oxygen. The left ventricle pumps oxygenated blood around the rest of the body.
The heart has four chambers: two atria at the top receive blood; two ventricles at the bottom pump it out. Valves stop blood flowing backwards.
The left ventricle has a thicker muscular wall because it pumps blood all round the body.
The heart muscle gets its own oxygen from the coronary arteries.
The natural resting heart rate is controlled by a group of pacemaker cells in the right atrium. An artificial pacemaker can correct an irregular heart rate.

Arteries carry blood away from the heart at high pressure. They have thick, muscular, elastic walls.
Veins carry blood back to the heart at low pressure. They have thinner walls, a wide lumen and valves to stop backflow.
Capillaries are tiny vessels with walls one cell thick, so substances diffuse easily between the blood and body cells.
Rate of blood flow = volume of blood ÷ time.

Blood is a tissue made of plasma, red blood cells, white blood cells and platelets.
Plasma carries blood cells, dissolved glucose, amino acids, carbon dioxide, urea, hormones and proteins.
Red blood cells carry oxygen using haemoglobin. They are biconcave discs with no nucleus, giving more room for haemoglobin and a large surface area.
White blood cells defend against pathogens. Platelets are cell fragments that help blood to clot.

Air passes down the trachea, into two bronchi, then into bronchioles, ending in millions of alveoli.
Oxygen diffuses from the alveoli into the blood in the capillaries, and carbon dioxide diffuses the other way.
Alveoli are adapted with a huge surface area, thin walls, a moist lining and a rich blood supply.

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.
Stents are tubes that hold arteries open. Statins are drugs that lower blood cholesterol, slowing the build-up of fatty material.
Faulty valves can be replaced with biological or mechanical valves. Heart failure may be treated with a donor heart or an artificial heart.
Each treatment has risks and side effects; for example, surgery carries a risk of infection and statins may cause side effects.

Double circulatory system: A system in which blood passes through the heart twice in each full circuit: once to the lungs and once to the body.
Atrium: An upper chamber of the heart that receives blood.
Ventricle: A lower chamber of the heart that pumps blood out.
Artery: A blood vessel with thick, muscular walls that carries blood away from the heart.
Vein: A blood vessel with thin walls and valves that carries blood back to the heart.
Capillary: A tiny blood vessel with walls one cell thick where substances are exchanged.
Coronary arteries: The arteries that supply the heart muscle with oxygenated blood.
Haemoglobin: The red protein in red blood cells that carries oxygen.
Plasma: The pale yellow liquid part of blood that carries cells and dissolved substances.
Platelets: Cell fragments that help the blood to clot.
Stent: A mesh tube put inside an artery to keep it open.
Statin: A drug that reduces the level of cholesterol in the blood.