Blood transports materials around the body and protects against disease. It consists of cells, solutes and liquid.

Blood is pumped away from the heart at high pressure in arteries and returns to the heart at low pressure in veins.

Heart problems include narrow rigid arteries, leaking heart valves and a faulty pacemaker.

Blood

Blood is used to transport materials around the body, and to protect against disease.

It's a mixture of cells, solutes and liquid:

Blood cells

• Red blood cells transport oxygen.
• White blood cells protect against disease.
• Blood platelets help the blood to clot.

Red blood cells

These carry oxygen from the lungs to tissues. Oxygen transport is efficient because:

• there are huge numbers of red blood cells
• the cells are tiny to allow them to pass through narrow capillaries
• the cells have a flattened disc shape to increase surface area allowing rapid diffusion of oxygen
• they contain haemoglobin which absorbs oxygen in the lungs and releases oxygen in the rest of the body

Higher tier only

Red blood cells have a large surface area to volume ratio to allow rapid diffusionof oxygen. Haemoglobin absorbs oxygen to form bright red oxyhaemoglobin in oxygen rich environments. In oxygen deficient environments this processes is reversed.

White blood cells

White blood cells can engulf bacteria and other pathogens by phagocytosis. They can change shape easily and produce enzymes that digest the pathogens.

Blood plasma

Plasma is a straw-coloured liquid. It transports dissolved substances around the body, including:

• hormones
• antibodies
• nutrients, such as water, glucose, amino acids, minerals and vitamins
• waste substances, such as carbon dioxide and urea

Blood is pumped at high pressure away from the heart in arteries. It travels through networks of thin capillaries, where it can exchange materials with the tissues. It's then collected up and returned to the heart at low pressure in veins.

Artery carry blood away from heart at high pressure thick, elastic, muscular walls to withstand pressure and to exert force (pulse) Capillary allow exchange of materials between blood and tissues thin permeable walls Vein return low pressure blood to heart Large diameter to offer least flow resistance. Valves to prevent back flow.

The heart is a pump that sends some blood to the lungs and some blood to the rest of the body each time it beats. The blood on the left side is kept separatefrom the blood on the right side. This is called double circulation and is a more efficient way of delivering oxygen to the tissues than single circulation.

Blood enters the heart through a vein and collects in an atrium. The atrium is emptied into a ventricle which contracts to put the blood under pressure. The blood is forced out through an artery as a valve prevents it flowing back to the atrium. The artery also contains a valve to stop blood flowing back to the ventricle when the ventricle relaxes.

Deoxygenated blood

Deoxygenated blood passes through these blood vessels, valves and parts of the heart:

1. vena cava
2. right atrium
3. tricuspid
4. right ventricle
5. semilunar
6. pulmonary artery
7. lungs

Oxygenated blood

Oxygenated blood passes through these blood vessels, valves and parts of the heart:

1. pulmonary vein
2. left atrium
3. bicuspid
4. left ventricle
5. semilunar
6. aorta
7. body

The left ventricle exerts more pressure than the right ventricle, and so it has a thicker more muscular wall. The atria (plural of atrium) exert less pressure than the ventricles so they have a thinner muscular wall.

Narrow rigid arteries need more pressure to pump blood through them. Increased blood pressure can lead to heart problems over time. Cholesterol contributes to the narrowing of arteries, so a change in diet can lead to a healthier heart.

Leaking heart valves make blood circulation inefficient. They can be replaced in open heart surgery by transplanted valves from a donor, or mechanical valves.

A faulty pacemaker causes irregular beating of the heart which in turn causes blood circulation to be inefficient. Artificial pacemakers powered by a battery can be fitted without needing open heart surgery. The problems of fitting mechanical or electrical heart components include:

• rejection by the immune system
• finding a way of reducing the size of the components to fit inside the body
• providing a power supply for pacemakers

In some cases a heart transplant may be needed. It's difficult to find suitable donors with healthy hearts that match the tissue type of the patient that needs them. People with heart transplants need to take drugs to stop their immune system from rejecting the heart for the rest of their lives. This can lead to greater risk from infections.