In IGCSE biology curriculum, humans have a double circulatory system. The heart pumps blood to the lungs to pick up oxygen which is done by the pulmonary circulatory system. Oxygenated blood then flows back to the heart which is then pumped around the body. This is called the systemic circulatory system.

There are three main types of blood vessels in IGCSE biology:

• Arteries - carry oxygenated blood away from the heart, except for the pulmonary artery which carries deoxygenated blood to the lungs. Arteries have thick muscular walls to smooth out the pulses of blood flow from the heart. 
• Veins - carry deoxygenated blood back to the heart from the  tissues, except the pulmonary  vein in the heart which carried oxygenated blood from the lungs to the left atrium. Veins are  large to carry blood under low pressure  back to the heart.
• Capillaries - substances diffuse in and out of the blood. They have very thin walls so that the distance for diffusion is very short and  efficient. 

Blood enters the atria of the heart. These contract to force blood into the ventricles. When the ventricles contract, blood leaves the heart to go to the lungs and around the body. The heart beats around 70 times per minute. The beating is controlled by a pacemaker region in the heart. The rate of heartbeat changes as the demands for the body change.

It is important in IGCSE biology curriculum, a high fat diet, obesity, high blood cholesterol etc are all risk factors to the heart and can lead to coronary heart disease. The coronary arteries which supply food and oxygen to the heart become narrower or blocked.

When the chambers of the heart contract and force the blood into the ateries, the heart is systole. When it relaxes and fills with blood it is diastole. the blood pressure in the arteries changes as the heart empties and fills - it is higher as the heart pumps and empties and lower when it relaxes and fills. Normal systolic blood pressure is 120 mmHg. Normal diastolic blood pressure is 80 mmHg.

In IGCSE biology curriculum, there are four components that you need to know:

• Plasma - this is a tellow liquid which transports dissolved food molecules, carbon dioxide and urea as well as all the blood cells. It is mainly water.
• Red blood cells - contain haemoglobin and transport oxygen.
• White blood cells - defend body against microbes. Lymphocytes produce antibodies to destroy microorganisms and memory lymphocytes give us immunity to specific diseases. Phagocytes engulf and digest microorganisms
• Platelets - cell fragments which help with blood clotting. Blood clotting involve several stages.

The white blood cells protect the body against pathogens. About 70% are phagocytes. Pathogens have antigens on their cell surfaces. Lymphocytes make antibodies in response to these antigens. In IGCSE biology, the antibodies stick to the antigens and destroy the pathogen in on e of several ways:

• making pathogens stick together so that phagocytes can engulf and digest them more easily
• acting as a label so that phagocytes recognise them more easily
• causing bacterial cells to burst open
• neutralising toxin produced by bacteria

Some lymphocytes form memory cells so that if the pathogen gets into your body again in can be dealt with quickly before symptoms occur.

You can be given artificial immunity to a disease by a vaccination. You are injected with a dead or weakened strain of pathogen so that your lymphocytes can produce antibodies to the antigens of the pathogen  and memory cells are ready for a secondary immune response. However, you are not at risk from the disease.

The end of part II ! (remember to read part III)

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