Blood

Blood is the delivery and waste collection system of your body. It also protects you against invading pathogens. It is pumped round the body by the heart at high pressure through the arteries and returns at lower pressure via the veins. Between the two, blood flows through capillaries which allow it to reach every single cell in your body.

Blood consists of a fluid called plasma in which red blood cells, white blood cells and platelets are suspended. Blood plasma also transports dissolved chemicals; carbon dioxide from the organs to the lungs, hormones, soluble products of digestion from the small intestine and urea from the liver to the kidneys.

Red blood cells are extremely small so that they can pass through the capillaries. The contain an amazing protein called hemoglobin. This can pick up oxygen where the concentration is high (in the lungs) and release it where the concentration is low (as it passes cells). Oxygen is important for aerobic respiration. Red blood cells are the only cells in your body not to have a nucleus.

The platelets are there to help your blood to clot when you cut yourself. They combine with substances in the plasma called clotting factors to make the blood much thicker and stickier. There are some people (heamophiliacs) who don't have these clotting factors and it is possible that they could bleed to death from even a minor cut.

The white blood cells are your body's first line of defense. There are hundreds of different types, each with a specific job to do. Some engulf the invading cells and slowly digest them, dying in the process and forming pus at the site of the infection. Others produce antibodies that either destroy or render the pathogen inactive and some produce antitoxins. They are triggered by specific antigens which are protein molecules on the surface of the pathogens. When an antigen is detected, the matching type of white blood cell rapidly multiplies to deal with the pathogen. Your immune system contains specialized cells that learn and remember how to deal with the different antigens, which is the reason why vaccination works.