Blood
You have between 5 and 7 litres of blood in your body.
It carries oxygen, carbon dioxide, urea and nutrients around your body. Blood also contains hormones, and it helps to regulate the body's atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow and atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow content.
Blood is made up of 4 main components.
1. Plasma - this is a pale atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow liquid that contains hormones, nutrients and proteins.
2. Red blood cells - these contain the atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow pigment haemoglobin which carries atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow. Their special shape means they have a large surface area, and they have no nucleus - both these adaptations mean they are well suited for their job.
3. White blood cells - there are several different shapes of these. They fight against atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow.
4. Platelets - these are small fragments of cells, and they stick together to make blood atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow to stop bleeding after an injury.

Blood Vessels
1. Arteries - these carry blood atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow the heart. They have atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow muscular walls as the blood is at atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow pressure. They divide into smaller vessels called capillaries.
2. Capillaries - these spread through the tissues to take blood to all the cells. They have very atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow walls which allows plasma to leak out. Capillaries join up into veins.
3. Veins - these carry blood atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow the heart. They have atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow walls and are wider than arteries. Inside them, blood pressure is atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow and the blood flows slowly. They have special atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow which only allow the blood to flow in one direction.

Heart
The heart has four chambers. The top two are called atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow. The bottom two are called atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow. The heart has its own blood supply carried by the atriaaway fromback toclotscoronarydiseasehighlowoxygenredtemperaturethickthinthinnervalvesventricleswateryellow arteries. If this supply is blocked, a heart attack occurs.