The human circulatory system supplies the cells of the body with food and [?] and carries [?] away from the cells. It also protects the body from [?] .
The heart is situated in the centre of the [?] and it has four chambers: two upper chambers - the and two lower chambers - the .
There are three major types of blood [?]: arteries that carry blood from the , veins that return blood to the heart, capillaries - extremely [?] vessels connecting the arteries and the veins.
Blood consists chiefly of a liquid called . Floating in the plasma are: 1) the red blood cells, which contain [?] and carry oxygen and carbon dioxide throughout the body, they have no [?] and have a relatively short life of about four months. White blood cells have a nucleus, often quite large, and they are able to move around and pass through the walls of capillaries into all parts of the body. Their main function is to infection and help protect the body from disease. Platelets are small fragments of cells whose function is to enable blood to [?] and prevent the loss of from damaged vessels.