The Blood
Blood is a tissue. It consists of plasma, which is a fluid that contains red blood cells, white blood cells and platelets.
Plasma transports:
Carbon dioxide from: the organs to: the lungs
Soluble products of digestion from: the small intestine to: other organs
Urea from: the liver to: the kidneys
Red blood cells transport:
Oxygen from: the lungs to: the organs
Red blood cells have no nucleus, which allows them to contain a larger volume. They contain haemoglobin, which is a red pigment. Haemoglobin combines with oxygen in the lungs to form oxyhaemoglobin. In the other organs (to which it is transported in the bloodstream), the oxyhaemoglobin splits up again into haemoglobin and oxygen.
White blood cells do have a nucleus. Their function is to help to protect the body against micro-organisms that pose a threat e.g. infection (these are called pathogens). They do this by surrounding and ingesting them
Platelets do not have a nucleus. They are little fragments of cells and their function is to help the blood to clot. This is important because if the skin is broken, the blood needs to clot in order to prevent the body from losing too much blood.