Blood is a complex liquid tissue. Its function is to supply oxygen and nutrients to the body's cells. In addition, blood transfers heat to different parts of the body, keeping our body temperature even.

The body of an average adult contains approximately five litres of blood. This blood consists of two distinct parts: a liquid part (= blood plasma, 55% of blood's total volume) and a cellular part (45% of blood's total volume).

90% of blood plasma is water, in which all other substances are dissolved. There are three types of blood cells: red blood cells, white blood cells and platelets. Red blood cells carry oxygen, white blood cells control pathogens, and platelets are involved in blood clotting.

Each person belongs to one of four blood types: O, A, B or AB. The blood type of a person must be known if a blood transfusion is needed during surgery. The wrong blood type would lead to the immune system attempting to destroy any foreign proteins on the red blood cells of the wrong blood type.

Changes in different parts of the blood can give clues about various diseases - for example, in anemia, the number of red blood cells is low.

​The composition of blood.