Blood is the liquid pumped by the heart throughout the body. The cells are produced in the bone marrow.

The chief function of blood is to bring oxygen and nutrients to tissues in the body, and to carry away carbon-dioxide, lactic acid and other waste material. It also transports hormones and other substances between organs and body tissues.

Blood is made up of the following main components:

• Plasma - This is the fluid of blood. It is mostly water, with about 4% proteins including albumin, as well as antibodies, hormones and blood clotting factors.
• Red blood cells - These make up most of the blood cells. They contain a red protein pigment called hemoglobin which combines with oxygen so it can be carried around the body. They also contain the proteins which determine blood types.
• White blood cells - These are part of the immune system, and defend the body against infection.
• Platelets - These are sacks of blood clotting chemicals.

• Our red blood cells do not have a nucleus or organelles, which all cells have. So, according to the definition, they are actually not really cells!
• Most medical terms related to blood begin with hemo- or hemato- (or haemo- and haemato-) which comes from the Greek word haima which means blood.
• Blood makes up about 7% of our body weight, and over half of that 7% is plasma.

• Our blood type (or blood group) is determined by substances on the surface of our red blood cells.