Fluid pumped by the heart, that circulates in the arteries, veins, and capillaries of vertebrate animals forming the bloodstream. The term also refers to the corresponding fluid in those invertebrates that possess a closed circulatory system. Blood carries nutrients and oxygen to each body cell and removes waste products such as carbon dioxide. It is also important in the immune response and, in many animals, in the distribution of heat throughout the body.

The adult human body contains about 5.5 l/10 pt of blood (about 5% of the body weight). It is composed of a fluid called plasma, in which are suspended microscopic cells of three main varieties:

Red blood cells (erythrocytes) form nearly half the volume of the blood, with about 6 million red blood cells in every millilitre of an adult's blood. They transport oxygen around the body. Oxygen is absorbed into the millions of blood capillaries surrounding the tiny air sacs of the lungs and is carried in the blood by haemoglobin, a red protein within the red blood cells.

White blood cells (leucocytes) are of various kinds. Some (phagocytes) ingest invading bacteria and so protect the body from disease; these also help to repair injured tissues. Others (lymphocytes) produce antibodies, which help provide immunity by binding to disease-causing bacteria and destroying them. Yet others can kill cells infected by viruses.

Blood platelets (thrombocytes) assist in the clotting of blood.