Blood

Blood is a specialized bodily liquid that is composed of a liquid called blood plasma and blood cells balanced within the plasma. The blood cells near in blood are red blood cells also called RBCs or erythrocytes, white blood cells with both leukocytes and lymphocytes and platelets also called thrombocytes. Plasma is mostly water containing dissolved proteins, salts and many other substances; and makes up about 55% of blood by volume. Mammals have red blood, which is bright red when oxygenated, due to hemoglobin. Some animals, such as the horseshoe crab use hemocyanin to carry oxygen, instead of hemoglobin.

By far the most plentiful cells in blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates carrying of oxygen by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is almost entirely transported extracellularly dissolved in plasma as bicarbonate ion. White blood cells help to stand firm infections and parasites, and platelets are important in the clotting of blood.

Blood is circulated around the body through blood vessels by the pumping action of the heart. Arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.

Medical terms related to blood often begin with hemo- or hemato- from the Greek word "αἷμα" for "blood." Anatomically and histologically, blood is considered a specific form of connective hankie, given its origin in the bones and the incidence of potential molecular fibers in the form of fibrinogen.