Blood is a bodily fluid found in most higher animals. It is a liquid that serves to diffuse the oxygen necessary to life throughout the body, and to remove waste products.
In vertebrates, blood is red. It owes its colour to hemoglobin, a respiratory pigment containing iron, to which oxygen binds. Hemoglobin is the most efficient respiratory pigment found in nature.
Other respiratory pigments with iron are myoglobin (red), chlorocruorin (green) and haemerythrin (violet). Some respiratory pigments use other elements. Haemocyanin (blue) uses copper. Sea squirts (among others marine life) use a vanadium chromagen (bright green, blue, or orange) for its respiratory pigment.
The hemoglobin is carried in specialized red blood cells, also called erythrocytes. Other blood cells include leukocytes for immune and infection-fighting action, and platelets for clotting.
Plasma (biology) is the protein-rich cell-free fluid that carries blood cells. It contains a variety of immune substances and chemical messengers (i.e., cytokines).
Blood is circulated by the muscular pump of the heart. It passes to the lungs (or in fish, the gills) to be oxygenated, and then is circulated throughout the body by the arteries. It diffuses its oxygen by passing through tiny blood vessels called capillaries. In its deoxygenated state, its bright red colour fades. It then returns to the heart through the veins.
Blood also removes poisons and waste products to the liver, the kidneys, and the intestines to allow them to be rejected from the body in urine and faeces.