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Blood is a body fluid that is set in motion (sucked in and out) by the heart and circulates through all arteries, veins and capillaries. It is composed of plasma, white blood cells (leukocytes), red blood cells (erythrocytes) and platelets (thrombocytes). Blood is one of the three main fluids in the body (the other two are the fluids around and within cells). It carries oxygen, nutrients and hormonal messengers to and from each of the body's cells. It also plays a role in the body's defense against foreign bodies. An adult has about 3,1019 blood cells. Each cubic millimeter of blood contains 4.5 to 5.5 million red blood cells and an average of 7,500 white blood cells. Blood is made up of four main components: red blood cells, white blood cells, platelets and plasma. Red and white blood cells are continuously destroyed and the body is constantly making new ones. About 2.5 million red blood cells die every second and about 2.5 million new cells are made at the same time.