Many small organisms rely upon the process of diffusion for the exchange of materials and gases across their body surface. In larger animals, this process is not sufficient to meet demand, so many have evolved a circulatory system, which consists of blood, blood vessels and a heart. Blood is composed of cells and plasma, and is used to transport metabolites, waste products, gases, hormones, and other materials around the body. Most of the cells in the blood are erythrocytes; they are also called…
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