• In unicellular and tiny multicellular organisms, diffusion is sufficient to transport materials in their bodies.
• In large organisms, a mass flow transport system carries materials throughout their bodies.
• In most animals, blood is the main medium of transport. Intercellular fluid and lymph are a part of blood and play a vital role in transporting materials to individual body cells.
• In most animals, the mass flow transport system is called a circulatory system. It consists of (i) a circulating fluid, usually blood (ii) a pumping device, the heart, and (iii) a system of branches vessels connected to the heart for circulating blood.
• In an open circulatory system, the heart pumps blood into spaces in the body directly bathing the organs. Blood flow is sluggish. In more efficient closed circulatory system found in vertebrates, blood is contained in blood vessels.
• Fish have a single circulation, while other vertebrates have a more efficient double circulation, one to the lungs and the other to the rest of the body.
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