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Circulation in Protozoans
Protozoans are one of the first animals and are invertebrates. They are heterotrophic and consume other cells or food particles. Gas and nutrient movement occurs by simple diffusion within the cell.
Circulation in Cnidarians
Cnidarians (includes hydras) are invertebrates that have a body composed of two layers of cells. all cells are in contact with either the internal or external environment, making a specialized circulatory system unnecessary.
Circulation in Arthropods
Arthropods are invertebrates that have an open circulatory system where blood and interstitial fluid (hemolymph) is in direct contact with body tissue in the hemocoel's open cavity. Circulation is dependent on body and/or heart movement. Exchange occurs when the fluid flows through the dorsal vessel (tubular heart) and then enters sinuses (open ended vessels).
Circulation of Annelids
Annelids are invertebrates and use a closed circulatory system to deliver nutrients to cells not directly in contact with the environment. The system is composed of a series of closed vessels
Circulation in Humans
Blood begins by being pumped out of the aorta, then branches into arteries, then arterioles and finally capillaries. In the capillaries, gas and nutrient exchange occurs and waste products are removed between blood an cells. The blood, now deoxygenated flows out into venules then veins and back to the heart.
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