Biology For Dummies
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The circulatory system is the method of transport in plants and animals. Animals must have nutrients and oxygen reach every cell in their body. Plants must have nutrients and carbon dioxide reach every cell in their “body.”

Both plants and animals must also have waste products removed from their systems. The circulatory system is how these things are carried around the living organism. Animals can have one of two kinds of circulatory systems: open or closed.

  • Open circulatory system: This type of system is found in animals such as insects and some mollusks (snails, clams). Inside these animals is an open cavity called a hemocoel into which a blood-like fluid called hemolymph is pumped (hemo = blood).

    A heart does the pumping, and it has holes called ostia through which the hemolymph is pumped in and out. The hemolymph carries the oxygen and nutrients, and when it fills the hemocoel, the tissues of the organism are flooded with the fluid. No vessels holding in the fluid here. Simple, yet effective.

  • Closed circulatory system: This is the type of system you are personally familiar with. Closed systems are said to be closed because they have vessels that contain the fluid — in these animals, blood.

    In small animals, such as insects, the nutrient- and oxygen-filled fluid does not have far to travel to bathe all the tissues of the organism. But, in larger animals, the organisms are too big for diffusion to work from a fluid-filled cavity to all tissues and cells. You have a network of “highways” that perform the transportation and keep the blood from seeping out.

In animals, each blood vessel in the network is responsible for transporting nutrients and oxygen to cells and for removing wastes and carbon dioxide from cells. The blood vessels are the arteries, arterioles, capillaries, venules, and veins. Besides humans, all other vertebrates have closed circulatory systems, as do birds and some invertebrates, such as earthworms and squids.

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