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chapter42 - Chapter 42 CIRCULATION AND GAS EXCHANGE...

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Chapter 42 CIRCULATION AND GAS EXCHANGE Organisms must exchange materials and energy with its environment and this exchange ultimately occurs at the cellular level. Cells live in aqueous surroundings. The materials they need must move across the plasma membrane into the cytoplasm, and metabolic wastes must move out. CIRCULATORY SYSTEMS REFLECT PHYLOGENY Internal transport and gas exchange are functionally related in most animals. Diffusion is not enough for transporting substances over long distances (more than a few millimieters) in animals. The time it takes to diffuse a substance is a function of the square of the distance. The same amount will take 1 sec to diffuse 1μm; 100 sec for 1 mm; 3 hours for 1 cm. INVERTEBRATES In all animals, fluid between the cells, called interstitial fluid or tissue fluid, bathes the cells and provides a medium for diffusion of oxygen and nutrients. Sponges, cnidarians, ctenophorans, platyhelminthes, etc. depend on diffusion for internal transport. The gastrovascular cavity serves for both digestion and distribution of substances throughout the body. Triploblastic animals with more complex body plan use open and closed circulatory systems. Both systems have a circulatory fluid (blood), a set of tubes (blood vessels) and a pump (heart). Heart creates blood pressure that acts as the motive force to move the fluid through the set of tubes. Arthropods and mollusks have an open circulatory system . Blood flows into a hemocoel bathing the tissues directly. The hemocoel is made of spaces or sinuses that surround the organs. The hemocoel is not part of the coelom. Hemolymph : blood and interstitial fluid are indistinguishable.
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When the heart contracts, hemolymph is pushed out of the tubes into the sinuses; when the heart relaxes, hemolymph is pulled into the tubes through openings called ostia . Hemocyanin : an oxygen-transporting pigment found in some mollusks and arthropods; contains copper. Some invertebrates (e.g. cephalopods, echinoderms, annelids) and vertebrates have a closed circulatory system. Blood is confined to the vessels. Blood is different from the interstitial fluid. Chemical exchange occurs between the blood and interstitial fluid. Nemerteans have a primitive circulatory system that is closed but does not have a pumping organ. Blood moves depending on the movements of the animal and contractions in the wall of the large blood vessels. VERTEBRATES Functions of the vertebrate circulatory system: 1. Transports oxygen, metabolic wastes, nutrients and hormones. 2. Helps maintain fluid balance. 3. Defends the body against invading microorganisms. 4. Distributes metabolic heat to maintain normal body temperature. 5.
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chapter42 - Chapter 42 CIRCULATION AND GAS EXCHANGE...

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