CH42 - Gas Exchange Circulation Chapter 42 Gas Exchange...

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Gas Exchange & Circulation Chapter 42 Gas Exchange & Circulation • Every organism must exchange materials and energy with its environment, and this exchange ultimately occurs at the cellular level. • Cells live in aqueous environments. • The resources that they need, such as nutrients and oxygen, move across the plasma • membrane to the cytoplasm. • Metabolic wastes, such as carbon dioxide, move out of the cell. Blood or Interstitial Fluid Most animals have organ systems specialized for exchanging materials with the environment, and many have an internal transport system that conveys fluid throughout the body. • Aquatic Organisms • Structures like gills present an expansive surface area to the outside environment. Oxygen dissolved in the surrounding water diffuses across the thin epithelium covering the gills and into a network of tiny blood vessels ( capillaries ). At the same time, carbon dioxide diffuses out into the water. Diffusion is Not Good Enough Diffusion alone is inadequate for transporting substances over long distances in animals • Diffusion is insufficient over distances of more than a few millimeters, because the time it takes for a substance to diffuse to one place to another is proportional to the square of the distance . • For example… • If it takes 1 sec for a given quantity of glucose to diffuse 100 microns • it will take 100 seconds for it to diffuse 1 mm • It will take almost 3 hours to diffuse 1 cm The circulatory system solves this problem by ensuring that no substance must diffuse very far to enter or leave a cell. Simple Internal Transport Systems Aquatic Invertebrates Gastrovascular Cavity (GVC) found in Cnidarians (Jellies, Hydra, Anemones) Planarians & other flatworms Circulatory Systems found in Mollusks Crustaceans Insects
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Gastrovascular Cavities • The body plan of a hydra and other cnidarians makes a circulatory system unnecessary. • A body wall only two cells thick encloses a central gastrovascular cavity that serves for both digestion and for diffusion of substances throughout the body. • The fluid inside the cavity is continuous with the water outside through a single opening, the mouth. • Thus, both the inner and outer tissue layers are bathed in fluid. • Planarians and most other flatworms also have gastrovascular cavities that exchange materials with the environment through a single opening. • The flat shape of the body and the branching of the gastrovascular cavity throughout the animal ensure that are cells are bathed by a suitable medium and diffusion distances are short. Circulatory Systems Large Animals • For animals with many cell layers, gastrovascular cavities are insufficient for internal distances because the diffusion transports are too great. • Two types of circulatory systems that overcome the limitations of diffusion
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CH42 - Gas Exchange Circulation Chapter 42 Gas Exchange...

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