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CHAPTER 42 - CIRCULATION AND GAS EXCHANGE Introduction 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. Most animals have organ systems specialized for exchanging materials with the environment, and many have an internal transport system that conveys fluid (blood or interstitial fluid) throughout the body. For 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. A. Circulation in Animals 1. Transport systems functionally connect the organs of exchange with the body cells: an overview Diffusion alone is not adequate for transporting substances over long distances in animals—for example, for moving glucose from the digestive tract and oxygen from the lungs to the brain of mammal. Diffusion is insufficient over distances of more than a few millimeters, because the time it takes for a substance to diffuse from one place to another is proportional to the square of the distance. For example, if it takes 1 second for a given quantity of glucose to diffuse 100 microns, it will take 100 seconds for it to diffuse 1 mm and almost three 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. The bulk transport of fluids throughout the body functionally connects the aqueous environment of the body cells to the organs that exchange gases, absorb nutrients, and dispose of wastes. For example, in the mammalian lung, oxygen from inhaled air diffuses across a thin epithelium and into the blood, while carbon dioxide diffuses out. Bulk fluid movement in the circulatory system, powered by the heart, quickly carries the oxygen-rich blood to all parts of the body. As the blood streams through the tissues within microscopic vessels called capillaries, chemicals are transported between blood and the interstitial fluid that bathes the cells. 2. Most invertebrates have a gastrovascular cavity or a circulatory system for internal transport 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.
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This note was uploaded on 12/14/2009 for the course BIOCHEM bIO taught by Professor Professor during the Spring '09 term at École Normale Supérieure.

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