Lecture 4: Circulatory Systems 1. All organisms need to exchange materials with the environment Gas exchange occurs by diffusion and this is only rapid, and efficient. ..- over short distances (1-2 cells)- with large surface areas for exchange ln larger, multicellular animals, circulatory systems link exchange surfaces with cells throughout the body o Circulatory systems minimise the distance substances must diffuse to enter/leave cell. e.g. Oz from inhaled air diffuses across only 2 cell layers in lungs o At the tissues Oz aBain diffuses only a short distance from blood in capillaries to cells 2. There are 3 basic architectures for systems of internal transport A. Gastrovaseularsystems Examples: hydra, cnidarians, flatworms . 2 layered body wall of cells, each exposed to aqueous solutions . lnternal layer doubles as a digestive (gastro-) and exchange (-vascular) surface. . Circulation in gastrovascular cavity is strictly through body movements B. Open drculatory systcms ' . Circulatory fluid
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