BSC-2010 Lecture Notes - Lecture 42: Extracellular Fluid, Arteriole, Pulmonary Circulation
Document Summary
Every organism must exchange materials with its environment. Exchanges ultimately occur at the cellular level by crossing the plasma membrane. In unicellular organisms, these exchanges occur directly with the environment. For most cells making up multicellular organisms, direct exchange with the environment is not possible. Gills are an example of a specialized exchange system in animals. O2 diffuses from the water into blood vessels. Co2 diffuses from blood into the water. Internal transport and gas exchange are functionally related in most animals. Circulatory systems link exchange surfaces with cells throughout the body. Diffusion is only efficient over small distances. In small and/or thin animals, cells can exchange materials directly with the surrounding medium. In most animals, cells exchange materials with the environment via a fluid-filled circulatory system. Some cnidarians, such as jellies, have elaborate gastrovascular cavities. A gastrovascular cavity functions in both digestion and distribution of substances throughout the body.