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L13 Int Epithelial Transport 2011

L13 Int Epithelial Transport 2011 - COPYRIGHT Mammalian...

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1/11 COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 Spring 2011 EPITHELIA: POLARITY AND VECTORIAL TRANSPORT 1) Excitable and epithelial cells. In general, one can distinguish between two broad classes of eukaryotic cells: excitable cells and epithelial cells. Nerve and muscle are examples of excitable cells, capable of generating and conducting action potentials. Epithelial cells lining the surfaces of multicellular animals are not excitable; they do not generate and they do not conduct action potentials (Fig. 1). However, non-excitable cells are not always epithelial cells as illustrated by erythrocytes, lymphocytes, endothelial cells of the vasculature, the cells of bone (osteoblasts, osteoclasts) and other organs such as the liver. Little appreciated is the fact that the number of epithelial cells in an animal equals or exceeds the number of excitable cells. The comparison should signal the importance of epithelial functions and, really, the continuity between the animal and its environment with “Every breath you take...” according to the “Police”. Fig. 1. Design of a multicellular animal from the perspective of excitable and epithelial cells. Excitable cells are bathed in an extracellular fluid compartment (the internal pond of Claude Bernard: plasma and interstitial fluid) that is maintained constant in composition and time by epithelial cells lining the surface of the body. In particular, the functional activities of epithelial cells are regulated by nerves and hormones. The regulation of the extracellular fluid compartment maintains the “private pond” constant in the support of excitable and other cells (homeostasis). 2) General functions of epithelia. Located at the surface of the body, epithelia are in the unique position to mediate three fundamental functions: 1. Barrier functions, 2. passive transport functions, and 3. active transport functions.
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2/11 a) Epithelial barrier. Like cell membranes, which are barriers that separate intracellular and extracellular environments, epithelial cells form barriers that separate internal and external environments of the animal. The barrier function of epithelia is best illustrated by the skin that prevents the physical leak of extracellular fluid, reduces water evaporation, and opposes equilibration with the external environment. b) Epithelial mediators of passive transport (diffusion and osmoses). Gills and lungs are the best examples of epithelia providing pathways for passive transport. No active transport is involved in the diffusion of gases across respiratory surfaces. Epithelial mediation of gas transport is passive, down concentration differences that have been generated by metabolism, i.e. the decrease of the [O 2 ] and the increase of [CO 2 ] in the blood.
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L13 Int Epithelial Transport 2011 - COPYRIGHT Mammalian...

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