Jones, David R. Department of Zoology, University of British Columbia, Vancouver, Canada.
Last reviewed:October 2019
- Open and Closed Circulatory Systems
- Physiology of Open and Closed Systems
- Components of Circulatory Systems
- Blood vessels
- Evolution of Vertebrate Circulation
- Related Primary Literature
- Additional Reading
Those processes by which metabolic materials are transported from one region of an organism to another. Ultimately, the essential gases, nutrients, and waste products of metabolism are exchanged across cell membranes by diffusion. Diffusion is the movement of material, by random motion of molecules, from a region of high concentration to one of low concentration. The amount of material moved from one place to another depends on the difference in concentrations and on the distance between the two points. The greater the distance, the less movement of material per unit time for a given difference in concentration. Consequently, in all but the smallest animals, convection (or bulk circulation) of materials to the cell must be employed to supplement diffusion.
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