Summary

Blood contains particles of many different sizes, shapes, and polarity. Some of these particles (e.g., proteins) are essential for the body; some (e.g., urea) must be removed from the blood and the body; others (e.g., many ions) must be maintained at certain concentrations. The body has many different methods for controlling the composition of the blood (including hemoglobin, ferritin, and buffers, which are discussed in other tutorials), but the kidneys have the primary responsibility for controlling the blood's chemical composition.

By filtration, reabsorption, and secretion mechanisms, the kidneys separated and regulate the components of the blood. Some of these components (e.g., proteins) are filtered from the fluid entering the tubule at the glomerulus, and remain in the blood. Other particles (e.g., water, ions, and sugar) are reabsorbed by the blood or secreted from the blood to maintain the proper concentrations; these processes occur while the fluid is flowing through the tubule. Any blood components that remain in the nephron when the fluid reaches the collecting duct (e.g., waste products such as urea) are excreted from the body.

The reabsorption and secretion of the blood components depend on the ability of these blood components to cross the nonpolar interior of the membrane surrounding the nephron tubule. The polar blood components can only pass through the membrane via special protein channels. These protein channels in the membrane are polar on the inside, to allow the passage of polar or charged particles through the membrane, and nonpolar on the outside, to interact with the nonpolar membrane interior. The size of the channel can determine which polar or charged particles will be able to cross the membrane through the channel.

The concentrations of the blood components are maintained by diffusion through the membrane (via the protein channels if the component is polar) and concentration gradients. Depending on the component, the concentration is maintained by passive diffusion (going "down" (with) the gradient), or by pumping (going "against" the gradient). Hence, the ability of the kidneys to remove harmful particles from the blood, and to regulate the concentration of other particles in the blood, depends on the chemical concepts of diffusion, polarity, and concentration gradients.

References:

Doyle, D.A. et al. "Potassium Channel (KCSA) From Streptomyces Lividans," (1998) Science, 280, 69. Potassium channel PDB coordinates, Brookhaven Protein Data Bank.

Guex, N. and Peitsch, M.C. Electrophoresis, 1997, 18, 2714-2723. (SwissPDB Viewer) URL: http://www.expasy.ch/spdbv/mainpage.htm.

Insight II graphical program; Molecular Simulations, Inc. URL: http://www.msi.com.

Persistence of Vision Ray Tracer (POV-Ray). URL: http://www.povray.org.

Vander, A. et al. Human Physiology, 7th ed. WCB McGraw-Hill, Boston, 1998, p. 503-533, 547-548.

Stryer, L. Biochemistry, 4th ed. W.H. Freeman and Co., New York, 1995, p. 261-278, 299-301.

Acknowledgements:

The authors thank Dewey Holten, Michelle Gilbertson, Jody Proctor and Carolyn Herman for many helpful suggestions in the writing of this tutorial.

The development of this tutorial was supported by a grant from the Howard Hughes Medical Institute, through the Undergraduate Biological Sciences Education program, Grant HHMI# 71199-502008 to Washington University.