In the process of evolution, the first simple forms of life developed in a fluid medium or environment of a constant composition, the sea. As time progressed, organisms evolved that were able to live first in fresh water and then on dry land. These organisms were able to face a hostile and ever changing external environment because they had developed mechanisms that enabled them to bathe their cells in a constant internal environment. To state it differently, these organisms could live relatively free and independent of changes in their external environment because of the constancy of the composition of their internal environment, their extracellular fluids. Organisms acquired this physiologic freedom principally because of the development of the kidney, the organ primarily responsible for the maintenance of the internal environment.

Fig. 1-1.  A cross-section of the renal architecture.

The kidneys accomplish this vital task in the following way. From the large volume of plasma that the circulation brings to the kidney daily, the glomeruli filter a fluid almost identical in composition to plasma except for protein. This fluid then flows through the approximately 2,000,000 nephrons in the kidneys. The cells lining these nephrons reabsorb specific substances from this fluid in varying quantities and return them to the blood. These cells also extract additional substances from the blood and secrete them into the urine. As the kidneys perform their task, the process of glomerular filtration and all the myriad tubular mechanisms respond to a variety of factors to return to the circulation a fluid with the composition and volume required to maintain the constancy of the internal environment. In a day's time, the kidneys process the equivalent of the extracellular fluid volume of the body some 15 times by filtering approximately 40 gallons of fluid, reabsorbing from it the necessary amount of various substances and water, and adding other substances to urine. Less than half a gallon of fluid with a vastly different composition is finally excreted.

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