Homeostasis and the Internal Environment:

The cells of each organ of the body exist and function in an internal fluid environment, the extracellular fluid (ECF), which remains relatively constant despite wide fluctuations in our external environment. This concept was first set forth by the French physiologist Claude Bernard in 1857. He later extended the concept stating that when the internal environment is disturbed compensatory mechanisms are set in motion to repair the disturbance. The American physiologist Walter Cannon further developed these concepts and coined the term homeostasis to describe the constancy of composition and volume of the internal environment. Maintaining the homeostatic state of the organism, with regard to both its volume and composition, is the primary function of the kidney.

Inherent in the idea of homeostasis is the concept of balance. To maintain homeostasis with regard to any constituent of the body, its input, the amount of that substance ingested or generated metabolically must be exactly balanced by its output, the amount of that substance excreted or metabolically destroyed. When input equals output the system is in balance. When input exceeds output the system is in positive balance. When output exceeds input the system is in negative balance.

Figure A illustrates the organization of the elements of a control system designed to maintain homeostasis.

Figure B illustrates the effect of a change in the temperature of the external environment. As external temperature rises body heat content increases (positive heat balance) and body temperature rises. Temperature control mechanisms (discussed in the section on Gastrointestinal Physiology) stimulate sweating resulting in increased heat loss to balance the heat gain and restore body temperature to normal. Sweating, however, incurs a loss of Na⁺ and H₂O (negative Na⁺ and H₂O balance) resulting in a decreased ECF volume and an increase in ECF Na⁺ concentration. The volume deficit is sensed as a drop in blood pressure and the increased Na⁺ concentration (due to loss of H₂0 > loss of Na⁺) is sensed as an increased ECF osmolarity. These changes lead to the release of hormones which act on the renal tubules to conserve Na⁺ and H₂O, and also stimulate thirst and salt appetite to replace the lost Na⁺ and H₂O and restore Na⁺ and H₂O balance and the ECF volume.

The mechanisms by which the kidney performs these functions will be discussed in detail in the following chapters.