Kidneys and Renal Physiology
Among the many physiological functions of the kidney—including those that are subject to feedback control—one is preeminent and omnipresent from birth to death. This is the need to maintain the homeostasis of body water and body electrolytes. Except for short-lived maladjustments, water and electrolyte balance among intracellular, extracellular, and intravascular spaces must be maintained twenty-four hours a day throughout a lifetime. Challenges to such a balancing act are presented by the daily cycles of hydration and dehydration we all experience. Consider the states of sleep and wakefulness. During the typical six to eight hours of sleep average adults get, we are steadily losing body water. This happens, in part, each time we exhale and lose water vapor to the atmosphere. It also occurs continuously with evaporation of water through the skin. Thus, during our sleep we lose a certain quantity of water but do not replace it. The net result is dehydration.
Normally from the moment we awake until about lunch time, we are consciously or unconsciously rehydrating. This is driven, in part, by eating breakfast, drinking multiple cups of java, and, earlier, by physiological feedback mechanisms promoting thirst: most notably a dry mouth and the afferent sensory signals it generates. During the sleeping hours, we steadily dehydrate and the homeostasis of water and electrolyte balance among the three compartments gets disturbed. Sensory signals from multiple locations throughout the body apprise the kidneys of these changes. The kidneys react via reflex mechanisms by conserving water. Water conservation during sleep takes place primarily at the distal tubules and collecting ducts of the renal nephrons. These tubular structures have specialized cells designed to transport water from the tubular fluid into the blood. In the membranes of such cells are proteins called aquaporins that form water channels. The number of such channels, that is, the production, release, and insertion of aquaporin proteins, is under physiological