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94 6 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 Kidneys and Renal Physiology control. Peter Agre of Johns Hopkins University shared the Nobel Prize in Chemistry in  for his discovery of water channels. As the body dehydrates, the osmolality of the blood gradually increases. The increased osmolality is sensed by osmoreceptors in the hypothalamus of the brain. The osmoreceptors cause the production and release of a hormone called antidiuretic hormone (ADH), or arginine vasopressin (AVP), from the posterior portion of the pituitary gland. Once in the systemic circulation, this hormone stimulates distal tubules and collecting ducts in the kidneys to increase the production of aquaporins. Then the kidneys are able to reabsorb more water, thereby excreting less water in the urine. This compensates to a limited extent for the dehydration that increased osmolality. This mechanistic response to imbalances in water and electrolyte distribution suffices until we awake and begin to rehydrate. Interestingly, we quite often overhydrate in the first several hours of wakefulness . The evidence of this is the number of trips we make to the bathroom between late morning and early afternoon, the progressively lighter hue of our urine as that time period progresses, and the reduced desire to drink fluids (thirst satiety). I will have more to say about this daily cycle later in the chapter. For now, once we have overhydrated, the opposite feedback to the one described above takes over to compensate for the dilution of osmolality in the body water compartments. Functional Morphology of the Kidneys As mentioned in chapter , the kidneys are among the best examples in humans of the relevance of structure to function. The kidneys are bilateral bean-shaped structures located at the back and outside of the abdominal cavity. The peritoneum is the inner epithelial and connective tissue lining of the abdominal cavity. Abdominal structures like the stomach, liver, and intestines rub against or come in contact with the peritoneum. The kidneys, on the other hand, are located on the outside of the peritoneum and therefore, technically speaking, are not abdominal organs. There are two of them, and they are found dorsally just below the lower ribcage. They are the approximate size of the individual’s loosely clenched fist, and in the average kg adult, they weigh about  grams each. Each kidney is encapsulated in a rigid sheath made of connective tissue and called the renal capsule. The renal hilus is the point of entry and exit of the ureter, blood vessels, and renal nerves. The top of the kidney where the adrenal gland is attached is called the upper or north pole, and the bottom is the lower or south pole...

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