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The Clinical Physiology of Water Metabolism-Part I: the Physiologic Regulation a Leicalprogress of Arginine Vasopressin Secretion and Thirst (Medical ______Progress)

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The Clinical Physiology of Water Metabolism-Part I: the Physiologic Regulation a Leicalprogress of Arginine Vasopressin Secretion and Thirst (Medical ______Progress) Refer to: Weitzman RE, Kleeman CR: The clinical physiology of water metabolism-Part I: The physiologic regulation A leicalProgress of arginine vasopressin secretion and thirst (Medical _______________________ Progress). West J Med 131:373-40 Nov 1979 The Clinical Physiology of Water Metabolism Part 1: The Physiologic Regulation of Arginine Vasopressin Secretion and Thirst RICHARD E. WEITZMAN, MD, Torrance, California, and CHARLES R. KLEEMAN, MD, Los Angeles Water balance is tightly regulated within a tolerance of less than 1 percent by a physiologic control system located in the hypothalamus. Body water homeostasis is achieved by balancing renal and nonrenal water losses with appropriate water intake. The major stimulus to thirst is increased osmolality of body fluids as perceived by osmoreceptors in the anteroventral hypothal- amus. Hypovolemia also has an important effect on thirst which is mediated by arterial baroreceptors and by the renin-angiotensin system. Renal water loss is determined by the circulating level of the antidiuretic hormone, arginine vas- opressin (AVP). AVP is synthesized in specialized neurosecretory cells located in the supraoptic and paraventricular nuclei in the hypothalamus and is trans- ported in neurosecretory granules down elongated axons to the posterior pituitary. Depolarization of the neurosecretory neurons results in the exocy- tosis of the granules and the release of AVP and its carrier protein (neurophy- sin) into the circulation. AVP is secreted in response to a wide variety of stimuli. Change in body fluid osmolality is the most potent factor affecting AVP secretion, but hypovolemia, the renin-angiotensin system, hypoxia, hypercap- nia, hyperthermia and pain also have important effects. Many drugs have been shown to stimulate the release of AVP as well. Small changes in plasma AVP concentration of from 0.5 to 4 AU per ml have major effects on urine osmo- lality and renal water handling. Dr. Weitzman is Assistant Professor of Medicine, UCLA School of Medicine; Division of Nephrology and Hypertension, Harbor-UCLA Medical Center, Torrance, CA; and Dr. Kleeman is Factor Family Foundation Professor of Nephrology and Medicine, Emeritus Chief, Division of Nephrology, Department of Medicine, Director, Center for Health Enhancement, Education and Research, UCLA Center for the Health Sciences, Los Angeles. Part I of a three-part article. Parts II and III will be published in subsequent issues. Supported by a grant from the Greater Los Angeles Affiliate of the American Heart Association, and the James Fleming Fund. Reprint requests to: Charles R. Kleeman, MD, Director, Center for Health Enhancement, Education and Research, Department of Medicine, UCLA Center for the Health Sciences, Los Angeles, CA 90024. THE WESTERN JOURNAL OF MEDICINE 373 CLINICAL PHYSIOLOGY OF WATER METABOLISM-PART I General Concepts of Water Metabolism ABBREVIATIONS USED IN TEXT IN NORMAL HUMANS, osmolality, or the concen- AVP= arginine vasopressin tration of osmotically active solutes in body CSF= cerebrospinal fluid fluids, is remarkably constant despite large vari- U/P=urinary to plasma (ratio) ations in water and solute intake and excretion. Every kilogram of body water contains 285 to 290 mOsm of solute, consisting primarily of salts intestinal tract. For the replacement of this extra- of sodium in extracellular fluid and of potassium renal loss, a person must rely on adequate water in intracellular fluid. The identical osmolality of ingestion. Humans and other mammals vary con- intracellular and extracellular fluids is produced siderably in their need for an exogenous source by free movement of water across all cellular and of water in their diet during maximal renal con- subcellular membranes, governed only by the servation. Of all mammals, only certain desert physical forces of osmosis and diffusion. A gain rodents can minimize their renal and extrarenal or loss of free water will be shared by all major water losses sufficiently to maintain their water body compartments (vascular, interstitial and in- balance from that available in the preformed and tracellular) in proportion to their relative sizes. oxidative water found in the desert plants they The single exception to this free movement is the ingest. These rodents can concentrate their urine control of water permeability of the distal portion to a level four to five times greater than that of of the mammalian nephron by arginine vasopres- humans, and by living in underground burrows sin (AvP), antidiuretic hormone. Among all ver- tebrates, only mammals (and to some extent, TABLE 1.-Body Water Compartments Throughout birds) possess the unique ability to excrete a Life Span* hyperosmotic urine and thus conserve free water Extra- Intra- Total when the supply of available water is limited. It Body cellular cellular Body Weight Water Water Water has been suggested that this ability of the mam- Age Sex (Kg) (Percent) (Percent) (Percent) malian to concentrate fluids kidney may have 0-11 days ... 41.6 34.8 76.4 played an important role in our ultimate domi- 11-180 days .. 34.9 37.9 72.8 over nation the dinosaurs and other reptiles at 1/2-2 years ... 27.5 34.7 62.2 the end of the Mesozoic era. There is a continu- 2-7 years ... 25.6 36.9 62.5 ous decrease in the proportion of total body water 7-14 years .. 17.5 46.7 64.2 (and particularly the extracellular compartment) 23-54 years .. a 72.5 23.4 30.9 54.3 with growth and aging, as well as a difference + 0.64 ± 0.89 1.39t between men and women (Table 1). 23-51 years .. 9 59.3 22.7 25.9 48.6 + 0.54 ± 0.96 1.47 The day-to-day fluctuations in total water body 71-84 years . a 68.1 25.4 25.4 50.8 in a normal person are very small, amounting to + 1.36 ± 0.58 1.55 approximately 0.2 percent of body weight per 24 61-74 years . 9 63.9 21.4 22.4 43.4 hours. Although infants have a relative excess of ± 0.45 ± 0.97 1.32 body water and extracellular volume as related *Modified from Parker HV, Olesen KH, McMurrey J, et al: In Ciba Foundation, Colloquia on Ageing, Vol 4. Boston, Little, to total body weight, the surface area, oxygen Brown and Company, 1958. consumption, cardiac output, insensible water tStandard error of the mean. loss, renal water excretion and overall metabolism are all high in relation to total water. There- TABLE 2.-Average Daily Intake and Output of Water body in a Normal Adult Human* fore, when one compares these fundamental met- abolic measurements with total body water, in- Water Intake (ml) Water Output (ml) fants and young children are seen to be more Oblig- Facul- Oblig- Facul- Source atory vulnerable to water deficit and dehydration than tative Source atory tative adults.' Drink ..... 650 1,000 Urine ..... 700 1,000 Preformed 750 .... Skin ...... 500 The average normal intake and loss of water in Oxidative . 350 .... Lungs ..... 400 a day by an adult human is given in Table 2. Feces ..... 150 Even with maximal renal conservation of water, Subtotals . 1,750 1,000 Subtotals 1,750 1,000 Total .... .... 2,750 Total .... 2,750 * ... the body is unable to prevent a continuous loss of *From Wolf AV: Thirst. Springfield, IL, Charles C Thomas, insensible fluid through the skin, lungs and gastro- 1958. 374 NOVEMBER 1979 * 131 * 5 CLINICAL PHYSIOLOGY OF WATER METABOLISM-PART I they reduce to a minimum their insensible pulmo- is surprising that pathologic processes do not nary and cutaneous water loss.2 Humans, in con- produce simultaneous disturbances in thirst reg- trast, even when forming maximally concentrated ulation and AVP release more frequently. Physi- urine, require some exogenous source of free ologically, these centers must be carefully inte- water to replace continued insensible and minimal grated in the maintenance of normal total body renal losses of water. water. It is not surprising, therefore, that the The day-to-day regulation of body water is major stimuli for thirst also cause the release of dependent, therefore, on the kidneys and on a AVP and the renal conservation of water-the receptor system for signaling the relative need for water repletion reaction. Those stimuli which in- water ingestion. This signal, thirst, plays an es- sential role in the regulation of the volume and TABLE 3.-Known Stimuli and Inhibitors of Thirst tonicity of the body fluids. The sensation of thirst and the Release of Arginine Vasopressin is dependent on excitation of the cortical centers of consciousness. Although the sensation of thirst Stimuli may be modified somewhat by stimuli arising in Osmotic Contracted intracellular volume of osmoreceptor neu- the oropharynx or gastrointestinal tract, it is rons due to hyperosmolality of extracellular fluids doubtful that in humans these local stimuli are (water loss or infusions of hypertonic solutions of of great importance.3 molecules that do not freely permeate the blood-brain barrier, such as sodium chloride or mannitol) Hypothalamic Thirst Centers Nonosmotic * Decreased arterial pressure (? pulse pressure) in Although thirst is a cortical or conscious sen- carotid and, possibly, aortic baroreceptors, resulting sation, it has been clearly established that there from vascular or extracellular volume contraction or exist in the hypothalamus specific nuclear centers fall in cardiac output * Decrease in tension in the left atrial wall and great essential for the integration of various types of pulmonary veins because of reduced intrathoracic signals altering water ingestion.3-5 The local in- blood volume (such as blood loss, quiet standing, stillation of hypertonic solutions of chloride or upright position and positive-pressure breathing) * Pain direct electrical stimulation of this area in goats * Psychosis leads to profound polydipsia culminating in se- * Increased temperature of blood perfusing the hypo- vere water intoxication, with an increase in body thalamus of * Drugs (acetylcholine and other cholinergic drugs, weight of as much as 40 percent.
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