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FLUID REGULATION IN WEIGHTLESS STATE.
Term Paper ID:23440
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Essay Subject:
Examines research studies & space flight experiments measuring body fluid changes for scientific & medical purposes.... More...
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10 Pages / 2250 Words
11 sources, 34 Citations,
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Paper Abstract:
Examines research studies & space flight experiments measuring body fluid changes for scientific & medical purposes.
Paper Introduction:
Fluid Regulation in a Microgravity Environment
The extraterrestrial environment presents human beings with innumerable physiologic challenges. Astronauts engaged in space flight may experience alteration of their bodily fluids and electrolyte balance. Weightlessness generally causes a reduction in total fluid volume. In addition, there is typically a cephalad redistribution of the blood and other body fluids. This cephalad shift initiates a series of compensatory mechanisms, many of which involve hormonal fluctuations. Unfortunately, data collected from human subjects during actual space flight is rather scarce. This paucity has led to the use of various ground-based weightlessness simulations. Two important experimental techniques include the water immersion and bed rest methods. Unfortunately though, despite the considerable research
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This movement of fluid may stimulate central cardiacvolume receptors (3:97). Changes in body fluid compartments during a 28-day bed rest. The state of weightlessness can perhaps be best explained byconsidering a person standing on a scale in an elevator. Inaddition, there is typically a cephalad redistribution of the blood andother body fluids. Journal of Clinical Pharmacology. J.; Gerzer, R. S.; Inners, L. Although no change wouldhave occurred in either the person's mass or the Earth's gravitationalfield, the external reactionary force would have been removed. This paucity has led to the use of various ground-basedweightlessness simulations. Such changes may involve anadaptive form of volume regulation (7:A74-A78). 7. Therefore,during the state of weightlessness, a "gravitational force is constantlyaccelerating the body in relation to the center of gravity of the Earthwithout the intervention of counteracting reactional forces" (1 :5 5-521). Whether or not natriuresis occursduring the initial hours of spaceflight remains unknown. The ambient water pressurethen simulates weightlessness by counteracting intravascular hydrostaticgradients. In fact, on one particular mission, anastronaut's extracellular fluid was found to be decreased 15 percent afterjust 24 hours of flight. Such circumstances exist, for example, when an objectis either in a free fall or engaged in a circular or elliptical trajectoryaround the planet. E.; Charles, J. Thisloss may or may not be augmented by increased renal fluid excretion.Typically though, a negative water balance results from astronauts'decreased fluid intake. The researchers observed a progressive decrease intotal body water among persons who remained in bed for 4 weeks (8:1 1-1 6). H.; Davis, J. Theseobservations may reflect the physiologic need to maintain body fluidvolumes above certain levels (1:S16-S19). E.; Vogel, J. This cephalad shift initiates a series of compensatorymechanisms, many of which involve hormonal fluctuations. S. The variability inherent to the datathus far collected may result from such things as differences inastronauts' ambient conditions, their activity levels, and induced stress(1 :5 5-521). Procedures such aswater immersion and bed rest techniques have proven useful tools forcardiovascular and renal researchers investigating adaptive physiologicresponses to weightlessness. Fortney et al. Gravitational stress and volume regulation. Moreover, while aldosterone levels have been found to increase,they may not do so for several days. (8), total body waterusually decreases by about 3.4 percent, or between 1.5 and 2. Normally, aldosterone is secreted following anyreduction in serum sodium. 351-362. Leach, C. 34:652-663; 1994.----------------------- 9 6. Such changes may both help the body adjust to stress, and might alsobe partly responsible for observed body mass losses (4:334-347). Yet another factor affecting research on weightlessness isthe paucity of data. Philadelphia, PA: Lea & Febiger; 1994; pp. In: Nicogossian, A. For example, variation in thyroxine, insulin, catecholamine, andcortisol levels might alter the metabolism of protein, carbohydrates, andfats. Fluid control mechanisms in weightlessness. Huntoon, C. Endocrine and biochemical functions. Clinical Physiology. About half of the weight lost during spaceflight is water. Thiscompartment has been found to be reduced practically every time that it hasbeen measured following a United States space mission (5:351). (2) demonstratedthat changes in body weight accompanying weightlessness were associatedwith consistently decreased natriuretic, or diuretic, kidney response.Finally, additional research is needed in order to determine the relativecontributions to observed body weight losses resulting from non-kidney-related pathways of fluid metabolism, evaporative water losses, and changesin oral intake can be quantified (2:678-686). D.; Charles, J. Overall, various observers have consistentlymeasured body mass losses of between 4 and 5 percent following exposure toweightlessness (2:678-686). Metabolic consequences of fluid shifts induced by microgravity. After eight days of flight, however, thatparticular astronaut's extracellular fluid compartment had normalizedsomewhat. Such effects generally occurduring the first few hours of weightlessness. Data from Skylabmissions indicate that fluid losses are caused by a "decrease in fluidintake and partly to an augmented renal water excretion (1 :5 5-521)."However, due to methodological problems, fluid balance data are oftenconflicting and difficult to interpret. Space travel typically causes a substantial loss of bodymass. Aviation, Space, and Environmental Medicine. L.; Pool, S. Clinical Investigator. reported on Space Shuttle studies using an isotope dilutiontechnique to determine total body water. A74-A79; 1987, September. Unfortunately,data collected from human subjects during actual space flight is ratherscarce. M.; Hyatt, K. With water immersion techniques, seated subjects are immersed inthermoneutral water (34.5?C) up to their necks. 71:678-686; 1993. Other hormonal changes observed in astronauts may represent the humanbody's adaptive response to environmental stress in addition to fluidshifts. Space sickness and fluid shifts: A hypothesis. The mean reductions for osmolality andsodium, for example, were 4 percent and 6 percent, respectively (4:334-347). Furthermore,other investigations found that some subjects experienced an additionalreduction of between .5 and 1. Therelatively large changes in plasma volume relative to the extracellularfluid may indicate that the interstitial fluid volume (i.e., the other four-fifths of the extracellular fluid) is conserved during weightlessness. Consequently, several ground-based simulations of weightlessness have been devised. On the Earth's surface, all objects are continuouslysubjected to a gravitational stress induced by the ground's reactionaryforce (1 :5 5-521). Inaddition, increased levels of plasma ANF have been observed. A.; Baartz, F. Decreases in total body fluids during weightlessness inevitablyaffect the extracellular compartment. B. Norsk, P. 5. Additionally, Shuttle mission experiments have observedincreased rates of potassium and calcium excretion. Reduced natriuresis during weightlessness. Further, under conditions ofweightlessness, the two hormones may not be "as closely coupled as would beexpected (1:S16-S19)." Studies which have examined the influence of weightlessness on otherhormones regulating fluid and electrolyte balances, have similarly providedinconsistent conclusions. Space Physiology And Medicine. In: Nicogossian, A. M. Unfortunately though, despitethe considerable research that has been performed on the effects ofweightlessness, the data currently available remain inconsistent. In addition to water, numerous electrolytes arealso excreted. Simanonok, K. These include both the aortic and carotidbaroreceptors, as well as the cardiac baroreceptors. Changes in total body water during spaceflight. When the elevatoris motionless, the scale reads the person's body weight. Fluid Regulation in a Microgravity Environment The extraterrestrial environment presents human beings withinnumerable physiologic challenges. 4. Hematologic and immunologic functions. D. Journal of Clinical Pharmacology. Such changes may bemediated through a number of different physiologic pathways (e.g. 3. For example, atrial natriuretic factor (ANF), isthought to regulate body fluid volume by increasing sodium excretion.Hence, an increase in ANF should result in reduced serum sodium. Aviation, Space, and Environmental Medicine. 9. Philadelphia, PA: Lea & Febiger; 1994; pp. Perhaps the most significant physiologic effect of space flightconsists of a reduction in total body fluid volume. F. During Skylabflights, increased urinary output of sodium, potassium, and chloride hasbeen demonstrated. One of the main hypotheses used to explain the observed fluid andvolume losses associated with spaceflight consists of the Henry-Gauerreflex. Due to mission constraints and the limited number ofsubjects, it is difficult to obtain statistically significant data underactual microgravity conditions (9:A8 -A85). Third Edition. Serum sodium is usually tightly controlledby various hormones. Cintrón, N. A.; Sams, C. However, it must be noted that if the Henry-Gauerhypothesis were correct, the onset of weightlessness would be clearlyassociated with increased renal diuresis and natriuresis. E.; Huntoon, C. Hence, weightlessness may be characterized as acondition of static equilibrium whereby there are no counteractingreactional forces. 31:1 1- 1 6; 1991. 8. kilograms throughout the entire course oftheir mission (1 :5 5-521). Astronauts who undergo weightlessness usually experience the rapiddevelopment of an "edematous 'puffy face (2:678-686).'" These featuresresult from a headward redistribution of the astronauts' body fluids.Without the pull of gravity, fluid flows from the legs to the chest andhead: The tissues of the lower limbs shrink; whereas, those of the faceswell (6:687-689). Fluid shifts into and out of superficial tissues under microgravity and terrestrial conditions. Huntoon, C. Overall, a 3 percentreduction in total body water was observed within 2 days of launch;moreover, several days later, the astronauts' body water was still found tobe reduced (1:S16-S19). Essentially, oxygen-18-labelledwater was used as a tracer in three crewmembers on one particular Shuttleflight. Body fluid volume is monitored by sensory receptors in thecardiovascular system. L.; Whitson, P. Within the Earth's gravitational field, weightlessnessmay be defined as "a condition where no other forces but gravity affects abody" (1 :5 5-521). Moreover,given that increased central blood volume stimulates ANF secretion,weightlessness should theoretically result in elevated plasma ANF.Unfortunately though, the experimental evidence varies. Over three decades ago, the initiation of manned space flightprograms in the United States and the former Soviet Union focusedconsiderable attention upon the physiologic effects of differentgravitational forces. Thedecreases typically occur soon after the onset of microgravity, but revertto preflight levels within 2 weeks of astronauts' return to Earth. The tracer was measured three times before the flight, twiceduring the flight, and several days after landing. Fortney, S. M.; Whitson, P. For many years now, investigators within the Space Medicine communityhave been studying the adaptation of fluid and electrolyte homeostasis todiffering gravitational fields. C.; Röcker, L. 71: 687-689; 1993. Initially, this redistribution causes in an increase inboth central blood volume and pressure. In the second stage ofmicrogravity adaptation, a physiologic plateau is attained: this morestable phase occurs subsequent to a change in the body's homeostatic "setpoints" (7:A74-A78)." These various physiologic adaptations toweightlessness may continue for between 4 and 6 weeks (4:334-347). Unfortunately, though, despiteconsiderable research effort, the data available are difficult tointerpret. For instance, insome cases, decreases in urinary output have been observed subsequent tothe onset of weightlessness. The Physiologist. This weightamounts to the force required to support the person's mass against theforce produced by the acceleration of gravity. Unfortunately,the experimental data gathered from actual space flights do notdefinitively support such a relationship. Through theactivation of baroreceptor reflexes--and other compensatory mechanisms--total body fluid volume can be reduced (1:S16-S19). For example, those hormonesusually associated with stress (e.g., angiotensin I, aldosterone, andcortisol) are sometimes found to be elevated. S. Each of thesealterations makes its own contribution to the overall processes of diuresisand natriuresis which occur during water immersion (1 :5 5-521). Drummer, C.; Heer, M.; Dressendörfer, R. Weightlessness would occurunder such circumstances if the elevator's cable were to be suddenly cut.The scale would almost immediately register zero. Leach, C. Research into gravitational stresses and fluid volume shifts hasdetermined that adaptation to microgravity occurs in two different stages.During the first stage, or the dynamic phase, the human cardiovascular andrenal/endocrine systems rapidly respond to their changed circumstances.This alteration may include, for example, a decrease in leg volume, aheadward shift of tissue fluids, blood, and other fluids, and an increasein central venous pressure (9:A8 -A85). Perhaps the most consistent hormonal fluctuation associated withspace flight involves antidiuretic hormone. Ingeneral, these decreases all occurred within a few days. 97-1 4; 1991, February. Then, in 199 ,Cintrón et al. M.; Lane, H. Although such losses usually occur overroughly the first 3 days of a spaceflight, they may be observed after amere 24 hours of weightlessness. Obviously, the specific effects of weightlessness on hormone levelshave yet to be precisely defined. L., eds. A.; Strasburger, C. According to this hypothesis, "increased central venous pressureactivates receptors in the atria of the heart that send impulses via thevagus nerve to the neurohypophysis (2:678-686)." These impulses ultimatelylead to the inhibition of antidiuretic hormone (ADH) secretion, withsubsequent diuresis. B. According to Leach et al. Observed losses of body water during weightlessness could result fromeither decreased intake, increased excretion, or both. 12:5 5-526; 1992.11. Weightlessness may also be simulated using bed rest. Montgomery (9)observed immediate and progressive calf volume changes associated with thedisplacement of fluid from the extremities to the thoracic/pelvic area.The study noted that such changes occurred within the first few hours ofbed rest (9:A8 -A85). Two important experimental techniques includethe water immersion and bed rest methods. L., eds. 2. A. Decreases in body fluids during weightlessness must be accompanied byelectrolyte losses from the relevant fluid compartments. E.; Huntoon, C. The data collected from water immersion research on humans generallyshow increased rates of renal fluid and sodium excretion. reported on subjects exposed toprolonged bed rest. liters, uponthe onset of weightlessness. Clinical Investigator. Furthermore, once the total body waterdeficits have developed, they will typically be maintained as long as theastronaut is in space (8:1 1-1 6). W.; Leach, C. Body volume changes during simulated weightlessness: An overview. One particularly important body fluid compartment which is usuallyaltered by spaceflight includes the blood plasma. It was initially just assumed that the observed rapid decreases inbody mass resulted from a loss of fluid (4:334-347). These two changes may cause atrialstretch, and may be interpreted physiologically as an increase in totalblood volume (11:652-662). the renin-angiotensin-aldosterone system and the sympathetic nervous system) (7:A74-A78). Perhaps with further space exploration, the differentphysiologic phenomena associated with weightlessness will be more clearlydelineated. For example, levels of angiotensin I, aldosterone,ADH, and plasma norepinephrine have been found to be suppressed. These losses occur primarily through renal excretion. Perhaps the electrolyte reduction most consistently observed amongastronauts is that of sodium. During the UnitedStates' Skylab program, for instance, inflight measurements of body massdemonstrated a decrease of between 2 and 3 kilograms in all subjects. Astronauts engaged in space flight mayexperience alteration of their bodily fluids and electrolyte balance.Weightlessness generally causes a reduction in total fluid volume. In addition though,an increase in plasma volume due to the filtration of fluid out of theinterstitial space and into the intravascular space might also influencethe renal response. Various studies have indicated that weightlessness may alter thebodily fluids. J.; Gunga, H. As a result, blood is redistributed from the caudad portions ofthe body into the intrathoracic circulation. Literature Cited 1. During Skylabflights, astronauts have been found to experience reductions in plasmaosmolality, sodium, and chloride. Angiotensin II--the precursor of which is renin, orangiotensin I--stimulates the secretion of the "sodium-retaining" hormone,aldosterone (1:S16-S19). Kirsch, K. Further, spaceflight has been associated with an increase in theplasma and urine levels of certain hormones. 334-35 . Third Edition. Plasma volume normallyconsists of about one-fifth of the extracellular fluid volume. L.; Cintrón, N. 33: S16-S19; 199 . L.; Pool, S. However, studies involving astronauts havefound diminished levels of angiotensin I during the first few days of spaceflight. This most likelyresults from the stimulation of atrial baroreceptors. Furthermore, Drummer et al. Montgomery, L. This causes increases incentral venous pressure, stroke volume, and arterial pulse pressure whichpersist throughout the duration of the procedure (i.e., 3-6 hours) (1 :5 5-521). These may include sodium, chloride, and other minerals.Such changes may cause decreased serum osmolality and decreased serumsodium. Aviation, Space, and Environmental Medicine. Obviously, innumerable hormones are involved in thesephysiologic processes. Space Physiology And Medicine. A8 -A85; 1987; September.1 . On both 28-day and 84-day Skylabmissions, astronauts' extracellular fluid volumes were found to be reducedapproximately 2 percent. Plasma ADH has been found tobe elevated practically every time it has been measured. While ANF mayinitially promote a hyponatremic state, it's precise effects throughoutlonger exposures to weightlessness remain in question (1:S16-S19).
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