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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Renal sodium reabsorption following induction of and recovery from volume expansion

In the rat, infusion of a volume of isotonic saline equal to 2% of body weight resulted in an 82% increase in the delivery of filtrate out of the proximal tubule but little or, in some animals, no change in the urinary excretion of sodium. By contrast, further degrees of volume expansion resulted in lesser increases in the distal delivery of filtrate, but were associated with a marked increase in the urinary excretion of sodium. Sixty minutes following completion of volume expansion, while the animals were still in positive sodium balance, the urinary excretion of sodium decreased 52% compared to a decrease of only 24% in the distal delivery of filtrate. During the course of progressive volume expansion and during the recovery phase, there was a dissociation between alterations in sodium reabsorption in the proximal convoluted tubule and in the whole kidney. These studies indicate that although the proximal tubule is more sensitive to changes in the extracellular fluid volume, distal nephron sites are ultimately responsible both for the natriuresis of volume expansion and the relative antinatriuresis of the recovery periods.

Knight, T. F.↗
NASA NTRS1988

Cardiovascular effects of weightlessness and ground-based simulation

A large number of animal and human flight and ground-based studies were conducted to uncover the cardiovascular effects of weightlessness. Findings indicate changes in cardiovascular function during simulations and with spaceflight that lead to compromised function on reambulation and/or return to earth. This altered state termed cardiovascular deconditioning is most clearly manifest when in an erect body state. Hemodynamic parameters inidicate the presence of excessive tachnycardia, hypotension (leading to presyncope in one-third of the subjects), decreased heart volume, decreased plasma and circulating blood volumes and loss of skeletal muscle mass, particularly in the lower limbs. No clinically harmful effects were observed to date, but in-depth follow-ups were limited, as was available physiologic information. Available data concerning the causes for the observed changes indicate significant roles for mechanisms involved with body fluid-volume regulation, altered cardiac function, and the neurohumoral control of the control of the peripheral circulation. Satisfactory measures are not found. Return to preflight state was variable and only slightly dependent on flight duration. Future progress awaits availability of flight durations longer than several weeks.

Sandler, Harold↗
NASA NTRS1986

Mechanism for negative water balance during weightlessness An hypothesis

The mechanism for the apparent decrease in body fluid volume in astronauts during spaceflight remains obscure. The widespread postulate that the hypohydration is the result of the Henry-Gauer reflex, a diuresis caused by inhibition of vasopressin secretion resulting from increased left and perhaps right atrial (central) venous pressure, has not been established with direct measurements on astronauts. An hypothesis is proposed to account for fluid-electrolyte shifts during weightlessness. A moderate but transient increase in central venous pressure occurs when orbit is entered that is insufficient to activate the Henry-Gauer reflex but sufficient to stimulate the release of atrial natriuretic peptides. Increased sodium excretion would facilitate some increased urinary water loss. The resulting relatively dilute plasma and interstitial fluids would cause fluid to shift into the cellular space, resulting in edema in the head and trunk and inhibition of thirst and drinking. Thus, the negative water balance in astronauts would be caused by a gradual natriuresis and diuresis coupled with reduced fluid intake.

Greenleaf, J. E.↗
NASA NTRS1998

Beta-Adrenergic Blockade Does not Prevent Polycythemia or Decrease in Plasma Volume in Men at 4300 m Altitude

When humans ascend to high altitude (ALT) their plasma volume (PV) and total blood volume (BV) decrease during the first few days. With continued residence over several weeks, the hypoxia-induced stimulation of erythropoietin increases red cell production which tends to restore BV. Because hypoxia also activates the beta-adrenergic system, which stimulates red blood cell production, we investigated the effect of adrenergic beta-receptor inhibition with propranolol on fluid volumes and the polycythemic response in 11 healthy unacclimatized men (21-33 years old exposed to an ALT of 4300 m (barometric pressure 460 Torr) for 3 weeks on Pikes Peak, Colorado. PV was determined by the Evans blue dye method (PV(sub EB)), BV by the carbon monoxide method (BV(sub CO)), red cell volume (RCV)was calculated from hematocrit (Hct) and BV(sub CO), and serum erythropoietin concentration ([EPO]) and reticulocyte count, were also determined. All determinations were made at sea level and after 9-11 (ALT-10) and 9-20 (ALT-20) days at ALT. At sea level and ALT, six men received propranolol (pro, 240 mg/day), and five received a placebo (pla). Effective beta-blockade did not modify the mean (SE) maximal values of [EPO] [pla: 24.9 (3.5) vs pro: 24.5 (1.5) mU/ml] or reticulocyte count [pla: 2.7 (0.7) vs pro: 2.2 (0.5)%]; nor changes in PV(sub EB)[pla: -15.8 (3.8) vs pro: -19.9 (2.8)%], RCV(sub CO) [pla: +7.0 (6.7) vs pro: +10.1 (6.1)%], or BV(sub CO) [pla: -7.3 (2.3) vs pro: -7.1 (3.9)%]. In the absence of weight loss, a redistribution of body water with no net loss is implied. Hence, activation of the beta-adrenergic system did not appear to affect the hypovolemic or polycythemic responses that occurred during 3 weeks at 4300 m ALT in these subjects.

Grover, R. F.↗
NASA NTRS1993

Physiological water model development

The water of the human body can be categorized as existing in two main compartments: intracellular water and extracellular water. The intracellular water consists of all the water within the cells and constitutes over half of the total body water. Since red blood cells are surrounded by plasma, and all other cells are surrounded by interstitial fluid, the intracellular compartment has been subdivided to represent these two cell types. The extracellular water, which includes all of the fluid outside of the cells, can be further subdivided into compartments which represent the interstitial fluid, circulating blood plasma, lymph, and transcellular water. The interstitial fluid surrounds cells outside of the vascular system whereas plasma is contained within the blood vessels. Avascular tissues such as dense connective tissue and cartilage contain interstitial water which slowly equilibrates with tracers used to determine extracellular fluid volume. For this reason, additional compartments are sometimes used to represent these avascular tissues. The average size of each compartment, in terms of percent body weight, has been determined for adult males and females. These compartments and the forces which cause flow between them are presented. The kidneys, a main compartment, receive about 25 percent of the cardiac output and filters out a fluid similar to plasma. The composition of this filtered fluid changes as it flows through the kidney tubules since compounds are continually being secreted and reabsorbed. Through this mechanism, the kidneys eliminate wastes while conserving body water, electrolytes, and metabolites. Since sodium accounts for over 90 percent of the cations in the extracellular fluid, and the number of cations is balanced by the number of anions, considering the renal handling sodium and water only should sufficiently describe the relationship between the plasma compartment and kidneys. A kidney function model is presented which has been adapted from a previous model of normal renal function in man. To test the validity of the proposed kidney model, results predicted by the model will be compared to actual data involving injected or ingested fluids and subsequent urine flow rates. Comparison of the model simulation to actual data following the ingestion of 1 liter of water is shown. The model simulation is also shown with actual data following the intravenous infusion of hypertonic saline.

Doty, Susan↗
NASA NTRS1995

Pros and Cons of Using Water Immersion to Simulate Physiological Responses to Microgravity

Head-out water immersion (HOI) has been employed as a remedial treatment for various ills and ailments for many millennia, and total body immersion even longer as protective encapsulation for the mammalian fetus. Two discrete differences between stimuli induced by true microgravity (10(exp -4) g) and HOI are readily apparent. External water pressure on the skin and accompanying negative pressure breathing cause blood to shift headward. Secondly, the gravitational force is ever present during immersion and microgravity, but its effect is essentially neutralized during Earth orbital flight. Thus, the physiological responses to immersion should not be expected to match those during microgravity. Immersion has been used mainly to study and understand kidney function and associated cardiovascular responses for control of body fluid volume and osmotic content, with some application to and simulation of microgravity responses. There is a plethora of data from human HOI studies, but relatively few controlled data from microgravity studies. In general, it appears that physiological responses occur more quickly with water immersion than in microgravity, but this may be due to less rigorous control (voluntary and involuntary) of the preflight state of crew members. The central venous pressure-vasopressin (Gauer-Henry) reflex control for fluid balance may not be of prime importance in microgravity. Gross functions such as reduced body weight and water, level of hypovolemia, decreased isokinetic strength, and lower nitrogen balance found during immersion are qualitatively similar in microgravity, but the mechanisms controlling these and other functions are, for the most part, unclear. Only acquisition of data from well-controlled microgravity experiments will resolve this discrepancy.

Greenleaf, J. E.↗
NASA NTRS1988

Changes in plasma volume during bed rest - Effects of menstrual cycle and estrogen administration

The effect of increased blood estrogen concentration, caused either during normal menstrual cycles or by exogenous administration of premarin, on the bed-rest (BR) induced decrease in plasma volume (PV) was investigated. In women who underwent duplicate 11-day BR without estrogen supplementation, the PV was found to decrease significantly, during the first 5 days of BR, to a lower level at which it remained for the rest of the BR period. In women who began BR in the periovulatory stage of the menstrual cycle, the loss of PV was delayed, while women who began BR during other stages of the cycle exhibited the usual trend of the PV decrease during the BR. In women who underwent a single 12-day BR period while taking premarin (1.25 mg/day), PV was found to decrease during the first 4-5 days of BR, but then returned toward the pre-BR level during the remainder of the BR, indicating that estrogens have a role in stabilizing body fluid volume.

Fortney, S. M.↗
NASA NTRS1993

Development of a model to assess orthostatic responses

A major change for crewmembers during weightlessness in microgravity is the redistribution of body fluids from the legs into the abdomen, thorax, and head. The fluids continue to be sequestered in these areas throughout the flight. Upon reentry into gravity on landing, these same body fluids are displaced again to their normal locations, however, not without hazardous incidence to the crewmembers. The problem remains that upon landing, crewmembers are subject to orthostasis, that is, the blood flowing into the legs reduces the blood supply to the brain and may result in the crewmember fainting. The purpose of this study was to develop a model of testing orthostatic responses of blood pressure regulating mechanisms of the cardiovascular system, when challenged, to maintain blood pressure to the brain. To accomplish this, subjects' responses were assessed as they proceeded from the supine position of progressive head-up tilt positions of 30 deg, 60 deg, and 90 deg angles. A convenience sample consisted of 21 subjects, females (N=11) and males (N=10), selected from a list of potential subjects available through the NASA subject screening office. The methodology included all non-invasive measurements of blood pressure, heart rate, echocardiograms, cardiac output, cardiac stroke volume, fluid shifts in the thorax, ventricular ejection and velocity times, and skin blood perfusion. The Fischer statistical analysis was done of all data with the significance level at .05. Significant differences were demonstrated in many instances of changes of posture for all variables. Based on the significance of the findings of this study, this model for assessing orthostatic responses does provide an adequate challenge to the blood pressure regulatory systems. While individuals may use different adaptations to incremental changes in gravity, the subjects, in aggregate, demonstrated significant adaptive cardiovascular changes to orthostatic challenges which were presented to them.

Rubin, Marilyn↗
NASA NTRS1966

Segmented ball valve is easy to open and close

Segmented ball valve and flowmeter in the same spherical housing provide a valve that will handle large fluid volume without bulkiness and weight of blade valves or conventional ball valves. The valve is easily opened or closed and the flowmeter remains stationary, so errors are eliminated.

Prono, E.↗
NASA NTRS1974

Proceedings of the 1972 Lyndon B. Johnson Space Center Endocrine Program Conference

Subjects covered during the Endocrine Program Conference include the following: (1) endocrine/metabolic studies on the Apollo 16 crewmen; (2) changes in glucose, insulin, and growth hormone levels associated with bed rest; (3) circadian rhythms of heart rate and body temperature during 56 days of bed rest; (4) stress-induced changes in corticosteroid metabolism in man; (5) present status of physiological studies on parathyroid hormone and vitamin D; (6) antagonistic effect of lithium on antidiuretic hormone action; (7) proposed Skylab body-fluid volumes study; (8) daily rhythmic changes in serotonin content in areas of the mouse brain and norepinephrine content in areas of the hamster brain; (9) studies of sodium homeostasis during simulated weightlessness; and (10) application of the water immersion model to man.

Source record↗
NASA NTRS1974

Adrenocortical responses of the Apollo 17 crew members

Changes in adrenal activity of the three Apollo 17 crew members were studied during the 12.55-day mission and during selected post-recovery days. Aldosterone excretion was normal early and elevated later in the mission, probably causing a loss in total body exchangeable potassium. There was decreased 17-hydroxycorticosteroid excretion only during the early mission days for the two moon landers and throughout the mission for the other astronaut. Cortisol excretion was elevated on physically stressful mission days. At recovery, plasma ACTH was elevated without a similar increase in plasma cortisol. Angiotensin I activity was elevated at recovery in only one crewman. This crewman was the only one with a decreased extracellular fluid volume. These results indicate that the mission and its activities affect adrenal function of the crewmen.

Leach, C. S.↗
NASA NTRS1975

The importance of the renin-angiotensin system in normal cardiovascular homeostasis

Studies were carried out on adult mongrel dogs (20 to 30 kilograms) to investigate the importance of the renin-angiotensin system. Results indicate that the renin-angiotensin system plays a major role in the maintenance of circulatory homeostasis when extracellular fluid volume is depleted. It was also found that angiotensin II concentration, in addition to renal perfusion pressure, is a factor in the regulation of renin release.

Haber, E.↗
NASA NTRS1979

System parameters for erythropoiesis control model: Comparison of normal values in human and mouse model

The computer model for erythropoietic control was adapted to the mouse system by altering system parameters originally given for the human to those which more realistically represent the mouse. Parameter values were obtained from a variety of literature sources. Using the mouse model, the mouse was studied as a potential experimental model for spaceflight. Simulation studies of dehydration and hypoxia were performed. A comparison of system parameters for the mouse and human models is presented. Aside from the obvious differences expected in fluid volumes, blood flows and metabolic rates, larger differences were observed in the following: erythrocyte life span, erythropoietin half-life, and normal arterial pO2.

Source record↗
NASA NTRS1980

Characterization of renal response to prolonged immersion in normal man

?jDuring the initial phase of space flight, there is a translocation of fluid from the lower parts of the body to the central vascular compartment with a resultant natriuresis, diuresis, and weight loss. Because water immersion is regarded as an appropriate model for studying the redistribution of fluid that occurs in weightlessness, an immersion study of relatively prolonged duration was carried out in order to characterize the temporal profile of the renal adaptation to central hypervolemia. Twelve normal male subjects underwent an immersion study of 8-h duration in the sodium-replete state. Immersion resulted in marked natriuresis and diuresis which were sustained throughout the immersion period. The failure of that natriuresis and diuresis of immersion to abate or cease despite marked extracellular fluid volume contraction as evidenced by a mean weight loss of -2.2 + or - 0.3 kg suggests that central blood volume was not restored to normal and that some degree of central hypervolemia probably persisted.

Epstein, M.↗
NASA NTRS1980

Effects of the space environment on the health and safety of space workers

Large numbers of individuals are required to work in space to assemble and operate a Solar Power Satellite. The physiological and behavioral consequences for large groups of men and women who perform complex tasks in the vehicular or extravehicular environments over long periods of orbital stay time were considered. The most disturbing consequences of exposure to the null gravity environment found relate to: (1) a generalized cardiovascular deconditioning along with loss of a significant amount of body fluid volume; (2) loss of bone minerals and muscle mass; and (3) degraded performance of neutral mechanisms which govern equilibrium and spatial orientation.

Hull, W. E.↗
NASA NTRS1982

Water immersion and its computer simulation as analogs of weightlessness

Experimental studies and computer simulations of water immersion are summarized and discussed with regard to their utility as analogs of weightlessness. Emphasis is placed on describing and interpreting the renal, endocrine, fluid, and circulatory changes that take place during immersion. A mathematical model, based on concepts of fluid volume regulation, is shown to be well suited to simulate the dynamic responses to water immersion. Further, it is shown that such a model provides a means to study specific mechanisms and pathways involved in the immersion response. A number of hypotheses are evaluated with the model related to the effects of dehydration, venous pressure disturbances, the control of ADH, and changes in plasma-interstitial volume. By inference, it is suggested that most of the model's responses to water immersion are plausible predictions of the acute changes expected, but not yet measured, during space flight. One important prediction of the model is that previous attempts to measure a diuresis during space flight failed because astronauts may have been dehydrated and urine samples were pooled over 24-hour periods.

Leonard, J. I.↗