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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Leaf gas exchange of Andropogon gerardii Vitman, Panicum virgatum L., and Sorghastrum nutans (L.) Nash in a tallgrass prairie

Net CO2 assimilation as a function of internal CO2 and stomatal conductance to water vapor were measured on blades of the C4 grasses Andropogon gerardii Vitman, Panicum virgatrum L., and Sorghastrum nutans (L.) Nash in northeast Kansas over two growing seasons to determine the comparative physiological responses of these dominant grasses of the tallgrass prairie to environmental variables. The response of dark respiration to temperature and of net assimilation to CO2 concentration and absorbed quantum flux differed little among species. A. gerardii had lower potential photosynthetic rates at internal CO2 concentrations below saturation than P. virgatum and S. nutans, but net assimilation under ambient conditions was similar in the three species. Net assimilation and both the initial slope of assimilation versus internal CO2 curves and the maximum potential assimilation rate decreased as leaf water potential declined in blades of A. gerardii and S. nutans. Changes in assimilation capacity were paralleled by changes in stomatal conductance that were similar in all three species. The strong correlations among processes regulating leaf CO2 assimilation and transpiration in A. gerardii, P. virgatum, and S. nutans suggest that the processes are tightly and similarly coupled in these grasses over a wide range of environmental conditions encountered in the tallgrass prairie.

Polley, H. W.↗
NASA NTRS1995

The components of crop productivity: measuring and modeling plant metabolism

Several investigators in the CELSS program have demonstrated that crop plants can be remarkably productive in optimal environments where plants are limited only by incident radiation. Radiation use efficiencies of 0.4 to 0.7 g biomass per mol of incident photons have been measured for crops in several laboratories. Some early published values for radiation use efficiency (1 g mol-1) were inflated due to the effect of side lighting. Sealed chambers are the basic research module for crop studies for space. Such chambers allow the measurement of radiation and CO2 fluxes, thus providing values for three determinants of plant growth: radiation absorption, photosynthetic efficiency (quantum yield), and respiration efficiency (carbon use efficiency). Continuous measurement of each of these parameters over the plant life cycle has provided a blueprint for daily growth rates, and is the basis for modeling crop productivity based on component metabolic processes. Much of what has been interpreted as low photosynthetic efficiency is really the result of reduced leaf expansion and poor radiation absorption. Measurements and models of short-term (minutes to hours) and long-term (days to weeks) plant metabolic rates have enormously improved our understanding of plant environment interactions in ground-based growth chambers and are critical to understanding plant responses to the space environment.

NASA Discipline Life Support Systems↗
NASA NTRS2008

Redesigned Human Metabolic Simulator

A design has been formulated for a proposed improved version of an apparatus that simulates atmospheric effects of human respiration by introducing controlled amounts of carbon dioxide, water vapor, and heat into the air. Denoted a human metabolic simulator (HMS), the apparatus is used for testing life-support equipment when human test subjects are not available. The prior version of the HMS, to be replaced, was designed to simulate the respiratory effects of as many as four persons. It exploits the catalytic combustion of methyl acetate, for which the respiratory quotient (the molar ratio of carbon dioxide produced to oxygen consumed) is very close to the human respiratory quotient of about 0.86. The design of the improved HMS provides for simulation of the respiratory effects of as many as eight persons at various levels of activity. The design would also increase safety by eliminating the use of combustion. The improved HMS (see figure) would include a computer that would exert overall control. The computer would calculate the required amounts of oxygen removal, carbon dioxide addition, water addition, and heat addition by use of empirical equations for metabolic profiles of respiration and heat. A blower would circulate air between the HMS and a chamber containing a life-support system to be tested. With the help of feedback from a mass flowmeter, the blower speed would be adjusted to regulate the rate of flow according to the number of persons to be simulated and to a temperature-regulation requirement (the air temperature would indirectly depend on the rate of flow, among other parameters). Oxygen would be removed from the circulating air by means of a commercially available molecular sieve configured as an oxygen concentrator. Oxygen, argon, and trace amounts of nitrogen would pass through a bed in the molecular sieve while carbon dioxide, the majority of nitrogen, and other trace gases would be trapped by the bed and subsequently returned to the chamber. If, as recommended, the oxygen concentrator were of a rotating twelve-bed design, then variations in the product stream could be made very small. Carbon dioxide would be added directly to the circulating air by simple injection from a supply tank. The rate of injection would be maintained at the required rate by use of a mass flowmeter/controller. In the same way, nitrogen would be added to make up for the small amount of nitrogen lost through the oxygen concentrator. Water vapor would be added to the circulating air by heating the corresponding required flow of water to steam in a heat exchanger. More heat, required to complete the simulation of the thermal effect of respiration, would be added through another heat exchanger. Heat would be supplied to both heat exchangers via a hot-oil loop.

Duffield, Bruce↗
NASA NTRS1992

Micrometeorological measurements of CH4 and CO2 exchange between the atmosphere and subarctic tundra

Eddy correlation flux measurements and concentration profiles of total hydrocarbons (THC) and CO2 were combined to provide a comprehensive record of atmosphere-biosphere exchange for these gases over a 30-day period in July-August 1988 in the Yukon-Kuskokwin River Delta of Alaska. Over 90 percent of net ecosystem exchanges of THC were due to methane. Lakes and wet meadow tundra provided the major sources of methane. The average fluxes from lake, dry tundra, and wet tundra were 11 +/- 3, 29 +/- 3, and 57 +/- 6 mg CH4/sq m/d, respectively. The mean remission rate for the site was 25 mg/sq m/d. Maximum uptake of CO2 by the tundra was 1.4 gC/sq m/d between 1000 and 1500 hrs, and nocturnal respiration averaged 0.73 gC/sq m/d. Net uptake of CO2 was 0.30 gC/sq m/d for the 30 days of measurement; methane flux accounted for 6 percent of CO2 net uptake.

Fan, S. M.↗
NASA NTRS1992

Determining the potential productivity of food crops in controlled environments

The quest to determine the maximum potential productivity of food crops is greatly benefitted by crop growth models. Many models have been developed to analyze and predict crop growth in the field, but it is difficult to predict biological responses to stress conditions. Crop growth models for the optimal environments of a Controlled Environment Life Support System (CELSS) can be highly predictive. This paper discusses the application of a crop growth model to CELSS; the model is used to evaluate factors limiting growth. The model separately evaluates the following four physiological processes: absorption of PPF by photosynthetic tissue, carbon fixation (photosynthesis), carbon use (respiration), and carbon partitioning (harvest index). These constituent processes determine potentially achievable productivity. An analysis of each process suggests that low harvest index is the factor most limiting to yield. PPF absorption by plant canopies and respiration efficiency are also of major importance. Research concerning productivity in a CELSS should emphasize: (1) the development of gas exchange techniques to continuously monitor plant growth rates and (2) environmental techniques to reduce plant height in communities.

Bugbee, Bruce↗
NASA NTRS1984

On the measurement of pilot perceptual workload - A comparison of assessment techniques addressing sensitivity and intrusion issues

A flight simulator-based study was conducted to examine fourteen distinct mental workload estimation measures, including opinion, secondary task, physiological, and primary task measures. Both the relative sensitivity of the measures to changes in mental workload and the differential intrusion of the changes on primary task performance were assessed. The flight task was varied in difficulty by manipulation of the presentation rate and complexity of a hazard-perception task that required each of 48 licensed pilots to rely heavily on their perceptual abilities. Three rating scales (Modified Cooper-Harper, Multi-descriptor, and Workload-Compensation-Interference/Technical Effectiveness), two secondary task measures (time estimation and tapping regularity), one physiological measure (respiration frequency), and one primary task measure (danger-condition response time) were reliable indicants of workload changes. Recommendations for applying the workload measures are presented.

Casali, J. G.↗
NASA NTRS1997

System identification of closed-loop cardiovascular control: effects of posture and autonomic blockade

We applied system identification to the analysis of fluctuations in heart rate (HR), arterial blood pressure (ABP), and instantaneous lung volume (ILV) to characterize quantitatively the physiological mechanisms responsible for the couplings between these variables. We characterized two autonomically mediated coupling mechanisms [the heart rate baroreflex (HR baroreflex) and respiratory sinus arrhythmia (ILV-HR)] and two mechanically mediated coupling mechanisms [the blood pressure wavelet generated with each cardiac contraction (circulatory mechanics) and the direct mechanical effects of respiration on blood pressure (ILV-->ABP)]. We evaluated the method in humans studied in the supine and standing postures under control conditions and under conditions of beta-sympathetic and parasympathetic pharmacological blockades. Combined beta-sympathetic and parasympathetic blockade abolished the autonomically mediated couplings while preserving the mechanically mediated coupling. Selective autonomic blockade and postural changes also altered the couplings in a manner consistent with known physiological mechanisms. System identification is an "inverse-modeling" technique that provides a means for creating a closed-loop model of cardiovascular regulation for an individual subject without altering the underlying physiological control mechanisms.

NASA Discipline Number 18-10↗
NASA NTRS1993

Sealed Plant-Growth Chamber For Clinostat

Laboratory chamber for growing plants used to measure photosynthesis and respiration in simulated microgravity. Holds plant specimens while rotated on clinostat, see article, "Clinostat Delivers Power To Plant-Growth Cabinets" (KSC-11537). Provides way of comparing gas-exchange rates of plants rotated horizontally on clinostat with those of stationary or vertically rotated plants. Gas extracted for analysis without stopping clinostat. Chamber includes potlike base and cylindrical cover, both made of transparent acrylic pipe. Gasket forms seal between cover and bottom plate of base. Cover bolted to pot baseplate, which in turn bolted to clinostat.

Brown, Christopher S.↗
NASA NTRS2004

Sulfur Cycling Mediates Calcium Carbonate Geochemistry in Modern Marine Stromatolites

Modem marine stromatolites forming in Highborne Cay, Exumas (Bahamas), contain microbial mats dominated by Schizothrix. Although saturating concentrations of Ca2+ and CO32- exist, microbes mediate CaCO3 precipitation. Cyanobacterial photosynthesis in these stromatolites aids calcium carbonate precipitation by removal of HS+ through CO2 use. Photorespiration and exopolymer production predominantly by oxygenic phototrophs fuel heterotrophic activity: aerobic respiration (approximately 60 umol/sq cm.h) and sulfate reduction (SR; 1.2 umol SO42-/sq cm.h) are the dominant C- consuming processes. Aerobic microbial respiration and the combination of SR and H2S oxidation both facilitate CaCO3 dissolution through H+ production. Aerobic respiration consumes much more C on an hourly basis, but duel fluctuating O2 and H2 depth profiles indicate that overall, SR consumes only slightly less (0.2-0.5) of the primary production. Moreover, due to low O2 concentrations when SR rates are peaking, reoxidation of the H2S formed is incomplete: both thiosulfate and polythionates are formed. The process of complete H2S oxidation yields H+. However, due to a low O2 concentration late in the day and relatively high O2 concentrations early in the following morning, a two-stage oxidation takes place: first, polythionates are formed from H2S, creating alkalinity which coincides with CaCO3 precipitation; secondly, oxidation of polythionates to sulfate yields acidity, resulting in dissolution, etc. Vertical profiles confirmed that the pH peaked late in the afternoon (greater than 8.8) and had the lowest values (less than 7.4) early in the morning. Thus, the effect of this S-cycling through alkalinity production, followed by acidification during H2S oxidation, results in a six times stronger fluctuation in acidity than photosynthesis plus aerobic respiration accomplish. This implies that anaerobic processes play a pivotal role in stromatolite formation.

Visscher, P. T.↗
NASA NTRS2003

Bursting into space: alterations of sympathetic control by space travel

AIM: Astronauts return to Earth with reduced red cell masses and hypovolaemia. Not surprisingly, when they stand, their heart rates may speed inordinately, their blood pressures may fall, and some may experience frank syncope. We studied autonomic function in six male astronauts (average +/- SEM age: 40 +/- 2 years) before, during, and after the 16-day Neurolab space shuttle mission. METHOD: We recorded electrocardiograms, finger photoplethysmographic arterial pressures, respiration, peroneal nerve muscle sympathetic activity, plasma noradrenaline and noradrenaline kinetics, and cardiac output, and we calculated stroke volume and total peripheral resistance. We perturbed autonomic function before and during spaceflight with graded Valsalva manoeuvres and lower body suction, and before and after the mission with passive upright tilt. RESULTS: In-flight baseline sympathetic nerve activity was increased above pre-flight levels (by 10-33%) in three subjects, in whom noradrenaline spillover and clearance also were increased. Valsalva straining provoked greater reductions of arterial pressure, and proportionally greater sympathetic responses in space than on Earth. Lower body suction elicited greater increases of sympathetic nerve activity, plasma noradrenaline, and noradrenaline spillover in space than on Earth. After the Neurolab mission, left ventricular stroke volume was lower and heart rate was higher during tilt, than before spaceflight. No astronaut experienced orthostatic hypotension or pre-syncope during 10 min of post-flight tilting. CONCLUSION: We conclude that baseline sympathetic outflow, however measured, is higher in space than on earth, and that augmented sympathetic nerve responses to Valsalva straining, lower body suction, and post-flight upright tilt represent normal adjustments to greater haemodynamic stresses associated with hypovolaemia.

NASA Program Biomedical Research and Countermeasur↗
NASA NTRS1993

Net exchange of CO2 in a mid-latitude forest

The eddy correlation method was used to measure the net ecosystem exchange of carbon dioxide continuously from April 1990 to December 1991 in a deciduous forest in central Massachusetts. The annual net uptake was 3.7 +/- 0.7 metric tons of carbon per hectare per year. Ecosystem respiration calculated from the relation between nighttime exchange and soil temperature, was 7.4 metric tons of carbon per hectare per year, implying gross ecosystem production of 11.1 metric tons of carbon per hectare per year. The observed rate of accumulation of carbon reflects recovery from agricultural development in the 1800s. Carbon uptake rates were notably larger than those assumed for temperate forests in global carbon studies. Carbon storage in temperate forests can play an important role in determining future concentrations of atmospheric carbon dioxide.

Wofsy, S. C.↗
NASA NTRS1992

Energy expenditure in space flight (doubly labelled water method) (8-IML-1)

The objective of the Energy Expenditure in Space Flight (ESS) experiment is to demonstrate and evaluate the doubly labeled water method of measuring the energy expended by crew members during approximately 7 days in microgravity. The doubly labeled water technique determines carbon dioxide production which is then used to calculate energy expenditure. The method relies on the equilibrium between oxygen in respiratory carbon dioxide and oxygen in body water. Because of this equilibrium, the kinetic of water turnover and respiration are interdependent. Under normal conditions, man contains small but significant amounts of deuterium and oxygen 18. Deuterium is eliminated from the body as water while oxygen 18 is eliminated as water and carbon dioxide. The difference in the turnover rates in the two isotopes is proportional to the carbon dioxide production. Deliberately enriching the total body water with both of these isotopes allows the isotope turnovers to be accurately measured in urine, plasma, or saliva samples. The samples are taken to the laboratory for analysis using an ion-ratio spectrometer.

Parsons, Howard G.↗
NASA NTRS2003

Influence of Microgravity on Arterial Baroreflex Responses Triggered by Valsalva's Maneuver

When astronauts return to Earth and stand upright, their heart rates may speed inordinately, their blood pressures may fall, and some returning astronauts may even faint. Since physiological adjustments to standing are mediated importantly by pressure-regulating reflexes (baroreflexes), we studied involuntary (or autonomic) nerve and blood pressure responses of astronauts to four, 15-second periods of 15- and 30-mmHg straining (Valsalva'.~ maneuver). We measured the electrocardiogram, finger blood pressure, respiration, and muscle sympathetic nerve activity in four healthy male astronauts before and during the 16-day Neurolab Space Shuttle mission. We found that although microgravity provoked major autonomic changes, no astronaut experienced fainting symptoms after the mission. Blood pressure fell more during straining in space than on Earth (the average reduction of systolic pressure with 30-mmHg straining was 49 mmHg during and 27 mmHg before the mission). However, the increases of muscle sympathetic nerve activity that were triggered by straining were also larger in space than on Earth. As a result, the gain of the sympathetic baroreflex, taken as the total sympathetic nerve response divided by the maximum pressure reduction during straining, was the same in space as on Earth. In contrast, heart rate changes, which are mediated by changes of vagus nerve activity, were smaller in space. This and earlier research suggest that exposure to microgravity augments blood pressure and sympathetic adjustments to Valsalva straining and differentially reduces vagal, but not sympathetic baroreflex responsiveness. The changes that we documented can be explained economically as a consequence of the blood volume reduction that occurs in space.

Eckberg, Dwain L.↗
NASA NTRS2010

Southeastern U.S.A. Continental Shelf Respiratory Rates Revisited

Respiratory rates on the U. S. southeastern continental shelf have been estimated several times by different investigators, most recently by Jiang et al. (Biogeochemistry 98:101-113, 2010) who report lower mean rates thanwere found in earlier work and attribute the differences to analytical error in all methods used in earlier studies. The differences are, instead, attributable to the differences in the geographical scope of the studies. The lower estimates of regional organic carbon flux of Jiang et al. (Biogeochemistry 98:101-113, 2010) are a consequence of their extrapolation of data from a small portion of the shelf to the entire South Atlantic Bight. This comment examines the methodologies used as well as the variability of respiratory rates in this region over space and time.

Microbial respiration↗
NASA NTRS1989

Assessment of autonomic response by broad-band respiration

We present a technique for introducing broad-band respiratory perturbations so that the response characteristics of the autonomic nervous system can be determined noninvasively over a wide range of physiologically relevant frequencies. A subject's respiratory bandwidth was broadened by breathing on cue to a sequence of audible tones spaced by Poisson intervals. The transfer function between the respiratory input and the resulting instantaneous heart rate was then computed using spectral analysis techniques. Results using this method are comparable to those found using traditional techniques, but are obtained with an economy of data collection.

NASA Discipline Cardiopulmonary↗
NASA NTRS1993

Important influence of respiration on human R-R interval power spectra is largely ignored

Frequency-domain analyses of R-R intervals are used widely to estimate levels of autonomic neural traffic to the human heart. Because respiration modulates autonomic activity, we determined for nine healthy subjects the influence of breathing frequency and tidal volume on R-R interval power spectra (fast-Fourier transform method). We also surveyed published literature to determine current practices in this burgeoning field of scientific inquiry. Supine subjects breathed at rates of 6, 7.5, 10, 15, 17.1, 20, and 24 breaths/min and with nominal tidal volumes of 1,000 and 1,500 ml. R-R interval power at respiratory and low (0.06-0.14 Hz) frequencies declined significantly as breathing frequency increased. R-R interval power at respiratory frequencies was significantly greater at a tidal volume of 1,500 than 1,000 ml. Neither breathing frequency nor tidal volume influenced average R-R intervals significantly. Our review of studies reporting human R-R interval power spectra showed that 51% of the studies controlled respiratory rate, 11% controlled tidal volume, and 11% controlled both respiratory rate and tidal volume. The major implications of our analyses are that breathing parameters strongly influence low-frequency as well as respiratory frequency R-R interval power spectra and that this influence is largely ignored in published research.

NASA Discipline Cardiopulmonary↗
NASA NTRS1998

Controlled breathing protocols probe human autonomic cardiovascular rhythms

The purpose of this study was to determine how breathing protocols requiring varying degrees of control affect cardiovascular dynamics. We measured inspiratory volume, end-tidal CO2, R-R interval, and arterial pressure spectral power in 10 volunteers who followed the following 5 breathing protocols: 1) uncontrolled breathing for 5 min; 2) stepwise frequency breathing (at 0.3, 0.25, 0.2, 0.15, 0.1, and 0.05 Hz for 2 min each); 3) stepwise frequency breathing as above, but with prescribed tidal volumes; 4) random-frequency breathing (approximately 0.5-0.05 Hz) for 6 min; and 5) fixed-frequency breathing (0.25 Hz) for 5 min. During stepwise breathing, R-R interval and arterial pressure spectral power increased as breathing frequency decreased. Control of inspired volume reduced R-R interval spectral power during 0.1 Hz breathing (P < 0.05). Stepwise and random-breathing protocols yielded comparable coherence and transfer functions between respiration and R-R intervals and systolic pressure and R-R intervals. Random- and fixed-frequency breathing reduced end-tidal CO2 modestly (P < 0.05). Our data suggest that stringent tidal volume control attenuates low-frequency R-R interval oscillations and that fixed- and random-rate breathing may decrease CO2 chemoreceptor stimulation. We conclude that autonomic rhythms measured during different breathing protocols have much in common but that a stepwise protocol without stringent control of inspired volume may allow for the most efficient assessment of short-term respiratory-mediated autonomic oscillations.

Non-NASA Center↗