A Strategy for Research in Space Biology and Medicine in the New Century (1998) / Chapter Skim
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8 Cardiovascular and Pulmonary Systems
8 Cardiovascular and Pulmonary Systems
Pages 118-131
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From page 118... ...
The cardiovascular and pulmonary systems are linked to other systems controlling plasma volume and red blood cell mass through afferent autonomic signaling, and also through neurohormonal substances released in response to chamber and vessel distension, blood flow, and oxygen content at other sites in the body. The pulmonary system includes the trachea (windpipe)
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From page 119... ...
The perfusion pressure of deoxygenated blood entering the pulmonary artery from the right ventricle is relatively low, typically 10 to 20 mm Hg. In an individual standing upright, this pressure may be insufficient to overcome hydrostatic gradients, and so very little flow reaches the upper regions of the lungs, but a relatively large portion of pulmonary blood flow perfuses the dependent portions of the lungs.
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From page 120... ...
The cardiac output gradually declines during flight, and by 7 to 10 days in orbit, values approach, but do not quite reach, outputs seen with humans in the upright position in 1 g. There are various causes for the decline in cardiac output over time, including progressive decreases in plasma volume and red blood cell mass, altered autonomic cardiovascular control, and continued changes in body fluid distribution.~5 ~6 It is of interest that the resting hemodynamic state achieved by humans in microgravity closely approximates that seen on Earth when compared with measurements taken of humans in the upright position.
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From page 121... ...
It was postulated that the absence of gravity gradients in pulmonary blood flow distribution provided a much greater effective capillary surface area and increased alveolar-capillary gas exchange. Analysis of the DLCo results showed that increases were due mainly to increased membrane diffusion surface area, while increased pulmonary capillary blood volume had only a minor contribution.43 End-tidal pCO2 reflects the concentration of carbon dioxide at the end of expiration and is dependent on alveolar CO2 concentrations.
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From page 122... ...
Pulmonary capillary blood volume Diffusing capacity of alveolar membrane Pulmonary blood flow distribution Pulmonary ventilation 4 4 4 4 7 18% increase 4% increase 28% increase 28% increase 27% increase More uniform but some inequality remained Respiration frequency 8 9% increase Tidal volume 8 15% decrease Alveolar ventilation 8 Unchanged Total ventilation 8 Small decrease Ventilatory distribution 7 More uniform but some inequality remained Maximal peak expiratory flow rate 7 Decreased by <12.5% early in flight but then returned to normal Pulmonary gas exchange O2 uptake 8 Unchanged CO2 output 8 Unchanged End-tidal PO2 8 Unchanged End-tidal PCO2 8 Small increase when CO2 concentration in spacecraft increased Lung volumes Functional residual capacity 4 15% decrease Residual volume 4 18% decrease Closing volume 7 Unchanged as measured by argon bolus NOTE: Pulmonary blood flow in normal subjects is the same as cardiac output, the amount of blood pumped by the heart per minute. Stroke volume is the volume of blood pumped per beat.
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From page 123... ...
Careful hemodynamic measurements of astronauts immediately postflight show that heart rate and stroke volume are relatively well maintained initially, but a failure to increase total peripheral resistance adequately leads to a fall in blood pressure, insufficient brain blood flow, and an inability to stand quietly for more than 5 to 10 minutes postflight. Decreased red blood cell mass and plasma volume (see the section "Hematology" in Chapter 9)
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From page 124... ...
The major hemodynamic defect is an inadequate stroke volume, which is decreased by one-third or more from preflight levels and leads to proportionate decreases in cardiac output and skeletal muscle oxygen delivery. Undoubtedly, some component of skeletal muscle atrophy and decreased neuromuscular coordination contribute to the decreased aerobic capacity, but these are probably minor factors comoared with decreased blood volume and autonomic nervous system dYsreculation.
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From page 125... ...
Further studies have not been conducted. Liquid-filled cooling garments used during EVA decrease thermally induced increases in skin blood flow and could be used during reentry to maintain cardiac output and blood pressure.75 76 Modified anti-g suits, similar to those used in high-performance military aircraft, have been employed to decrease the lower extremity and abdominal venous pooling that occurs during reentry hypergravity, and their use appears to produce the desired hemodynamic improvement.
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From page 126... ...
These are described briefly: · Automated recording devices should be used extensively to capture physiological data with minimal additional astronaut involvement. Examples include exercise equipment that records astronaut identification, date, time, and workload; a simple body-mass measuring device; a dietary log system that does not require a manual logbook entry each time food or drink is consumed; and an automated urine measurement system that records void volumes, but also makes and records simple measures of electrolytes, creatinine levels, and so on.
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From page 127... ...
· Determine whether microgravity-induced changes in local perfusion cause changes in vascular or vasomotor control (vascular proliferation or atrophy, secretion of endothelially derived vasoactive substances and microcirculatory autoregulatory mechanisms, and so on) or organ function (pulmonary gas exchange, renal clearance mechanisms, blood-brain barrier and cerebral pressure, etc.~.
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From page 128... ...
1993. Pulmonary diffusing capacity, capillary blood volume, and cardiac output during sustained microgravity.
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From page 129... ...
1993. Pulmonary diffusing capacity, capillary blood volume, and cardiac output during sustained microgravity.
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From page 130... ...
1993. Pulmonary diffusing capacity, capillary blood volume, and cardiac output during sustained microgravity.
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From page 131... ...
1991. Development of lower body negative pressure as a countermeasure for orthostatic intolerance.
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