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The Cardiovascular System in Microgravity

Donald E. Watenpaugh, Alan R. Hargens

10.1002/cphy.cp040129

Source: Supplement 14: Handbook of Physiology, Environmental Physiology

Originally published: 1996

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Fundamental Concepts
2 Acute Effects of Microgravity
2.1 Parabolic Flight of Aircraft
2.2 Spaceflight and the Prelaunch Posture
2.3 Peripheral Circulation
2.4 Central Circulation
2.5 Effective Circulatory Compliance
2.6 Psychological Stress
3 Acclimation to Microgravity
3.1 Leg Blood Flow
3.2 Leg Compliance
3.3 Splanchnic Circulation
3.4 Upper Body Vascular Adjustments
3.5 Transcapillary Fluid Balance and Microcirculatory Responses
3.6 Central Circulation
3.7 Cardiac Rhythm
3.8 Extracellular Fluid Volume and Composition
3.9 Hematopoiesis
3.10 Integrated Cardiovascular Function
3.11 Interrelationships among Systemic Adjustments to Microgravity
3.12 Validity of Ground‐Based Models
4 Postflight Readjustment to Gravity
4.1 Orthostatic Intolerance
4.2 Reduced Upright Exercise Capacity
5 Countermeasures
6 Summary and Conclusions
Figure 1. Figure 1.

Blood pressure gradients hypothesized for a large terrestrial dinosaur and tree‐climbing snake (Pa = arterial pressure, Pc = capillary pressure, Pv = venous pressure).
Figure 2. Figure 2.

Expected distributions of tissue fluid (shading) and mean arterial pressure (numerical values in mm Hg) at head, heart, and feet during preflight standing posture on Earth (left), microgravity (middle), and postflight standing on Earth (right). During microgravity, all gravitational blood pressure gradients disappear, and only viscous blood pressure gradients exist between the arteries, capillaries, and veins.

Modified from Hargens 111 with permission
Figure 3. Figure 3.

Illustration of how regional vascular compliances (C1: upper body, and C2: lower body) affect cardiac filling pressure. Relatively stiff, noncompliant lower body tissues normally “push” the hydrostatic indifference level (HIL) towards heart level, such that height H1 produces only moderate reduction of cardiac filling pressure when orthostatic. If chronic, microgravity‐induced equilibration of regional vascular transmural pressures leads to relatively uniform regional compliances, the HIL shifts further away from the heart (H2 > H1). Upon subsequent orthostasis in gravity, cardiac filling pressure would thus be compromised, because it decreases in proportion to the height between the HIL and heart 85.
Figure 4. Figure 4.

Typical parabolic flight profile as flown by the National Aeronautics and Space Administration's KC‐135 based at Ellington Field, Texas. KIAS, knots indicated air speed.
Figure 5. Figure 5.

Middle cerebral artery blood flow velocity index (F mean of max.), mean arterial blood pressure (MAP), and heart rate in one seated subject during parabolic flight microgravity. Data represent means ± SD from five parabolas.
Figure 6. Figure 6.

Typical leg volume reduction during spaceflight, and post‐flight recovery of leg volume. Pre‐ and postflight measurements were made with the subject standing.

Modified from Moore and Thornton 183 with permission
Figure 7. Figure 7.

Illustration of importance of central venous pressure (CVP) to cardiovascular and fluid regulatory function, and thus to physiologic adjustments to microgravity (see text). ANP, atrial natriuretic peptide; ADH, antidiuretic hormone.
Figure 8. Figure 8.

Central venous pressure (CVP) in one crewmember before, during and after launch and orbit insertion in the Shuttle Columbia on the SLS‐1 flight. “Suit room” baseline data were collected in upright sitting posture. CVP was measured directly via a catheter with an ambulatory system designed for use in spaceflight conditions 36. SLS‐2 289 and D‐2 75 results were similar (n = 3).

From Buckey et al. 34 with permission
Figure 9. Figure 9.

Use of vascular compliance curves to illustrate two alternative explanations for reduced venous pressure in microgravity: A: increase in compliance due to reduced venous tone; B: reduction of mean circulatory filling pressure due to reduction of extravascular pressure.
Figure 10. Figure 10.

Reduction of end‐expiratory gastric (intra‐abdominal) pressure (Pga in cm H2O) during parabolic flight microgravity in four seated male subjects (X ± SEM). These data illustrate a positive linear relationship between extravascular pressure and gravity, and thus quantify how microgravity could indirectly reduce intracirculatory pressures.

Modified from Estenne et al. 74
Figure 11. Figure 11.

Resting leg compliance increased during and after spaceflight in the three SLS‐1 Shuttle crew‐members studied, relative to preflight supine conditions 272. Compliance was assessed with venous occlusion plethysmography 277 using hardware developed specifically for flight experiments 36 and a standardized protocol (20, 40, 60 and 80 mm Hg venous occlusions held for 1, 2, 3, and 4 min, respectively, with 1 min breaks between occlusions; 273).
Figure 12. Figure 12.

Comparison of mean middle cerebral arterial blood flow velocity between upright‐seated control, head‐down tilt (HDT), and upright recovery (X ± SEM; n = 8). *, significantly different from the corresponding value of upright‐seated control at same time of day, P < 0.05.

[From Kawai et al. 142 with permission.]
Figure 13. Figure 13.

Hypothesized regional distributions of tissue fluid (shading) and capillary blood pressure (numerical values in mm Hg) before, during, and after exposure to microgravity.

From Hargens et al. 111, and Levick and Michel 165 with permission
Figure 14. Figure 14.

Starling pressures which regulate transcapillary fluid balance. Pressure parameters which determine direction and magnitude of transcapillary exchange include capillary blood pressure (Pc), interstitial fluid pressure (Pt) (directed into capillary when positive or directed into tissue when negative), plasma colloidal osmotic pressure (πc), and interstitial fluid colloidal osmotic pressure (πt). (Precapillary sphincters (P.S.) regulate Pc and capillary flow. It is generally agreed that a hydrostatic pressure gradient (Pt > lymph pressure [Pt]) drains off excess interstitial fluid under conditions of net filtration. Relative magnitudes of pressure in resting tissues are depicted by the size of the arrows.

From Hargens 105 with permission
Figure 15. Figure 15.

Capillary blood pressure increased significantly in the lip within the first half hour of head‐down tilt (HDT) relative to seated control values, and remained elevated throughout HDT and in the seated recovery period. Lower bar indicates period of HDT.

From Parazynski et al. 204 with permission
Figure 16. Figure 16.

Change in echocardiographic left ventricular volume index in four Shuttle crewmembers during and after spaceflight, relative to preflight supine (top) and standing (bottom) levels. Letters (A–D) represent data from individual crewmembers; lines and dots represent mean data.

Modified from Charles and Lathers 52 with permission
Figure 17. Figure 17.

Percent of preflight resting heart rate in chronic micro‐gravity (1g baseline posture supine).

Modified from Charles and Lathers 52 with permission
Figure 18. Figure 18.

Fluid balance during the first 10 days of all Skylab flights, as compared to preflight control fluid balance (n = 9).

Modified from Leach et al. 157 with permission
Figure 19. Figure 19.

Simplified, hypothetical mechanisms whereby early existence in microgravity leads to reduced plasma volume. Acute reduction of interstitial fluid pressure in microgravity would elicit net capillary filtration, especially in relatively compliant upper body tissues.
Figure 20. Figure 20.

Heart rate response to simulated orthostasis (lower body negative pressure [LBNP]) before and during spaceflight. Data are from Shuttle (STS; n = 6) and Skylab (n = 9) flights.

Modified from Charles et al. 51 with permission
Figure 21. Figure 21.

Illustration of how reduction of extracellular fluid volume (ECFV) increases the heart rate necessary to maintain a given cardiac output (Q) for maintenance of blood pressure and flow, and thus metabolism and homeostasis. Chronic microgravity elicits actual reduction of ECFV. This reduction is functionally appropriate in flight, as long as no gravity‐like stress exists. Simulated (LBNP) or actual gravity, with or without exercise, elicits effective reduction of ECFV at the central circulation by displacing fluids footward. This simplified scheme ignores the important influence of vasoconstriction in maintaining blood pressure. With increasing age, vasoconstriction may become relatively more important than heart rate elevation for maintaining blood pressure during orthostasis 239.
Figure 22. Figure 22.

Preflight (upright) and in‐flight peak oxygen consumption of the three Skylab 4 crewmen during cycling exercise (X ± SEM).

Data from 179179; figure modified from Convertino 55 with permission
Figure 23. Figure 23.

Echocardiographic assessment of cardiac output, stroke volume, ejection fraction, end‐diastolic volume, and end‐systolic volume during rest and two levels of cycling exercise in two Salyut space station crewmen before and during a 237 day spaceflight (X ± SEM).

Preflight posture presumed upright; modified from Atkov et al. 10 with permission
Figure 24. Figure 24.

An example of potential interrelationships among systemic adjustments to microgravity: somato‐ and sympatho‐motor pathways by which vestibular effects of microgravity might influence cardiovascular function.
Figure 25. Figure 25.

Gz force during Shuttle re‐entry to Earth gravity from orbit, and corresponding heart rates of the Commanders (CDR) of the first four Shuttle flights. (Flight durations in parentheses)

Modified from Bungo and Johnson 41 with permission
Figure 26. Figure 26.

Supine‐to‐standing changes in leg volume (ml), total peripheral resistance (dynes X s X cm−5), stroke volume (ml), and heart rate (beats/min) before and on landing day after the SLS‐1 flight (n = 6; X ± SE). Standing data were collected at 5 to 6 min of standing. All supine‐to‐standing changes are significant except postflight leg volume. *, significant difference pre‐ to postflight at a given posture, P < 0.05.
Figure 27. Figure 27.

Pre‐ to postflight percent reduction in plasma volume, red cell mass, and total hemoglobin after flights of 10 to 96 days.

Data compiled in Ref. 55
Figure 28. Figure 28.

Negative linear relationship between pre to postflight percent reduction of blood volume and pre to postflight percent increase in orthostatically stressed heart rate. This association suggests that microgravity‐induced loss of blood volume accentuates heart rate elevation during actual or simulated (LBNP) postflight orthostatic stress. SMEAT: Skylab Medical Experiments Altitude Test, which was the ground‐based control study for the reduced atmospheric pressure maintained on Skylab.

Modified from Hoffler 117 with permission
Figure 29. Figure 29.

Negative linear relationship between pre to postflight percent reduction of left ventricular end‐diastolic volume index and preflight left ventricular end‐diastolic volume index. This relationship indicates that individuals with relatively large preflight weight‐specific end‐diastolic volumes (and probably relatively high aerobic fitness) exhibit greater postflight reduction of end‐diastolic volume than individuals with lower preflight end‐diastolic volume indices (posture presumed left lateral decubitus; n = 19).

Modified from Charles and Lathers 52 with permission
Figure 30. Figure 30.

Postflight decrease in cerebral blood flow pulsatility at 6 min of 30° head‐down tilt. Data are expressed as a percentage of preflight values for the same intervention. The ratio of variable to basal tissue impedance components provides the index of pulsed blood perfusion, which is considered proportional to vascular tone. Therefore, head‐down tilt elicits greater cerebral vasoconstriction after spaceflight, and the effect increases with flight duration.

Modified from Gazenko et al. 87 with permission
Figure 31. Figure 31.

Stroke volume during submaximal upright exercise after the Skylab flights, expressed as a percentage of preflight (n = 3 per flight). The authors hypothesized that increased in‐flight exercise during Skylab 4 (84 days; the longest Skylab flight) helped preserve blood and cardiac stroke volumes.

Modified from Michel et al. 179 with permission
Figure 32. Figure 32.

Interstitial fluid and foot venous pressures during lower body negative pressure (LBNP) with and without saline ingestion. The lower rectangle labeled LBNP in each graph represents the time period during which 30 mm Hg LBNP was applied to each subject. The subdivisions at the beginning and end of the rectangle represent 10 mm Hg increments and decrements of chamber pressure. The rectangle in each graph labeled saline represents the time period over which each subject drank 1 liter of isotonic saline. Values significantly different from baseline (time = 0) are indicated as solid squares or circles. Time points between the saline and no saline trials were not significantly different (X ± SEM; 6 ≥ n ≥ 4).

From Aratow et al. 3 with permission
Figure 33. Figure 33.

Four treatments employed by Breit et al. 31 to investigate effects of actual and simulated gravity on regional microvascular blood flow. Hypothetical regional arterial pressures are shown for each treatment. Treatment intensity was standardized to provide 1 Gz at the feet. Actual short‐arm G gradient was slightly less than illustrated, due to variation in subject height.
Figure 34. Figure 34.

Average normalized cutaneous microvascular blood flows measured by laser Doppler flowmetry at the neck, thigh and shin of humans during application and removal of gravitational stress. Eight male and seven female subjects (n = 15) underwent: 1) whole‐body tilting; 2) LBNP; 3) long‐arm centrifugation (radius = 7.6m); and 4) short‐arm centrifugation (radius = 2.4m) stepwise (0.2G steps; 30 s intervals) from 0 Gz (supine posture) to 1 Gz at the feet and back to 0 Gz, followed by a 5 min recovery period. One hundred mm Hg LBNP is approximately equivalent to 1 Gz at the feet.

From Breit et al. 31 with permission
Figure 35. Figure 35.

Comparison of representative recordings during upright exercise in 1 g and supine exercise in LBNP. Exercise consisted of full range of motion ankle plantar‐ and dorsiflexion at 25 cycles/min. a and b: reaction force waveforms; c: exercise‐induced LBNP chamber pressure oscillations; d and e: soleus intramuscular pressure (IMP: an index of muscle contraction force; 2); f and g: tibialis anterior (TA) IMP. Negative pressure transmission into muscles from LBNP was quantified and added to experimentally obtained LBNP IMP data.

From Murthy et al. 189 with permission
Figure 36. Figure 36.

Distribution of mean arterial pressure during running exercise. Pressure ranges represent inertial variations due to z‐direction acceleration forces associated with treadmill running up a 5° incline: (A) on Earth, and (B) in 100 mm Hg LBNP in microgravity. To provide a “normal” gradient of transmural pressure across blood vessels of the lower body, an anti‐LBNP suit compresses tissues from 70 mm Hg below the waist to 0 mm Hg at the foot.


Figure 1.

Blood pressure gradients hypothesized for a large terrestrial dinosaur and tree‐climbing snake (Pa = arterial pressure, Pc = capillary pressure, Pv = venous pressure).



Figure 2.

Expected distributions of tissue fluid (shading) and mean arterial pressure (numerical values in mm Hg) at head, heart, and feet during preflight standing posture on Earth (left), microgravity (middle), and postflight standing on Earth (right). During microgravity, all gravitational blood pressure gradients disappear, and only viscous blood pressure gradients exist between the arteries, capillaries, and veins.

Modified from Hargens 111 with permission


Figure 3.

Illustration of how regional vascular compliances (C1: upper body, and C2: lower body) affect cardiac filling pressure. Relatively stiff, noncompliant lower body tissues normally “push” the hydrostatic indifference level (HIL) towards heart level, such that height H1 produces only moderate reduction of cardiac filling pressure when orthostatic. If chronic, microgravity‐induced equilibration of regional vascular transmural pressures leads to relatively uniform regional compliances, the HIL shifts further away from the heart (H2 > H1). Upon subsequent orthostasis in gravity, cardiac filling pressure would thus be compromised, because it decreases in proportion to the height between the HIL and heart 85.



Figure 4.

Typical parabolic flight profile as flown by the National Aeronautics and Space Administration's KC‐135 based at Ellington Field, Texas. KIAS, knots indicated air speed.



Figure 5.

Middle cerebral artery blood flow velocity index (F mean of max.), mean arterial blood pressure (MAP), and heart rate in one seated subject during parabolic flight microgravity. Data represent means ± SD from five parabolas.



Figure 6.

Typical leg volume reduction during spaceflight, and post‐flight recovery of leg volume. Pre‐ and postflight measurements were made with the subject standing.

Modified from Moore and Thornton 183 with permission


Figure 7.

Illustration of importance of central venous pressure (CVP) to cardiovascular and fluid regulatory function, and thus to physiologic adjustments to microgravity (see text). ANP, atrial natriuretic peptide; ADH, antidiuretic hormone.



Figure 8.

Central venous pressure (CVP) in one crewmember before, during and after launch and orbit insertion in the Shuttle Columbia on the SLS‐1 flight. “Suit room” baseline data were collected in upright sitting posture. CVP was measured directly via a catheter with an ambulatory system designed for use in spaceflight conditions 36. SLS‐2 289 and D‐2 75 results were similar (n = 3).

From Buckey et al. 34 with permission


Figure 9.

Use of vascular compliance curves to illustrate two alternative explanations for reduced venous pressure in microgravity: A: increase in compliance due to reduced venous tone; B: reduction of mean circulatory filling pressure due to reduction of extravascular pressure.



Figure 10.

Reduction of end‐expiratory gastric (intra‐abdominal) pressure (Pga in cm H2O) during parabolic flight microgravity in four seated male subjects (X ± SEM). These data illustrate a positive linear relationship between extravascular pressure and gravity, and thus quantify how microgravity could indirectly reduce intracirculatory pressures.

Modified from Estenne et al. 74


Figure 11.

Resting leg compliance increased during and after spaceflight in the three SLS‐1 Shuttle crew‐members studied, relative to preflight supine conditions 272. Compliance was assessed with venous occlusion plethysmography 277 using hardware developed specifically for flight experiments 36 and a standardized protocol (20, 40, 60 and 80 mm Hg venous occlusions held for 1, 2, 3, and 4 min, respectively, with 1 min breaks between occlusions; 273).



Figure 12.

Comparison of mean middle cerebral arterial blood flow velocity between upright‐seated control, head‐down tilt (HDT), and upright recovery (X ± SEM; n = 8). *, significantly different from the corresponding value of upright‐seated control at same time of day, P < 0.05.

[From Kawai et al. 142 with permission.]


Figure 13.

Hypothesized regional distributions of tissue fluid (shading) and capillary blood pressure (numerical values in mm Hg) before, during, and after exposure to microgravity.

From Hargens et al. 111, and Levick and Michel 165 with permission


Figure 14.

Starling pressures which regulate transcapillary fluid balance. Pressure parameters which determine direction and magnitude of transcapillary exchange include capillary blood pressure (Pc), interstitial fluid pressure (Pt) (directed into capillary when positive or directed into tissue when negative), plasma colloidal osmotic pressure (πc), and interstitial fluid colloidal osmotic pressure (πt). (Precapillary sphincters (P.S.) regulate Pc and capillary flow. It is generally agreed that a hydrostatic pressure gradient (Pt > lymph pressure [Pt]) drains off excess interstitial fluid under conditions of net filtration. Relative magnitudes of pressure in resting tissues are depicted by the size of the arrows.

From Hargens 105 with permission


Figure 15.

Capillary blood pressure increased significantly in the lip within the first half hour of head‐down tilt (HDT) relative to seated control values, and remained elevated throughout HDT and in the seated recovery period. Lower bar indicates period of HDT.

From Parazynski et al. 204 with permission


Figure 16.

Change in echocardiographic left ventricular volume index in four Shuttle crewmembers during and after spaceflight, relative to preflight supine (top) and standing (bottom) levels. Letters (A–D) represent data from individual crewmembers; lines and dots represent mean data.

Modified from Charles and Lathers 52 with permission


Figure 17.

Percent of preflight resting heart rate in chronic micro‐gravity (1g baseline posture supine).

Modified from Charles and Lathers 52 with permission


Figure 18.

Fluid balance during the first 10 days of all Skylab flights, as compared to preflight control fluid balance (n = 9).

Modified from Leach et al. 157 with permission


Figure 19.

Simplified, hypothetical mechanisms whereby early existence in microgravity leads to reduced plasma volume. Acute reduction of interstitial fluid pressure in microgravity would elicit net capillary filtration, especially in relatively compliant upper body tissues.



Figure 20.

Heart rate response to simulated orthostasis (lower body negative pressure [LBNP]) before and during spaceflight. Data are from Shuttle (STS; n = 6) and Skylab (n = 9) flights.

Modified from Charles et al. 51 with permission


Figure 21.

Illustration of how reduction of extracellular fluid volume (ECFV) increases the heart rate necessary to maintain a given cardiac output (Q) for maintenance of blood pressure and flow, and thus metabolism and homeostasis. Chronic microgravity elicits actual reduction of ECFV. This reduction is functionally appropriate in flight, as long as no gravity‐like stress exists. Simulated (LBNP) or actual gravity, with or without exercise, elicits effective reduction of ECFV at the central circulation by displacing fluids footward. This simplified scheme ignores the important influence of vasoconstriction in maintaining blood pressure. With increasing age, vasoconstriction may become relatively more important than heart rate elevation for maintaining blood pressure during orthostasis 239.



Figure 22.

Preflight (upright) and in‐flight peak oxygen consumption of the three Skylab 4 crewmen during cycling exercise (X ± SEM).

Data from 179179; figure modified from Convertino 55 with permission


Figure 23.

Echocardiographic assessment of cardiac output, stroke volume, ejection fraction, end‐diastolic volume, and end‐systolic volume during rest and two levels of cycling exercise in two Salyut space station crewmen before and during a 237 day spaceflight (X ± SEM).

Preflight posture presumed upright; modified from Atkov et al. 10 with permission


Figure 24.

An example of potential interrelationships among systemic adjustments to microgravity: somato‐ and sympatho‐motor pathways by which vestibular effects of microgravity might influence cardiovascular function.



Figure 25.

Gz force during Shuttle re‐entry to Earth gravity from orbit, and corresponding heart rates of the Commanders (CDR) of the first four Shuttle flights. (Flight durations in parentheses)

Modified from Bungo and Johnson 41 with permission


Figure 26.

Supine‐to‐standing changes in leg volume (ml), total peripheral resistance (dynes X s X cm−5), stroke volume (ml), and heart rate (beats/min) before and on landing day after the SLS‐1 flight (n = 6; X ± SE). Standing data were collected at 5 to 6 min of standing. All supine‐to‐standing changes are significant except postflight leg volume. *, significant difference pre‐ to postflight at a given posture, P < 0.05.



Figure 27.

Pre‐ to postflight percent reduction in plasma volume, red cell mass, and total hemoglobin after flights of 10 to 96 days.

Data compiled in Ref. 55


Figure 28.

Negative linear relationship between pre to postflight percent reduction of blood volume and pre to postflight percent increase in orthostatically stressed heart rate. This association suggests that microgravity‐induced loss of blood volume accentuates heart rate elevation during actual or simulated (LBNP) postflight orthostatic stress. SMEAT: Skylab Medical Experiments Altitude Test, which was the ground‐based control study for the reduced atmospheric pressure maintained on Skylab.

Modified from Hoffler 117 with permission


Figure 29.

Negative linear relationship between pre to postflight percent reduction of left ventricular end‐diastolic volume index and preflight left ventricular end‐diastolic volume index. This relationship indicates that individuals with relatively large preflight weight‐specific end‐diastolic volumes (and probably relatively high aerobic fitness) exhibit greater postflight reduction of end‐diastolic volume than individuals with lower preflight end‐diastolic volume indices (posture presumed left lateral decubitus; n = 19).

Modified from Charles and Lathers 52 with permission


Figure 30.

Postflight decrease in cerebral blood flow pulsatility at 6 min of 30° head‐down tilt. Data are expressed as a percentage of preflight values for the same intervention. The ratio of variable to basal tissue impedance components provides the index of pulsed blood perfusion, which is considered proportional to vascular tone. Therefore, head‐down tilt elicits greater cerebral vasoconstriction after spaceflight, and the effect increases with flight duration.

Modified from Gazenko et al. 87 with permission


Figure 31.

Stroke volume during submaximal upright exercise after the Skylab flights, expressed as a percentage of preflight (n = 3 per flight). The authors hypothesized that increased in‐flight exercise during Skylab 4 (84 days; the longest Skylab flight) helped preserve blood and cardiac stroke volumes.

Modified from Michel et al. 179 with permission


Figure 32.

Interstitial fluid and foot venous pressures during lower body negative pressure (LBNP) with and without saline ingestion. The lower rectangle labeled LBNP in each graph represents the time period during which 30 mm Hg LBNP was applied to each subject. The subdivisions at the beginning and end of the rectangle represent 10 mm Hg increments and decrements of chamber pressure. The rectangle in each graph labeled saline represents the time period over which each subject drank 1 liter of isotonic saline. Values significantly different from baseline (time = 0) are indicated as solid squares or circles. Time points between the saline and no saline trials were not significantly different (X ± SEM; 6 ≥ n ≥ 4).

From Aratow et al. 3 with permission


Figure 33.

Four treatments employed by Breit et al. 31 to investigate effects of actual and simulated gravity on regional microvascular blood flow. Hypothetical regional arterial pressures are shown for each treatment. Treatment intensity was standardized to provide 1 Gz at the feet. Actual short‐arm G gradient was slightly less than illustrated, due to variation in subject height.



Figure 34.

Average normalized cutaneous microvascular blood flows measured by laser Doppler flowmetry at the neck, thigh and shin of humans during application and removal of gravitational stress. Eight male and seven female subjects (n = 15) underwent: 1) whole‐body tilting; 2) LBNP; 3) long‐arm centrifugation (radius = 7.6m); and 4) short‐arm centrifugation (radius = 2.4m) stepwise (0.2G steps; 30 s intervals) from 0 Gz (supine posture) to 1 Gz at the feet and back to 0 Gz, followed by a 5 min recovery period. One hundred mm Hg LBNP is approximately equivalent to 1 Gz at the feet.

From Breit et al. 31 with permission


Figure 35.

Comparison of representative recordings during upright exercise in 1 g and supine exercise in LBNP. Exercise consisted of full range of motion ankle plantar‐ and dorsiflexion at 25 cycles/min. a and b: reaction force waveforms; c: exercise‐induced LBNP chamber pressure oscillations; d and e: soleus intramuscular pressure (IMP: an index of muscle contraction force; 2); f and g: tibialis anterior (TA) IMP. Negative pressure transmission into muscles from LBNP was quantified and added to experimentally obtained LBNP IMP data.

From Murthy et al. 189 with permission


Figure 36.

Distribution of mean arterial pressure during running exercise. Pressure ranges represent inertial variations due to z‐direction acceleration forces associated with treadmill running up a 5° incline: (A) on Earth, and (B) in 100 mm Hg LBNP in microgravity. To provide a “normal” gradient of transmural pressure across blood vessels of the lower body, an anti‐LBNP suit compresses tissues from 70 mm Hg below the waist to 0 mm Hg at the foot.

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Article Title: The Cardiovascular System in Microgravity
 

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Donald E. Watenpaugh, Alan R. Hargens. The Cardiovascular System in Microgravity. Compr Physiol 2011, Supplement 14: Handbook of Physiology, Environmental Physiology: 631-674. First published in print 1996. doi: 10.1002/cphy.cp040129