**3.2 Decrease in the circulatory blood volume**

The centralization of body fluid induces dehydration to adapt to the microgravity environment. The cephalad fluid shift causes an increase in venous return and marked increase in the stroke volume, inducing the alterations in the autonomic and endocrine systems to control the cardiovascular system.

On the first day of microgravity exposure, urine volume does not increase, but the circulatory blood volume suddenly decreases by 17%, probably due to the shift of water from the intravascular to the interstitial spaces and finally to the intracellular space. This induces an increase in the hematocrit level, which suppresses erythropoietin production and reduces the erythrocyte volume. Reductions in the circulatory plasma volume and erythrocyte volume equal an 11% reduction in the total blood volume, and this stabilizes the central blood volume to a new equilibrium, which nearly equals the central blood volume in the standing position at 1 G on Earth.

Upon the above alterations, the autonomic nervous system stabilizes the blood pressure by suppressing the sympathetic functions and activating the vagal functions by reducing the heart rate and suppressing muscle sympathetic nerve activity. Alterations include suppression of vasopressin by the Henry-Gauer reflex, facilitation of α-natriuretic peptide secretion, and suppression of the renin-angiotensinaldosterone system, all of which facilitate urination. Thus, centralized body fluid is excreted, accounting for 10–15% of the circulatory blood volume, increased hematocrit level, and adaptation of the cardiovascular system 5–7 days after microgravity exposure. This ameliorates the facial edema and jugular distension. This adaptation causes cardiovascular deconditioning, including orthostatic intolerance, after returning to 1 G on Earth.

### **3.3 Reduced heart size**

Once exposed to microgravity, the volume and pressure stimuli disappear. Constant postural change of lying down from upright standing on Earth loads intermittent volume on the heart, which ceases during microgravity. In addition, microgravity reduces the overall pressure load on the heart depending on the content of the countermeasure program. The mean arterial pressure slightly decreases. During spaceflight, the myocardial volume decreases by 8–10%.

#### **3.4 Cardiovascular system after stabilization**

After adaptation to microgravity, the cardiovascular system stabilizes, and the blood pressure is either unchanged or slightly lower [19]. Ambulatory blood pressure recording for 24 hours in eight astronauts revealed that the systolic, diastolic, and mean arterial pressures (mean ± se) in space were reduced by 8 ± 2 mmHg (*p* = 0.01; ANOVA), 9 ± 2 mmHg (P < 0.001), and 10 ± 3 mmHg (*p* = 0.006), respectively, with a maintained nocturnal dip of 8 ± 3 mmHg (*p* = 0.015). The cardiac stroke volume and output increased by 35 ± 10% and 41 ± 9% (*p* < 0.001),

**17**

thetic tone.

*Effects of Microgravity on Human Physiology DOI: http://dx.doi.org/10.5772/intechopen.90700*

**3.5 Alteration of aerobic exercise capacity**

output.

respectively, the heart rate and catecholamine concentrations were unchanged, and

Microgravity exposure reduces the circulatory blood volume; however, the maximal oxygen uptake is maintained after a short duration of spaceflight. During long-term spaceflight, the aerobic capacity decreases without countermeasures, but aerobic exercise training can maintain it, although standard exercise only markedly reduces the postflight maximal oxygen uptake. After a short duration of spaceflight (9–14 days), the maximal oxygen uptake decreased by 22%, probably due to decreases in the maximal stroke volume and maximal cardiac output without alterations in the maximal heart rate, blood pressure, or wholebody arteriovenous oxygen. This decrease in maximal oxygen uptake is believed to be due to the decreases in intravascular blood volume, stroke volume, and cardiac

As crew members are expected to work on the Moon/Martian surface, and they are exposed to extensive heat stress in the extravehicular suits, this aerobic capacity

Sympathetic neural traffic indirectly measured by the plasma noradrenaline level has been reported to increase during spaceflight from the preflight control level [14, 20], and vagal activity estimated by power spectral analysis of heart rate

Microneurographically recorded neural traffic in humans is known to be muscle and skin sympathetic nerve activity (MSNA and SSNA), and MSNA controls the vasomotor function of the muscular bed, responding to blood pressure changes against gravitational stress [23–25]. MSNA was suppressed during exposure to short-term microgravity induced by parabolic flight [26], mild lower body positive pressure (10–20-mmHg LBPP) [15], and thermoneutral head-out water immersion [27] responding to the loading or unloading of cardiopulmonary receptor-stimulated cephalad fluid shift. On the other hand, MSNA was increased after exposure to long-term microgravity in spaceflight and its simulation induced by dry immersion [28] or 6° head-down tilt bedrest [17] due to different mechanisms, including

Orthostatic intolerance is usually observed after returning to 1 G on Earth. The definition of orthostatic intolerance usually includes simple syncope, lightheaded-

Less important factors for postflight orthostatic intolerance are reduced compliance of the lower legs, reduced baroreflex sensitivity, and increased basal sympa-

Astronauts usually notice orthostatic intolerance during prolonged upright standing rather than while standing up. Just before fainting, they sometimes have tachycardia, suggesting that they have postural orthostatic tachycardia syndrome (POTS). This phenomenon is due to a state in which fluid shift easily triggers tachycardia, which also easily triggers Bezold-Jarisch reflex, and the vagal response suppresses the systolic blood pressure. Although all astronauts stood upright for

systemic vascular resistance was reduced by 39 ± 4% (*p* < 0.001).

is considered to be significant after landing on the Moon/Mars.

variability was reduced after long-term spaceflight [21, 22].

**3.6 Alterations in sympathetic neural traffic under microgravity**

plasma volume loss, changes in baroreflex, and vascular compliance.

ness, or >20-mmHg reduction in systolic blood pressure.

10 min, 63% were unable to finish the stand test in 10 min.

**3.7 Postflight orthostatic intolerance**

*Beyond LEO - Human Health Issues for Deep Space Exploration*

**3.2 Decrease in the circulatory blood volume**

and endocrine systems to control the cardiovascular system.

a negative balance 2010 of decreased fluid intake and smaller reduction of urine output; (2) fast fluid shifts from the intravascular to interstitial spaces as a result of lower transmural pressure after reduced compression of all tissues by gravitational forces, especially of the thorax cage; and (3) fluid shifts from intravascular to muscle interstitial spaces because of lower muscle tone required to maintain body posture, and the attenuated diuresis during space flight is due to increased retention after stress-mediated sympathetic activation during the initial phase of space flight.

The centralization of body fluid induces dehydration to adapt to the microgravity environment. The cephalad fluid shift causes an increase in venous return and marked increase in the stroke volume, inducing the alterations in the autonomic

On the first day of microgravity exposure, urine volume does not increase, but the circulatory blood volume suddenly decreases by 17%, probably due to the shift of water from the intravascular to the interstitial spaces and finally to the intracellular space. This induces an increase in the hematocrit level, which suppresses erythropoietin production and reduces the erythrocyte volume. Reductions in the circulatory plasma volume and erythrocyte volume equal an 11% reduction in the total blood volume, and this stabilizes the central blood volume to a new equilibrium, which nearly equals the central blood volume in the standing position at 1 G

Upon the above alterations, the autonomic nervous system stabilizes the blood pressure by suppressing the sympathetic functions and activating the vagal functions by reducing the heart rate and suppressing muscle sympathetic nerve activity. Alterations include suppression of vasopressin by the Henry-Gauer reflex, facilitation of α-natriuretic peptide secretion, and suppression of the renin-angiotensinaldosterone system, all of which facilitate urination. Thus, centralized body fluid is excreted, accounting for 10–15% of the circulatory blood volume, increased hematocrit level, and adaptation of the cardiovascular system 5–7 days after microgravity exposure. This ameliorates the facial edema and jugular distension. This adaptation causes cardiovascular deconditioning, including orthostatic intolerance,

Once exposed to microgravity, the volume and pressure stimuli disappear. Constant postural change of lying down from upright standing on Earth loads intermittent volume on the heart, which ceases during microgravity. In addition, microgravity reduces the overall pressure load on the heart depending on the content of the countermeasure program. The mean arterial pressure slightly decreases.

After adaptation to microgravity, the cardiovascular system stabilizes, and the blood pressure is either unchanged or slightly lower [19]. Ambulatory blood pressure recording for 24 hours in eight astronauts revealed that the systolic, diastolic, and mean arterial pressures (mean ± se) in space were reduced by 8 ± 2 mmHg (*p* = 0.01; ANOVA), 9 ± 2 mmHg (P < 0.001), and 10 ± 3 mmHg (*p* = 0.006), respectively, with a maintained nocturnal dip of 8 ± 3 mmHg (*p* = 0.015). The cardiac stroke volume and output increased by 35 ± 10% and 41 ± 9% (*p* < 0.001),

During spaceflight, the myocardial volume decreases by 8–10%.

**3.4 Cardiovascular system after stabilization**

**16**

on Earth.

after returning to 1 G on Earth.

**3.3 Reduced heart size**

respectively, the heart rate and catecholamine concentrations were unchanged, and systemic vascular resistance was reduced by 39 ± 4% (*p* < 0.001).
