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One of the most profound circulatory changes that occur in man during exposure to microgravity is a cephalad redistribution of fluid. Central venous pressure (CVP) is not only an important standard for determining the cephalad redistribution of fluid but also the unique valuable means for measuring continuously cardiac filling pressure under simulated weightlessness (SW) and microgravity, and has a relationship with neurohumoral [correction of nerohumoral] regulation. With the development of manned spaceflight, the researches in space suggested that CVP did not increase, which is directly contradictory to the previous results under SW. This paper mainly discusses the changes of CVP and the effects of countermeasures on CVP under SW and microgravity, and the mechanisms of decreased CVP under microgravity. It is postulated that more suitable SW models, more studies about the area of low pressure including especially pulmonary circulation, and comprehensive studies in multiple systems are needed.

Objective. To accumulate data for further studies on the mechanisms of the decrease of cardiovascular function after weightlessness. Method. Cardiopulmonary circulatory function under supine position and head up tilting (HUT) for 20 min before and after exposure to 7 d 6 degrees head down tilt (HDT) in 6 healthy adults who aged 19-21 were measured with a XXH-2000 lesser (pulmonary) circulation and cardiac function instrument. Result. The 7 d HDT resulted in orthostatic intolerance of the subjects; the reduction of RVET (j-z) and the elevation of RPEP [RVET(Q-j)/(j - z)] indicated a decrease of the right heart function. An obvious decrease of the right heart reserves was also observed under HUT after HDT. Conclusion. hc and hc/hz were demonstrated to be useful parameters to evaluate orthostatic tolerance or to forecast orthostatic syncope. The lesser (pulmonary) circulation and cardiac function testing methods are valuable in evaluating the cardiopulmonary circulatory function during HUT.

Fluid is transferred cephalad in microgravity and simulated microgravity, and the pulmonary circulation is the first to be affected. Blood and fluid contents in the lungs increase, but unevenly distributed among various zones of the lungs. Blood vessels in the lungs are filled and distend. Capillary changes have been observed in the animal model of simulated microgravity. Studies about the changes of regulative function of pulmonary circulation are relatively rare. Observations of the reactivity of pulmonary vessels may help to understand the mechanisms of the changes in pulmonary circulation during microgravity.

Objective. Through the observations of dynamic changes of eNOSmRNA and iNOSmRNA in arterial endothelial cells of systematic circulation and pulmonary circulation under simulated weightlessness, to collect some data for studies of the adaptive mechanisms of local regulation in arterial systems. Method. Wistar rats were -30 degrees tail suspended to simulate the effects of weightlessness. The rats were randomly divided into three groups: control group (CON), 7-day tail suspension group (TS7) and 14-day tail suspension group (TS14). Changes of NOSmRNA expresses in endothelial cells of the thoracic aortae and pulmonary arteries were observed with in situ hybridization technique. Result. The eNOSmRNA and iNOSmRNA of thoracic aortic and pulmonary arterial endothelial cells in TS7 rats increased very significantly. The eNOSmRNA of thoracic aortic endothelial cells from TS14 rats returned to control level, but remained very significantly increased in pulmonary arteries. The iNOSmRNA in pulmonary arterial endothelial cells from TS14 rats decreased very significantly, but that in thoracic aortae returned to the control level. Conclusion. The responses of eNOSmRNA and iNOSmRNA in arterial endothelial cells of systematic circulation to tail suspension were similar, but they were different in pulmonary arterial endothelial cells, which might be due to the difference in the peak course of the shift of fluid from lower body entering the pulmonary or systematic circulation during initial period of simulated weightlessness. It could be a kind of important sign of depressed local regulative function under simulated weightlessness and might contribute to orthostatic intolerance after simulated weightlessness.

Electrocardiography (ECG) is still the only method for continuous medical observation during various stages of foreign manned spaceflight activities, and it also serves as an important item for medical monitoring in the project for manned spaceflight activities in our country. With electrocardiography, pulse or heart rate (one of the four life signs) and cardiac function can continuously be observed. Cardiac phase relative to ECG is a main index reflecting cardiac function. Understanding of the results of ECG and cardiac phase and their mechanisms during various stages of foreign manned spaceflight activities will be helpful in establishing emergency measures in the course of manned spaceflight activities in our country.

Arterial blood pressure (BP) is one of the four life signs in human beings, it could reflect cardiovascular function in some degrees. Thus, BP serves as an important cardiovascular parameter during various stages in foreign spaceflight activities. Although the reports about BP are not always in consistency during spaceflight, hypotension commonly occurs during orthostatic stress after spaceflight. The mechanisms of orthostatic intolerance after spaceflight are virtually the same as that of orthostatic hypotension after spaceflight. This paper simply reviewed the reports on BP during various stages in foreign spaceflight activities and the researches on the mechanism of orthostatic hypotension after spaceflight.

To clarify the effects of simulated microgravity on the local regulating function of the pulmonary circulation.Rat tail-suspension (TS) was used as the animal model to simulate the effects of microgravity. The changes in responses of pulmonary artery to several vasoactive agents were determined by using in vitro vessel rings perfusion technique.As for the endothelium-intact vessel rings from TS group compared with those from the control (C) group, the contractile responses of pulmonary artery to potassium chloride (KCl, 10 ~ 70 mmol/L) did not change obviously after 7 d tail-suspension, which were markedly diminished as to phenylephrine (PE, 10(-9) ~ 10(-5) mol/L). The vasodilatory responses to acetylcholine (ACh, 10(-9) ~ 10(-5) mol/L) were markedly enhanced, which did not change obviously as to captopril (Cap, 10(-5) ~ 3 x 10(-3) mol/L). The vasoreactivity of endothelium-removed pulmonary artery rings to all vasoactive agents did not change after 7 d tail-suspension as compared with those from control rats.The release of nitric oxide (NO) by rat pulmonary arterial endothelium was enhanced after 7 d tail-suspension. The vasodilatory function of pulmonary artery was enhanced.

The main differences between physiological effects of microgravity in spaceflight and simulated microgravity on humans appeared in the circulation of the low pressure side, in humoral and electrolyte metabolism. For a further understanding of the physiological effects of microgravity, some improvement of the current human experimental models are needed. It is possible to choose more adequate models closer to the situation in microgravity through measuring cardiovascular parameters, fluid regulation and renal excretion variables under head-up tilt (HUT) plus lower body positive pressure condition, or under head-down tilt (HDT) plus upper body negative pressure in various angles and pressure levels.

Orthostatic intolerance commonly occurs after spaceflight, but its mechanisms remain to be clarified. It is believed that the reduction of blood volume might be one of the important factors. So the current countermeasures against orthostatic intolerance are aimed to control the changes of blood volume. The main differences between physiological effects of microgravity and simulated microgravity on humans appear in the circulation of the low pressure side and in humoral and electrolyte metabolism. Reflexes elicited from the low pressure side are very important for regulation of the extracellular fluids. The authors postulated that for a further understanding of mechanisms of orthostatic intolerance after spaceflight and to establish appropriate countermeasures against orthostatic intolerance, it is important to study the course of changes of the circulation in the low pressure side and the reflexes thus elicited with more appropriate method of simulation.