The cardiovascular system has the primary function of circulating blood through the body, and is composed of two main parts: the heart and the circulatory system. Driven through the body by the cardiovascular system, blood supplies nutrients to and collects wastes from cells, and maintains the body's internal environment by regulating the acid/base balance, fluid content and temperature of the body.
One of the many changes that occurs in the body during space flight is an acute change in the cardiovascular system. In a microgravity environment, the fluid contained in the tissues and blood begins to pool in the upper body, because it is no longer pulled down towards the feet by the Earth's gravity. The headward fluid shift triggers sensors in the cardiovascular system that detect and eliminate excess fluid pressure in the upper body. Other short-term changes include an increase in heart rate and an increase in heart size due to excessive fluid volume.
Over several days, both the blood volume and heart volume decrease, due to a decrease in thirst and an increase in water output by the kidneys. Within 3 to 5 days in space, total body water stabilizes at about 2 to 4% below the normal level and plasma volume decreases about 22%. After one week in space, most studies show a decrease in heart rate and an increased cardiac output. Inflight exercise performance may be diminished, but exercise has proven to be the single most effective method for reducing readaptation effects and maintaining a healthy cardiovascular system in space, as on Earth.
Researchers are focusing on understanding the effects of space flight on the cardiovascular system by studying cardiac output, heart rate, blood vessel behavior, blood pressure, and blood volume during space flight and upon return to Earth. Further study is needed to determine precisely when fluid shifts occur, because they are believed to be the forerunner of other physiologic changes that occur in microgravity.
The loss of fluid volume and the resulting changes termed "cardiovascular deconditioning," are normal adaptations of the human body to an extreme environment change. Upon return to Earth, the body must again readapt to Earth gravity. Because the blood volume is reduced after multiple days in space, not enough blood remains in the head and upper torso when gravity again pulls fluid downwards. Fainting or light-headedness, called orthostatic intolerance, may occur when astronauts remain upright. Low blood pressure and a high heart rate result, but most cardiovascular responses return to normal within 1 to 3 weeks of return to "1-G " gravity. Scientists study methods to reduce these cardiovascular readaptation effects, as in the experiments listed below, so that pilots landing spacecraft are not impaired and astronauts do not have long recovery periods once back on Earth.
Aerobic Capacity Using Graded Bicycle Ergometer
Cardiovascular Investigations: Adaptive Changes in Cardiovascular Control at µG (E712)
Cardiovascular Investigations: Autonomic Mechanisms During Prolonged Weightlessness (E709)
Evaluation of Thermoregulation during Long-Duration Space Flight
Gas Analyzer System for Metabolic Analysis Physiology
Physiological Responses during Descent of the Space Shuttle
Studies of Mechanisms Underlying Orthostatic Intolerance Using Ambulatory Monitoring, Baroreflex Testing, and the Valsalva Maneuver
Studies on Orthostatic Tolerance with the Use of LBNP
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Page last updated: 07/16/1999