The cardiovascular system is a system which allows blood and lymph to circulate blood cells, oxygen, amino acids, etc., throughout the body. Although it continues to function in space, it is drastically altered. Firstly, the volume of blood greatly decreases, due to a reduction of plasma and red blood cells caused by the movement of albumin-containing fluids and low levels of erythropoietin (which controls red blood cell production). Also, the blood itself is redistributed due to the lack of gravity; the majority of blood shifts to upper body (whereas Earth’s gravity usually pulls it to the lower extremities). In addition, without the added stress inherently involved in counteracting gravity, the heart atrophies. Some astronauts even experience cardiac rhythm disturbance, due to stress, sleep loss, and/or electrolyte imbalance. In addition, aerobic capacity and orthostatic tolerance decrease, causing symptoms such as light-headedness, lower systolic blood pressure, and so on.
This last symptom is the most pressing for interplanetary missions, as astronauts may be required to commence work as soon as they land. Also, crew members may be required to perform prolonged EVA (Extravehicular activity) in suits with limited cooling. Such work would require superior endurance, heat tolerance, and exercise tolerance, all of which can be affected by the space environment.
Current countermeasures call for aerobic exercise to increase oxygen uptake and endurance, in preparation for demanding EVA or the possibility of an emergency. Such a regimen is accompanied by strength training, which can help maintain cardiac mass. Also, heat acclimatization (building up heat tolerance) could improve endurance and work capacity in hot temperatures. Intermittent artificial gravity would also be an effective solution, as it may minimize orthostatic intolerance.
LBNP (Lower Body Negative Pressure) is another possible solution which involves lower body suction. Such a device reduces central blood volumes (moving blood back to the feet) and increases fluid retention by applying suction to the lower body. It has an immediate effect on blood pressure, heart rate, and stroke volume, although it is time-consuming. In addition, drugs such as vasoconstrictors, erythropoietin, and fludrocortisone can be used to increase plasma volume, and mediate other issues with the cardiovascular system. Even simple things like exposure to low levels of oxygen, fluid loading, and body cooling can help.
Individual monitoring, a central idea of Astro-Omics, is also useful in assessing the effectiveness of an individual countermeasure program. Holter monitoring, ultrasounds, exercise examinations, and lower body negative pressure tests can reveal arrhythmias, changes in muscle mass, physical fitness trends, and changes in orthostatic function. Such tests should guide a personalized treatment plan, including exercise and medication.