Several weeks into their mission, four members of the crew on the International Space Station remained motionless while ultrasound probes mapped the paths of blood in their necks, shoulders, and legs. The scans, part of Expedition 73’s “Ultrasound 2” protocol, are meant to look for telltale signs of venous thrombosis a risk elevated in microgravity, where the fluid balance of the body is dramatically changed. Blood and other fluids redistribute upwards from the legs to the head in orbit, distending central veins and elevating cephalad vasculature pressure. This redistribution, as it decreases the workload of the heart, may compromise venous return and enhance the risk of clotting.
NASA’s Zena Cardman and Oleg Platonov were the first astronauts to have the scans this rotation, followed by Mike Fincke and JAXA’s Kimiya Yui. The information will enter an expanding database of studies in space-induced circulatory alterations, which replicate some aspects of aging on planet Earth. These studies have indicated that astronauts in long-duration missions can suffer a 12% loss in left ventricular mass, elevated arterial stiffness, and decreased baroreflex sensitivity alterations that are responsible for orthostatic intolerance during return to gravity.
In addition to the imaging, Expedition 73 astronauts are also engaging in CIPHER, a wide-ranging set of 14 human studies. Through comparisons between physiological and psychological measurements from short- and long-duration flights, CIPHER will chart the course of adaptation and deconditioning. This encompasses monitoring cardiovascular metrics like stroke volume, cardiac output, and heart rate variability, as well as cognitive function and measures of energy stress. The long-term aim is to hone countermeasures for space travel to deep space, where microgravity will be added to by cosmic radiation and extended isolation.
Exercise is still a foundation for mitigation. Cardman’s workout on the Advanced Resistive Exercise Device was overseen in real time by experts on the ground, who checked her cardiorespiratory health and muscle strength. Such aerobic and resistive regimens are essential: without them, microgravity hastens muscle atrophy and demineralization of bone, and compromises the vascular smooth muscle necessary to modulate blood flow. Even with daily exercise, studies have reported a 23% arterial stiffening and reduced orthostatic tolerance following months in space.
The cardiovascular adjustments to weightlessness are multifaceted. Originally, stroke volume is increased over 30% by augmented venous return, but within weeks, plasma volume decreases by 10–15%, and the heart also remodels to a more spherical, less effective form. For some astronauts, jugular vein cross-sectional area increases significantly, and instances of stagnant or even retrograde venous flow have been seen states associated with Spaceflight Associated Neuro-ocular Syndrome (SANS), leading to permanent vision disturbances. The ISS ultrasound treatments are intended to detect early symptoms of such vascular adaptations before they become irreversible.
For the just-returned SpaceX Crew-10 astronauts, the challenge is the opposite: re-adapting their cardiovascular systems to Earth’s gravity after five months in space. Rehabilitation at NASA’s Johnson Space Center involves restoring plasma volume, reinforcing muscle tone, and retraining the autonomic nervous system to respond to changes in blood pressure during posture changes. Without this, the risk of dizziness, fainting, and reduced cerebral perfusion remains high. Data from returning crews help validate in-flight countermeasures and guide postflight recovery protocols.
The engineering behind these investigations is just as important as the biology. Motorized probes and ground-controlled beam steering in ISS ultrasound systems take high-resolution images of blood vessels, allowing for reliable data collection by inexperienced operators in space. Exercise equipment such as ARED is intended to replicate Earth-like loading under microgravity conditions through the use of vacuum cylinders to generate up to 600 pounds of opposition. Collectively, the equipment constitutes a technological defense against the cardiovascular costs of space travel.
As the station’s medical research keeps going, the discoveries will benefit Mars mission planning as well as Earth’s medicine. The similarities between cardiovascular deconditioning due to microgravity and aging-related deterioration back on Earth mean that information gained in orbit could help create new treatments for heart failure, vessel stiffness, and clotting prevention. For the time being, with each ultrasound scan, each vial of blood and urine drawn under CIPHER, and each exercise session carefully monitored brings scientists closer to comprehending and defeating one of the most pernicious threats of space travel.
