NASA has a goal to send the first woman and first person of color to the moon. From there, a launch point will be established for the first human mission to our galactic neighbor, Mars. This is no small feat and would expose people to the perils of space for extended periods.

Space travel has a broad range of detrimental effects on human physiology. These range from muscle loss to problems with vision. Interestingly, some of these problems show up only after returning to Earth. One such problem is called “orthostatic intolerance.” This condition is characterized by symptoms that occur upon standing, including lightheadedness, dizziness or even fainting.

When we go from lying or sitting to standing up on Earth, gravity causes pooling of blood from the chest down to the legs, causing blood pressure to drop abruptly. When this happens, our body enacts certain reflexes to help our blood pressure get back to normal and maintain blood flow to the brain. When blood pressure is not maintained and there isn’t enough blood flow to the brain, symptoms of orthostatic intolerance may occur. Orthostatic intolerance has been widely observed among astronauts postflight and can happen quickly, in as little as eight days of spaceflight.

When gravity is removed from the equation of human physiology, such as during spaceflight, blood shifts away from the legs and toward the torso and face. Astronauts describe this as “puffy head bird legs.” This redistribution of blood from the lower extremities to the face can cause other problems such as blood clots and may also explain why orthostatic intolerance occurs. Small sensors called baroreceptors detect stretch within major blood vessels, including the aorta and carotid arteries. The baroreceptors respond to abrupt changes in blood pressure, such as going from sitting to standing. Past research has shown that in simulated microgravity in rats, baroreceptor sensitivity is impaired and the sensors are not able to respond normally to changes in blood pressure. In other words, after returning from space, the body may not be able to detect changes in blood pressure and adjust like it used to, which leads to orthostatic intolerance.

Fortunately, it seems as though orthostatic intolerance symptoms only happen when astronauts are standing still and not going about their daily activities. This is because the leg muscles act as a “pump” to prevent blood pooling in the legs and the subsequent large drops in blood pressure. Exercising during spaceflight seems to prevent symptoms of orthostatic intolerance, too. Another factor is that in space, we lose blood volume, which can cause abrupt drops in blood pressure. But in this case, returning to Earth and a normal blood volume can prevent symptoms.

Successfully treating conditions associated with spaceflight is only the beginning. To paraphrase astronaut Neil Armstrong, it’s “one small step” for humankind and “one giant leap” toward colonizing Mars.