The term cardiovascular means heart (cardio) and blood vessels (vascular). They work together in a synergistic and complimentary fashion to ensure adequate oxygenated blood supply to the entire body. There are many different factors that can influence how that happens, gravity being one.
In fact, if you think about it, its’ the very reason why the heart is such a strong muscle; and as to why it weakens in space. Let’s go through some fundamental physiology first. Gravity is a force which pulls everything downwards, including our blood. Blood therefore accumulates around our ankles and calf muscles when we stand up, and then disperses relatively evenly throughout our body when we lay down. Think about an empty bottle of water that you have, which you have now filled up by a quarter (for no other reason than display purposes). Picture how the water sits at the bottom of the bottle when it’s upright (equivalent to you standing). Now lay the bottle down and watch how the water evenly disperses (equivalent to you laying). This is a very simple but effective experiment to understand how our own blood distributes during different activities.
Now…if we don’t have blood in our head, we faint. So how do we get that blood all the way back up to our head? Well firstly we have a venous pump located near the calf muscles. This is a mechanism by which many of the veins in this portion of the leg will constrict, and essentially “push” blood upwards. Secondly, we have our very own pressure sensors located in and around our neck, called Baroreceptors. They sense that the pressure at the level of the neck has fallen, so they call upon the autonomic nervous system to increase heart rate and constrict numerous veins so that blood can get back up to the head level quickly.
Now let’s turn to spaceflight. With regards to the cardiovascular system, unlike the musculoskeletal system being primarily affected during spaceflight (see previous blog), most of the real issues are experienced after spaceflight, but as a result of certain adaptations during flight.
Within the first two days of being in space, blood moves from the lower body to the upper body because it doesn’t have gravity pushing down on it; in fact astronaut’s bodies are as though they are lying down, so blood is evenly dispersed as per the blurb above. So… spaceflight removes the requirement for blood to move from the lower limbs up to the head, which has knock on effects to all of those mechanisms which usually do so. 1. Those veins in the lower limbs that usually help to pump blood upwards can become pliable (elasticky) and therefore less effective. 2. Those baroreceptors have been so out of touch with not having to sense pressure changes that they become less sensitive to any changes. 3. The heart has been so used to not having to help pump blood back up to the head, that it becomes a little less efficient, and somewhat weaker (astronaut-dependant).
This all leads to one of the biggest issues for astronauts on return to Earth – the difficulty standing up without feeling faint (it’s not unusual for some to actually faint, or at least get close to it). All of a sudden, those mechanisms have to deal with the gravity-imposed regime of moving blood from the feet to the head quickly, and they just can’t cope. Thankfully, as quickly as the body adapts to weightlessness, it can adapt back to gravity, so within a couple of weeks, this resolves.
Although still an issue, the prevalence of “Orthostatic Intolerance” – the term used to describe this inability to remain in the upright posture – is far less nowadays compared to what it used to be. Countermeasures (which I have already mentioned in my previous blogs) are very advanced and help to reduce the severity of this problem. An entire blog piece on countermeasures will follow soon.
Interestingly, there are many different disorders on Earth that cause people to have Orthostatic Intolerance. Understanding what happens to astronauts in space is one of the many ways that we can help find advances in our understanding of them and the medical approach to help them.
Over and out… for now…