They always tell you to stretch and move your legs once a few hours on a flight to prevent blood clots, but why is this not a problem when we sleep daily and our legs are basically immobile for 8 hours?
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Your knees aren’t bent and your heart isn’t fighting gravity when you’re lying down. Bent knees can pinch blood vessels – not enough to stop blood flow, but enough to slow it down a bit and potentially trap clots enough for them to grow. Gravity also slows down your blood and causes that blood to pool in your legs. That lower circulation and pooling is what causes clots to start forming.
Yes, blood may pool a bit while you’re sleeping in whatever side of your body is down, but that’s distributed across a lot more of your body and without the restrictions of bent and crimped blood vessels. And, you’re a lot more mobile than you think at night. You may not be moving *much* but you’re still moving. You might roll over a bit and flex your muscles, which helps get your blood moving enough to prevent clotting.
The last big problem is when you stand up after a long plane ride you’re releasing all that pooled blood and potentially clotting blood all at once. Tiny *almost* clots are all moving together and may end up sticking together into a big clot. When you wake up, again any pooled blood is spread out so the tiny almost clots are spread out. You’re not getting up suddenly, you’re moving, stretching, rolling around, flexing muscles, sitting up…You’re giving a lot more time for your blood to start moving around and breaking up the almost clots before they can stick.
Gravity. Veins do not pump blood like arteries do. They have little flaps to keep the blood from flowing backwards. The blood is forced through them by contraction of your muscles and pressure from the arteries.
If you’re sitting for extended periods without using your leg muscles, gravity is holding the blood down and only blood pressure is pushing it up. If you were to flex your muscles while sitting your could induce some circulation. Getting up to walk naturally does that. If you don’t, blood pools in the legs and can clot.
When you’re laying down, there is no gravity pulling the blood down your legs and blood pressure can move it around easier.
does this relate to sitting in non flight environments too ? like sitting in an office chair or similar
Large passenger jets fly above clouds to avoid the weather, and in order to make it comfortable for the crew and passengers, the plane is pressurized.
Maybe not the full 14 psi near sea level, but there is “some” pressure. I also suspect that the atmosphere becomes slightly low in oxygen and high in CO2.
Obviously its not at dangerous levels, but those are the only factors that I think “might” have some effect…I don’t know how, but…it would seem nobody else does either.
Also, flying has a higher risk of worsening macular degeneration (vision). nobody knows why.
When I had surgery a few years ago they asked me to stop taking fish oil supplements because they thin the blood. Would they help prevent blood clots in this situation?
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