Red blood cells bound to one another by fibrin, our bodies' principle coagulative protein.
A favorite question on physicians' medical boards: "a lady comes into the ER short of breath and with sharp chest pain after a flight. What will you look for?"
Answer: a pulmonary embolism! A pulmonary embolism is a clot that forms in the veins (usually leg veins), that then travels through the venous system into the heart, which in turn pumps it into the pulmonary arteries. When a clot travels there, a portion of the lungs can't receive blood from the heart, causing shortness of breath.
But why after a flight?? 🤔 During a flight, we sit for hours at a time, allowing blood to pool in our leg veins. Well this pooling, termed "stasis", is actually breeding grounds for blood clot formation. When blood pools, rather than moving through vessels, cells have time ⏳⌛️to stick to one another. While arteries have a pump behind them to push blood through (aka the heart! ♥️), our veins have no such pump and thus rely on contractions of skeletal muscles to push blood back to the heart.
To prevent blood pooling on flights: focus on contracting the leg muscles, from scrunching toes, to squeezing calves, to extending thighs around every hour. But any sort of stretching or just walking to the lavatory 🚶🚶 works as well!