Drentel - If you think that is scary, read this. I saw a stat that said 5 percent of all plane travelers get blood clots. This is largely because of cabin pressure and dehydration which cause the blood to thicken.
That's why it is important to take something to drink when the flight attendants come around with the cart, and that's why you should get up and walk around on long flights. Also, you should be aware of blood clot symptoms.