Roger Seheult, MD
Some people have inherited tendencies to clot. You don't know unless you check. When you sit in the same position for long periods of time, the blood stagnates. This causes it to clot in your legs. When they break free, they are carried by the blood in the veins to the right side of the heart and then on to the branches of the pulmonary (lung) arteries which get smaller and smaller until the clot lodges and blocks the flow of blood to the lungs. This places a pressure on the right side of the heart which cannot tolerate that and it can stop. Sometimes people survive and other times they collapse. Coughing up blood is a symptom sometimes. This happens to even athletes: https://7news.com.au/sport/cycling/australian-olympian-michael-matthews-stops-all-training-amid-life-threatening-health-issue-c-19041322