Eli5 How does blood know when to clot?

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When does your blood know when its the right time to form a clot?
Its supposed to happen only when blood leaves the body. If it happens in the vessels its a pretty bad idea to clot.
My first thought was it must be the contact to oxygen that triggers the clotting process. But thats nonsense since blood tranaports oxygen, right?
So how does your blood “know” its leaving the body?

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7 Answers

Anonymous 0 Comments

The default state of blood is clotting. Blood vessels secrete a chemical that inhibits clotting. Once the blood leaves the vessel the chemical is no longer present and the clotting process starts.

When you donate blood there is an anti-coagulant in the bag because even though there is no exposure to oxygen (vein -> needle -> tube -> bag) the blood will still clot if left on its own. Found that out when I had a blood donation fail partway through and it rendered what had been taken unusable because there was too much anti-coagulant in the bag for the volume of blood.

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