The blood was painted on the door posts of the homes of the Israelites.
They put lamb's blood on their doors.
They were told to slay a lamb and smear the blood on the door of their houses , so that when the angel passes that way he will pass the houses with the blood on the door.
The "little door" refers to the valves that are present throughout the venous blood vessles, and in the heart chambers. They prevent blood from flowing the wrong way in the vessels.
There are valves in the blood vessels (like a one-way door) - that stop the blood flowing backwards.
Michael Gambon
It was a mark of blood from the Passover sacrifice (Exodus ch.12).It was on the lintel, the beam at the top of the door frame, not on the doorstep.Also on the door posts (the uprights on the sides of the door frame).And it was God who spared them, not the blood that saved them.
If a hamster is stuck in your door you should see blood and hear loud squeaking noises. yuytur
I believe it is the ventricles. It collects and expels blood. It is like an automatic door that closes and shuts itself.
Only if you smear goat blood over your door.
valves
The blood stains in Marco Polo's homebase in the level "The Dig" lead to a secret door that opens up when you interact with the blood stains. This door reveals a hidden passage for you to progress further in the game.