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Your arteries and veins form a pressurized system. If you seal a hole well enough (stronger than the systolic pressure of the heart), it will prevent any blood from leaking out. When you close your bathroom faucet it doesn't cause water to start leaking out somewhere else in your house.

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But if I cut the pipe leading to the faucet then merely shut the cupboard door, my cupboard is not going to be very happy.

I think he's comment was that the gel isn't necessarily sealing the arteries/veins, only the 'hole in your leg' - the assumption being this could still result in fatal internal bleeding - that more traditional methods such as pressure on the wound or a tourniquet could have had better success with.

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