eli5: Why do people who have major blood loss need transfusions?

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If a person gets an injury and has major blood loss they get a transfusion. How come their blood doesnt just replenish? If someone donates blood how come they dont need a transfusion after? In both scenarios people are losing blood but why cant someone who lost blood from an injury just naturally make more blood to fix that?

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Anonymous 0 Comments

Blood does replenish but it takes a while. It takes about 24 hours to replace the volume of a pint of blood and 4-6 weeks to replace a pints worth of red blood cells. If you’ve suffered major blood loss, you need blood NOW or you’re going to die in minutes.

When you donate blood, you only donate what you can spare. The average adult can spare 1 pint of blood to donate but that’s very different than getting shot or stabbed and losing 5 pints of blood.

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