Blood does replenish but if you're losing it faster than you can make it or lost a lot at once you need a transfusion to get enough blood to stay alive long enough to fix the problem and then make enough more of your own. The body needs time and resources to make more blood and can compensate for small losses like a donation or minor wounds but anything more and it's gonna need some help

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.

You need to have a blood pressure within a certain range for your organs to function properly. If there is no blood available for a transfusion, you should at least add some water to the system.

The amount of blood given in donation is well within the limits that the body can survive for a time without, and readily rebuild and come back from.

With traumatic injury, when massive amounts of blood are lost at once, there is not enough left to keep oxygen and nutrients flowing to the heart and other organs. The transfused blood brings the body back up to a functional level of blood, and then the body can take over and make more.

It does replenish by itself, but it takes time. If you’re bleeding out faster than you can make new blood, you need some help.

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The people who donate blood donate a much smaller amount than the people who lose it and need a transfusion. Also, the people who donate blood need to be in perfect health and they can't take meds. The blood needs to be 'pure'. You get asked a lot of questions before you donate blood and if you're eligible, then they give you a physical and test your haemoglobin. That's basically the iron. If it's too low, or if your blood pressure is too low, they won't let you donate blood because you can pass out and it's not healthy. Also, after some surgeries and accidents resulting in bloodloss, you don't need a transfusion. It really depends on the amount.

blood is the highway your body uses to transport oxygen and some other stuff to the organs and muscles to keep them functional. while the body does make blood over time, it takes time as well as water ingestion. a person who has quickly lost a large amount of blood cannot regenerate it in time to prevent death. they require it immediately or their systems will shut down and they will die.

tldr can't make more blood fast enough in such instances

An injury that leads to tons of blood loss is very traumatic for the body. A needle that extracts some blood barely registers. These aren't equivalent things.

People needing blood often need multiple transfusions as they are actively loosing blood. The transfused blood buys surgeons time to find and stop the bleed(s). A person donating blood is losing a controlled amount of blood that is safe to lose.

If you lose too much blood organs and tissue start to die due to lack of oxygen. We can't reverse this damage and depending on what died our body is also limited or incapable of repairing the damage too.

Transfusions try to maintain enough blood in the body so the heart and lungs or a bypass machine can keep your body saturated with oxygen. A person donating has no risk of this.

Your body can't just magic out liters of blood. Where would it come from?