The other commenter gave a good explanation so I’ll just provide some other insight about organs and transplant.
The bladder has incredibly complex wiring. Whenever you want to pee, there has to be a talk between the wiring in your body that you consciously control and the wiring in your body that you can’t consciously control. These different systems have a variety of receptors that then have to work in coordination to relax certain muscles and contract other muscles. This is much different from a lot of other systems in our bodies which are (for the most part) controlled by one wiring system or the other. Pooping is similar in that there is an automatic and voluntary aspect to it. This is why people who have neurologic damage or conditions will often have difficulty urinating or pooping even after recovery of other systems.
When you remove an organ you take all of its blood supply and nerves (wiring) with it. If you wanted to transplant a bladder you would have to rehook this complex wiring back together so the brain could then tell the new bladder when it wants to pee and when it doesn’t.
With other organs, wiring is not as much of an issue because the these organs work (for lack of a better term) by themselves or via processes that don’t require reconnecting the wires. The heart for example does not need input from the brain to pump, it will beat by itself. And if we need it to pump faster or slower, we can put in a device to make it do that.
That is a very simplistic way of explaining these complex physiological processes, but I hope that helps provide context.
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