I can see issues with the widely available market for blood donors, but I disagree about "paying down loans with blood, skin, and kidneys".
If you do physical work you already destroy some muscles, and if work is stressful you destroy your kidney trying to get relief by drinking yourself out. The skin on your hands also gets damaged from hard work. Or skin on your body after constant exposure to the sun.
We should strive for a society where we do not have to work ourselves out to pay off the loan, and not focus on preventing people from paying off loans destroying their bodies specifically, no matter if it is blood donation, or herniated disks, or destroyed lungs, or anything in between.
> We should strive for a society where we do not have to work ourselves out to pay off the loan
When we have such a society we can reevaluate. But right now we don't, and in the current environment creating more ways for inequality moves us away from such society.
That would be yet another way to increase inequality.
Do you really want to force people to pay down loans with blood, skin and kidneys? Because that will inevitably happen if allowed.