MSF,
Living kidney donors lose about half of their functioning kidney tissue immediately. ("About half" because kidneys are never perfectly equal. Most centers will take the smaller of the 2 kidneys, if the center can see on the CT angiogram or equivalent that 1 is larger that the other.) However, within about 1-2 months, the remaining kidney "grows" in size (expands), but more importantly increases its functioning. On average, the increase is to 65% functioning or so of when the donor had 2 kidneys. (To put in another way, the remaining kidney has increased its function by 30% above what it used to be -- because before donation the 2 kidneys shared the load and thus were not working at maximum possible function.)
Similarly, the
recipient's "new/used" kidney also increases its function by almost 30%, so that the recipient's kidney function may get to at least 50-60% of original function. That level is compared to the level of kidney function on dialysis -- that I have read is only about 10% of original function. No wonder most recipients feel much better -- more energy, etc. -- after receiving a kidney compared to being on dialysis.
In this and many other ways, our bodies are "pretty cool," no?
Bill
PS. The difference in percentage increases given by Michael & I come from differences found in different research studies of different donors. Not surprising in medical science!