If a person has to remove one kidney, or is an organ donor, what are the side effects of only having one kidney?
There are a number of reasons someone might live with just one kidney, including the reasons you mentioned. Donation and removal due to illness aren't the only reasons someone might live with just one kidney, though. In fact, about one in every seven-hundred and fifty people is born missing one of their kidneys! It is possible to live a long, healthy life with just one kidney, but a person in this condition must be careful not to put too much strain on the extant kidney.
People with just one kidney are at a higher risk for high blood pressure. This may require medication in some cases. Additionally, because just one kidney is performing a job typically done by two, a person may have trouble filtering metabolic waste from their blood stream. This can result in kidney disease if the levels of waste in the blood are too high. Some people may also develop a condition called proteinuria where too much protein is taken from the blood and expelled through urine. An imbalance like this can cause edema or fluid retention in the body.
Long term, someone who lives with just one kidney might experience a decrease in kidney function due to the amount of work this kidney has to do. Eating a balanced diet and drinking lots of water helps to maintain the appropriate levels of water and nutrients in the blood and can help prevent kidney disease. It is also important for people with a single kidney to protect it if they choose to play sports, so as not to damage the organ.