MSFinals-5700

A 35-year-old teacher is tested for compatibility to donate a kidney to his older brother, who has end-stage renal failure. To his joy, he is found to be a suitable match. The patient is then thoroughly counselled regarding the operative procedure, short- and long-term risks, and the implications of living with one healthy kidney. He is particularly interested to learn how his body will adapt to having only one kidney.
What will be decreased in the donor after the kidney transplant?