Kidneys, an internal organ, have played an important part of several characters' lives.
After being "found" by his estranged mother, John Locke met his father, Anthony Cooper. The two began to bond, until Locke discovered Cooper was on dialysis, and that it was unlikely that he would receive a transplant in time. Locke, a match, agreed to do the transplant with Cooper. However, after the surgery, Cooper disappeared, leading Locke to realize that Cooper had conned him for the kidney. ("Deus Ex Machina") Locke developed anger issues because of the "theft," attending support groups and finding other attendees' problems insignificant when compared to his own. ("Orientation")
Years later, while on the Island, Sayid Jarrah questioned Locke about his scar from the transplant. Locke lied, calling it a war wound. Several days later, Locke was shot in the abdomen by Benjamin Linus. ("The Man Behind the Curtain") Locke recovered incredibly quickly from this wound, attributing this speedy recovery to the island as well as the fact that the bullet had gone through the space where his kidney had once been. Locke states that if his kidney had still been there, then he would have likely been dead, which is quite ironic due to the pain that having his kidney stolen had caused him in the past. ("Confirmed Dead") Actor Michael Emerson later hinted in an interview that this was why Ben shot Locke in that area. (Season 4 Arrives!)
Jack, while conducting surgery on Ben Linus to remove a spinal tumor, decided to use the leverage he had to help free his captive friends, Kate and Sawyer. Jack made a small incision on Ben's kidney sac, which would be fatal within the hour if Jack did not suture him up. ("I Do") This allowed Kate and Sawyer time to escape, despite the interference of the Others. After making sure that the two were safe, Jack sutured up the nick on Ben's kidney and Ben recovered. ("Not in Portland")