We see that when people fail their apparition tests, there's a bit of splinching that happens. It can be funny (an eyebrow was left behind) to horrific and gory (Ron leaving a bit of himself behind in Deathly Hallows). My question is, can internal organs be left behind (like a bit of kidney or even brain).
In Half Blood Prince, Susan Bones is splinched during one of their lessons in the Great Hall. She leaves her entire leg behind. Therefore roughly a quarter or fifth of the body was removed and left standing there. A great, thick width of flesh, bone and skin was sliced through for this to happen.
Surely this, combined with the aforementioned quote about leaving half of themselves behind (@Valorum), and the grave warnings that come with Apparition lessons, is pretty strong evidence that internal organs are surely within considerable risk during Apparition. Some of them are far nearer to the skin than slicing a leg off. Few are protected by as much bone as would be broken to remove a leg. I don't see any reason that they should be protected - apart from the author not wanting to be too gross!