The skin is sometimes referred to as a 'third kidney,' because it can act the same way the kidney does to remove toxins and fluid from the body through perspiration. Patients who have put off dialysis way too long will develop all over their skin what is called 'uremic frost,' which is a layer of toxins drawn out of the blood stream by the skin in its role as substitute kidney. On a plane, the dry air will draw more moisture out of everyone, whether they have renal failure or not, which is why airline staff are always forcing fluids on patients. Under normal conditions a person loses about 0.5 kilograms of fluid a day through the breath and through perspiration via the skin, but in very warm or very dry conditions, this amount can be greatly increased. A friend of mine was a captain in the Israeli Army and one of his roles, while marching troops through the Negev Desert, was to require them to stop every 15 minutes and drink a prescribed amount of water whether they wanted to or not. This was because so much fluid can be lost from the human body in hot, dry conditions that the thirst sensors of the body cannot register how much replacement fluid is really needed.