The body can adjust to the lower concentration of oxygen and expansion of gas in the body.The body can adjust to the lower concentration of oxygen which is about 7% less than at sea level if the is not suffering from a heart,or blood disease. As cabin attitude increase and gas expand in the body as much as 25%,it can cause discomfort or problems in the abdomen [bloting or cramps],ear[crackling sounds or ear blockage] and respiratory/sinus difficuties.