Low levels of oxygen. There are a few different ways to measure oxygen levels in your blood. Simply put, oxygen is necessary for your body to produce energy (to stay alive!) if the levels of oxygen are a little bit low, your body will slowly suffer. If its really low, the body cannot survive for long. There are several organs involved in delivering oxygen to your body. Which one is not functioning?
Red blood cells. The red blood cells in your circulation carry oxygen. Oxygen is necessary for cells to function (produce energy, metabolize food, reproduce, etc.) when red cells aren't doing their job, the tissues they supply lack oxygen and function poorly and/or die. Lung disease, heart disease, anemia, carbon monoxide and certain inherited disorders can cause you to be deficient in oxygen. Clear?