Both humans and pandas are placed in the class Mammalia and therefore, there is little difference between their respiratory systems. Both can use the nostrils or mouth to take in oxygen which passes into the trachea or windpipe. From there, it passes to the bronchial tubes which lead to each lung. Within the lungs, the bronchial tubes become subdivided into smaller branches called bronchioles. At the end of these tiny tubes are alveoli or air sacs. These are thin and moist and the place for gas exchange between the respiratory system and the bloodstream. Oxygen is absorbed by diffusion at the alveoli and transported by hemoglobin in the red blood blood cells to all parts of the body. Carbon dioxide in the blood passes into the alveoli also by diffusion and is eventually exhaled. A sheet of internal muscle called the diaphragm aids in the inhalation and exhalation of gases. When the diaphragm is pulled down, during inhalation, the chest cavity expands and oxygen fills the lungs. Likewise, when the diaphragm moves up, the chest cavity decreases and the air is exhaled from the lungs.