Breathing, also called ventilation, refers to the movement of gasses in and out of the lungs. The process of breathing begins when the diaphragm contracts and moves downwards. This creates a negative pressure within the chest cavity, which causes air to enter the lungs through the nose or mouth. The air then travels down the respiratory tract and into the alveoli in the lungs.
The alveoli are small sacs where gas exchange takes place. Oxygen from the air diffuses into the blood and carbon dioxide diffuses out of the blood into the alveoli. The blood then carries the oxygen to the cells of the body where it is used for energy. The carbon dioxide is exhaled out of the body. Breathing control takes place in its centers in the brainstem to maintain the blood oxygen and carbon dioxide partial pressures within the normal limits even under widely varying conditions like exercise.