Pulmonary ventilation exchanges gases between atmosphere and air sacs (alveoli) of lungs. Respiration exchanges waste carbon dioxide for fresh oxygen. External respiration is the exchange of gases between the lungs and the blood. Internal respiration is the exchange of gases between body cells and the blood.
Inhalation (inspiration) is active. It requires muscle contraction. The diaphragm contracts and flattens and the ribs and sternum are elevated, expanding the rib cage. When relaxed, the diaphragm assumes a domed shape. Quiet exhalation (expiration) is passive. Muscles relax and the lungs recoil because they are elastic.
Increasing the volume of the thorax reduces pressure in the lungs and air flows in. Decreasing the volume of the thorax in exhalation increases pressure in the lungs and air flows out.
Making a Difference in the World 