The process in which oxygen rich air enters the lungs and carbon dioxide rich air leaves the lungs is called breathing. Adults breathe 16-18 times per minute and children breathe twice per minute. However, the rate of absorption increases during exercise. Respiration is done in two stages. These are-
1. Inspiration: The process by which oxygen-rich air enters the lungs is called inspiration or inhalation. During this the diaphragm and intercostal muscles contract. The peritoneum rises upwards and the diaphragm descends. As a result, the volume of the thoracic cavity increases. The lungs expand. Air pressure inside the lungs is low. But the air pressure in the outside environment is normal. As a result, oxygen-rich air from the outside environment enters the lungs.
2. Expiration: The process by which air rich in carbon dioxide is released from the lungs is called exhalation. During this time the diaphragm and intercostal muscles are stretched. The scrotum moves downward and the diaphragm moves upwards. As a result, the volume of the thoracic cavity decreases. The lungs are compressed. Air pressure inside the lungs increases (3-4 mmHg more). Air pressure in the lungs but the air pressure in the outside environment is normal. As a result, air rich in carbon dioxide is released from the lungs.