Describe the structure of the human gas exchange system and explain how we breath in and out.

The human gas exchange system includes the trachea, the bronchioles and the alveoli. During inhalation (breathing in) the diaphragm contracts and the external intercostal muscles contract. This increases the volume and decreases the pressure in the thoracic cavity to lower than that of the atmosphere. Therefore, air moves in to the thoracic cavity (inhalation).During exhalation, the diaphragm relaxes and the internal intercostal muscles contract. This causes the volume to decrease and the pressure to increase in the thoracic cavity to greater than atmospheric pressure. Therefore, air moves out of the thoracic cavity (exhalation).