Breathing in air is achieved by contracting the diaphragm and intercostal muscles so that the volume in the thorax increase. This creates a negative pressure in the lungs which draws air in through the nose and mouth. The air inthe nose (to a certain extent, the mouth) is heated and humidified before it passes down the pharynx. From the pharynx, the air passes over the larynx and epiglottis which controls the air passage to the trachea. Down the trachea, a 10-12 cm long tube, air passes to the right and left bronchus.
Through the bronchi, which branch off and become narrower and narrower, the air passes to the bronchioles and finally to the alveoli. The alveoli are surrounded by tiny capillaries, and between the air in the alveoli and the blood in the capillaries there is only a thin barrier so that oxygen and carbon dioxide gas can diffuse across. In addition to the capillaries, the alveoli are also surrounded by elastic fibers which together with the alveoli surface tension is pushing alveoli together. The pleural cavity, a small cavity between the lungs and the thorax wall is counteracting these forces and pulls the lungs up against the thorax by the attraction between the visceral and parietal pleural membranes. The visceral and parietal membranes are attached to the lung tissue and the inside of the thorax wall respectively. When the volume of the lungs expands during inspiration, the parietal membrane is pulled as well therefore expanding the lung volume.