• The architecture of the mammalian breathing apparatus is such that a single organ, the mammalian lung, is responsible for gas exchange as well as the mechanical act of ventilation. It is interesting to note that in some animals, such as birds, these functions are separated such that one organ possesses gas exchange tissue while a distinct organ actuates the flow of externally-derived oxygen-rich air over the former. In contrast, the mammalian lung not only possesses gas exchange tissue, but is designed to pull in oxygen-rich fresh air and expel carbon dioxide-rich air through a mechanism of cyclical expansion and contraction.
General Principles
  • Like the movement of blood, the movement of air between two points in a closed environment is actuated by differences in the relative pressure of air between those two points. As a rule, air moves from areas of higher pressure to areas of lower pressure. The mammalian lung takes advantage of this principle to actuate pulmonary ventilation and does so by cyclically increasing and decreasing the pressure of air within it above and below the atmospheric pressure. When the internal pressure of air within the lung is above atmospheric pressure, air is exhaled; in contrast, when the internal pressure of air within the lung is below atmospheric pressure, air is inhaled.
  • The cyclical changes in the lung's internal air pressure are achieved by the cyclical expansion and contraction of the lung's volume, achieved by motion of the chest wall]] and diaphragm as discussed in Breathing Biomechanics below. However, expansion and contraction of the mammalian lung is uniquely challenging given the fact that the organ is not physically attached to the chest wall or diaphragm and is instead separated from these moving structures by the intrapleural space. In fact, pulmonary expansion and contraction are the result of the dynamic relationship between the compliances of the lung and chest wall which are discussed separately in lung compliance and chest wall compliance and then considered together in integrated pulmonary compliance. The entire cyclical motion of breathing is summarized qualitatively in the breathing cycle page below and the effect of airflow resistance on breathing is considered in airflow resistance.

Breathing Occurs by Cyclical Increase and Decrease of Intrapulmonary Pressure
Air flows from areas of high pressure to areas of relatively lower pressure. As shown, breathing is achieved by cyclically increasing and reducing the intrapulmonary pressure (Pip) compared to the surrounding atmospheric pressure (Patm).