The mammalian respiratory system consists of a highly branching system of airways that lead to alveoli—
learning outcomes
You should be able to:
Explain the roles of surface tension and surfactant in breathing.
Explain how changes in volume and pressure in the thoracic cavity are involved in ventilation.
Describe the changes (in arbitrary units) in tension between the pleural membranes and the pressure in the alveoli, and air flow during a breathing cycle.
Why do some premature newborns have difficulty breathing?
Some premature infants may be born before cells in their alveoli have started to secrete surfactant. Without surfactant, inflation of the lungs requires extra effort to overcome surface tension in the fluid lining the alveoli.
Why does a puncture wound of the chest cause a collapsed lung?
There is always tension between the pleural membranes because of the outward pull of the chest wall and the inward pull of the lung tissue. This tension keeps the lungs partially inflated even between breaths. A puncture of the chest wall allows air to enter space between the pleural membranes, equilibrating its pressure with atmospheric pressure. As a result, there is no longer a force maintaining partial inflation of the lungs.
In the breathing cycle, what is the alveolar pressure when the tension between the pleural membranes is maximal? Explain your answer.
The tension between the pleural membranes is maximal at the peak of inhalation because it is maintaining maximum distension of the lung tissue, but once the lungs are filled, air flow ceases and the alveolar pressure is the same as atmospheric pressure.
Having discussed how respiratory gases get to and from the environmental side of the gas exchange membranes through ventilation, we will now look at how these gases get to and from the internal side of those membranes through perfusion.