Airflow through the respiratory system of mammals is tidal, meaning that air flows in by the same route that it leaves. When at rest, the average adult human breathes in and out about half a liter of air with each breath. This is called our tidal volume. When this fresh air enters the lungs, it mixes with stale air—typically 2 liters worth—before it hits the respiratory surfaces in the alveoli (air sacs) of the lungs. Because the stale air has a low partial pressure of oxygen, tidal breathing is not an optimally efficient means of gas exchange.
Breathing in mammals is driven by pressure changes in the thoracic cavity. In the accompanying animation, we focus on the mechanics of tidal breathing in humans, and, in particular, we examine the muscles and membranes that are important in this ventilation of the lungs.
The inhalation of a breath is an active process driven by the contraction of muscles. When the diaphragm contracts, it expands the thoracic cavity downward. When the muscles between the ribs contract, the ribs elevate and expand the thoracic cavity outward.
During an inhalation, the contraction of muscles is an active process, but the movement of air is passive. The movement of air follows pressure gradients. As the lungs expand, pressure within the air sacs (alveoli) drops. Air from the environment, which is at a higher pressure, then floods the lungs and equalizes the alveolar pressure with the atmospheric pressure.
When the diaphragm and the muscles between the ribs relax, they cause an exhalation. The chest cavity reduces in size and the lungs elastically recoil. As the lungs shrink, the alveolar pressure increases, forcing air out of the lungs and into the environment. This again equalizes the lung pressure with the external, atmospheric pressure.
Note that the pleural cavity is always slightly negative in pressure relative to the external pressure. This negative pressure maintains a constant suction on the lungs, keeping the alveoli partially inflated. If the sealed pleural cavity is ruptured, such as by a knife wound, air rushes into the pleural cavity, collapsing a lung.
Textbook Reference: Concept 31.3 The Mammalian Breathing System Is Anatomically and Functionally Elaborate