What makes lungs expand and contract? Are they muscles?
No, the lungs aren’t muscles. *Special* respiratory muscles “make” breathing happen. Normally an involuntary act, breathing can also be consciously controlled and altered, thank goodness. Although technically many muscles of the thorax can participate in breathing, any decent yogi will tell you that the diaphragm is the prime and proper breathing muscle (Vishnudevananda 235). Here’s how to do it:
Relaxed, the diaphragm is dome-shaped. Upon contraction it flattens inferiorly, expanding the thoracic cavity and lungs. This is inspiration. The muscles shown between the ribs in this illustration are the external intercostals; they aid inspiration by elevating the rib cage. In a full inspiration, the belly will protrude, the chest will expand up and out, and the shoulders may rise. The lungs expand as well, of course. Expiration is achieved as these muscles relax and the lungs recoil. Because lungs naturally recoil, it follows that expiration is normally a passive process. The diaphragm and the internal intercostals, muscles which run alongside but contrary to the external intercostals, may be voluntarily used to force exhalation. Yogis accentuate expiration to maximize release of stale air from the lungs (239).
The lungs expand and contract, but why does air go in and out?
Changes in volume cause changes in pressure! This is explained in Boyle’s Law, where pressure (P) is in mm Hg, volume (V) is mm3, and temperature is a constant:
When V = k1/P, k = constant, and P1V1 = P2V2,
Volume relates inversely with pressure, given constant temperature and mass. Constant k relates one Volume x Pressure condition set to another.
During normal, quiet inspiration, the lung volume increases on average by about 500mL. According to Boyle’s law, the system would respond to such an increase in volume (expanded thorax) with a decrease in pressure. A decrease in pressure corresponds to the fact that the space between gas molecules increases and collisions between them decrease in number. Because intrapulmonary (inner lung) pressure is less than atmospheric pressure, air rushes into the lungs.
During normal, quiet expiration, the lungs contract and the intrapulmonary volume decreases. The gas is “compacted,” collisions increase, and air moves out to equalize pressure.
In summary, breathing is an inverse relationship that obeys Boyle’s law: as you breath out, volume decreases and pressure increases. As you breath in, volume increases and pressure decreases. Gases instantaneously flow from areas of high pressure to low pressure and a constant is maintained: equilibrium between alveolar and atmospheric pressure.