5.2.1

# Ventilation: Inspiration and Expiration

Under quiet resting conditions the respiratory rate is about 12 breaths per minute. This air movement or ventilation is necessary from moment to moment to sustain life. From the metabolism chapter in BIO 264, we learned that to make sufficient ATP from glucose or other nutrient molecules, we have to inhale oxygen so that it might be reduced and accept hydrogens and electrons to form water. We also learned that carbon dioxide is a byproduct of nutrient molecule breakdown and that we actually "breathe out" part of the molecules of the food we eat (the carbons in CO2 you're breathing out were part of the glucose in the Snickers bar you ate). The process of moving air from the external environment into the lungs is called inspiration. Expiration is air moving from the lungs out of the body into the environment. As described previously, air will always move from a region of high pressure to a region of lower pressure. The question, then, is what actually brings about the pressure differences to ensure air movement into and out of the lungs? The answer lies in the relationship between pressure and volume described by Robert Boyle.

#### Boyle’s Law

Boyle’s law is P = k/V, where P is the pressure of a gas, V is the volume of the gas, and k is a constant. This equation shows that there is an inverse relationship between pressure and volume. It shows that if the volume of a container of gas decreases, the pressure exerted by that gas will increase.

Boyle’s Law.