# The Balloon Model for Human Ventilation

The balloon model is often used to show how ventilation works in the human lungs. You pull on the diaphragm, expanding the volume of the container, and the balloon begins to fill up. My understanding (which may be flawed) is this:

1. The air in the container is isolated from the air outside the container, so when the container’s volume is increased, the container’s gas pressure decreases in accordance to Boyle’s law.
2. There becomes an imbalance of forces between the side of the balloon wall that faces the container (i.e. at reduced pressure) and that which is open to the atmosphere (i.e. at atmospheric pressure). This pushes the balloon outward, increasing its volume.
3. The balloon’s expansion causes the container volume to decrease, increasing the container pressure again. At the same time, the gas pressure inside the balloon decreases as the balloon’s volume increases. This continues until the gas pressures on both sides of the balloon are equal.
4. The equilibrium pressure is lower than the atmospheric pressure, and soon atmospheric air rushes down a pressure gradient into the balloon.

I confused as to what would happen after this (if these events even occur; if my analysis is wrong, pray tell and explain). Does the pressure in the balloon increase? Does its volume? I’m thinking about the ideal gas law $PV=nRT$, and it’s clear that $RT$ is constant and $n$ is changing. However, which is the depending variable: $P$ or $V$?

(And though this part is probably more suited for the Bio SE, I’d also like to ask if this balloon model, in light of what I wrote above, is an accurate depiction of the lungs’ ventilation mechanism. There is no attachment between the balloon and container wall, yet in the human body, the alveoli are nestled in the lung tissue, the lung tissue is connected to the thoracic cavity via pleural membranes, and the thoracic cavity is bounded by the ribs and diaphragm, which whose muscles accomplish the volume expansion process.)

Try this: exhale, close your nose & mouth, and try to expand your chest. You won't be able to, or not very much. The point is that your body cannot maintain much of a pressure difference, so the $PV=nRT$ equation's operation consists of $\Delta n$ following $\Delta V$ pretty closely ($P$ remaining close to constant).