Air doesn't move in and out of your lungs by magic. It moves because of pressure differences. The main player is transpulmonary pressure ($P_{tp}$), which is the difference between the pressure inside the alveoli ($P_{alv}$) and the pressure in the space between your lungs and chest wall, the intrapleural pressure ($P_{ip}$).

Think of your lungs as a balloon inside a sealed glass jar. If you use a pump to suck some air out of the jar, the pressure inside the jar drops. This pressure difference between the inside of the balloon and the inside of the jar forces the balloon to expand. In your chest, your diaphragm and rib muscles expand the chest cavity, which makes the intrapleural pressure even more negative. This increased transpulmonary pressure pulls the lungs open, dropping the alveolar pressure below atmospheric pressure, and air rushes in.

How much your lungs expand for a given change in pressure is determined by their compliance, or stretchiness. Compliance ($C$) is simply the change in volume ($\Delta V$) divided by the change in transpulmonary pressure ($\Delta P$).

But it's not just the lungs we have to consider. The entire respiratory system has a compliance, which is a combination of the lung's compliance and the chest wall's compliance. These two components are in a constant tug-of-war. The lungs have elastic recoil, meaning they naturally want to collapse inward. The chest wall, however, naturally wants to spring outward.

The point where these two opposing forces are perfectly balanced is called the Functional Residual Capacity (FRC). This is the amount of air left in your lungs after a normal, relaxed exhalation. It's the system's resting state.

In restrictive lung diseases like pulmonary fibrosis, the lungs become stiff. This means their compliance is low, and it takes much more pressure to inflate them. The lung's inward recoil is stronger, so the FRC decreases. Conversely, in obstructive diseases like emphysema, the lung tissue loses its elastic recoil. Compliance becomes very high, the chest wall's outward pull dominates, and the FRC increases, trapping air in the lungs.

Breathing isn't free; it costs energy. The work of breathing is the energy spent to overcome two main forces: elastic recoil of the lungs and chest wall, and airway resistance.

Airway resistance is the friction that air encounters as it flows through the bronchi and bronchioles. Just like it's harder to drink a thick milkshake through a thin straw, it's harder for air to flow through narrow airways.

The most important factor determining resistance is the radius of the airway. This relationship is described by Poiseuille's Law for fluid flow, which shows that resistance is inversely proportional to the radius to the fourth power ($r^4$).

In diseases like asthma or chronic bronchitis, inflammation and mucus narrow the airways. According to Poiseuille's Law, this small decrease in radius dramatically increases airway resistance. Your body has to work much harder to move air, leading to shortness of breath.

In a healthy person, breathing is an efficient, almost effortless process. But when compliance drops or resistance climbs, the work of breathing can become exhausting, consuming a significant portion of the body's energy.