Why does air flow in when you expand your lungs?

Air flows into the lungs when you expand them because of the difference in air pressure between the lungs and the outside atmosphere. When you inhale, the muscles between your ribs and diaphragm contract, pulling the ribs up and out and flattening the diaphragm. This increases the volume of the thoracic cavity, causing the pressure inside the lungs to drop below the pressure of the air outside. Air then flows into the lungs to equalize the pressure.

Here's a more detailed explanation of what happens when you breathe in:

- The diaphragm, which is a dome-shaped muscle located at the bottom of the thoracic cavity, contracts. This pushes the diaphragm downward and increases the volume of the thoracic cavity.

- The external intercostal muscles, which are located between the ribs, also contract. This pulls the ribs up and out, further increasing the volume of the thoracic cavity.

- As the volume of the thoracic cavity increases, the pressure inside the lungs decreases. This creates a pressure gradient between the lungs and the outside atmosphere, with the pressure inside the lungs being lower than the pressure outside.

- Air flows into the lungs through the nose and mouth to equalize the pressure.

Exhalation is the opposite process of inhalation. The diaphragm and external intercostal muscles relax, allowing the lungs to recoil and expel air.