Lung Ventilation: Volume and Pressure Changes

Ventilation of the lungs is controlled primarily by the diaphragm, a thin sheet of muscle that separates the thoracic (chest) cavity from the abdomen. Along with the intercostal muscles, the diaphragm helps regulate the volume and pressure within the lungs to enable breathing.

Inhalation (Breathing In)

• When the diaphragm contracts, it moves downwards and flattens, increasing the volume of the thoracic cavity.
• At the same time, the external intercostal muscles contract, pulling the ribs up and out.
• This combined action further increases thoracic volume, leading to a decrease in air pressure inside the lungs compared to the air pressure outside the body.
• As a result, air is drawn into the lungs (down the pressure gradient).

Exhalation (Breathing Out)

• When the diaphragm relaxes, it returns to its domed shape, decreasing the volume of the thoracic cavity.
• Simultaneously, the external intercostal muscles relax, allowing the ribs to move down and in.
• These actions reduce the volume of the chest cavity, causing the air pressure inside the lungs to increase relative to the outside air.
• This forces air out of the lungs

Forced Exhalation

• During periods of increased physical activity, the body produces more carbon dioxide, which must be expelled quickly.
• The internal intercostal muscles contract, pulling the ribs down and in more forcefully.
• This further decreases the volume of the thoracic cavity and increases air pressure inside the lungs, causing air to be expelled more rapidly.
• This allows for faster gas exchange, helping to maintain the body’s oxygen and carbon dioxide balance.