And what phase of ventilation is the air pressure in the lungs greater than the air pressure in the atmosphere?
The correct answer and explanation is:
The phase of ventilation when the air pressure in the lungs is greater than the air pressure in the atmosphere is expiration (or exhalation).
During expiration, the diaphragm and intercostal muscles relax after contraction during inspiration. This relaxation causes the thoracic cavity to decrease in size, which increases the pressure inside the lungs. As the lungs compress, the air within them is forced out. The air pressure in the lungs rises above the atmospheric pressure, driving air out of the respiratory system.
This phase is often described as passive, although forced expiration can occur with the help of additional muscles like the abdominal muscles. Forced expiration increases the pressure inside the lungs even further, which helps expel air more rapidly, particularly during activities like exercise or when clearing the airways.
During expiration, the decrease in the lung volume causes an increase in lung pressure. The relationship between pressure and volume in the lungs is governed by Boyle’s Law, which states that at a constant temperature, the pressure of a gas is inversely proportional to its volume. As the lung volume decreases during expiration, the air pressure increases, leading to air being expelled from the lungs until the pressure inside the lungs is equalized with the atmospheric pressure.
The efficiency of expiration is critical for clearing carbon dioxide from the body. If the expiration phase is impaired, as in conditions like chronic obstructive pulmonary disease (COPD), it can lead to difficulties in breathing and reduced gas exchange.
This phase contrasts with inspiration, where the diaphragm contracts, expanding the lungs and lowering the lung pressure below atmospheric pressure, drawing air in.