How are all lung pressures expressed?

• A. Relative to lung volume
• B. Relative to intrapleural pressure
• C. Relative to atmospheric pressure
• D. Relative to venous pressure

Answer: (C)

Lung pressures are typically expressed relative to atmospheric pressure because this provides a standard reference point that allows for consistency in measurements and comparisons. Atmospheric pressure is the pressure exerted by the weight of air in the Earth's atmosphere, which is approximately 760 mmHg at sea level.

When lung pressures are described in relation to atmospheric pressure, it allows for clarity in understanding how the pressure within the lungs changes during different phases of the breathing cycle. For instance, during inhalation, the pressure within the lungs becomes negative relative to atmospheric pressure, allowing air to flow into the lungs. Conversely, during exhalation, the lung pressure becomes positive relative to atmospheric pressure, pushing air out of the lungs.

Other options do not serve as universal reference points for expressing lung pressures. For example, intrapleural pressure and venous pressure are physiological pressures relevant to specific contexts but do not provide the same broad applicability for expressing lung volumes and pressures during ventilation and gas exchange processes. Adopting atmospheric pressure as the reference ensures that the changes in lung pressures are understood in relation to the environment in which breathing occurs.