Question

Briefly describe the difference between atmospheric pressure, intrapulmonary pressure, and

intrapleural pressure. Which of these is always negative in a healthy individual during normal breathing? What happens if intrapleural pressure becomes equal to atmospheric pressure?

Answer 1

Atmospheric pressure is the pressure exterted by the air in the atmosphere. In physiological term, atmospheric pressure signifies the pressure exterted by the air in the atmosphere on the human body. Normally, at sea level, atmospheric pressure is equal to 760mm of Hg or 1 atmosphere.

Intrapulmonary pressure is the pressure exterted by the air in the alveoli of the lungs. It is also cslled as intra-alveolar pressure. Its value eventually becomes equal to that of atmospheric pressure during breathing.

Intrapleural pressure is the pressure in the pleural cavity of the lung. Pleural cavity is the space between the parietal and visceral pleura of the lung. Normally, its value is around -4mm of Hg.

In healthy individual during normal breathing, intrapleural pressure is always negative. Its value is around -4mm of Hg. During entire breathing cycle, intrapleural pressure remains negative (-4mm of Hg). Due to the pressure gradient between intrapleural pressure and atmospheric pressure and intrapulmonary pressure, the process of ventilation (inspiration and expiration) takes place.

If intrapleural pressure becomes equal to atmospheric pressure, the lungs collapse. Normally, intrapleural pressure is always negative (-4mm of Hg). This negative intrapleural pressure even after the expiratory phase, prevents the lungs from collapsing. Due to rupture of pleural cavity due to any cause, the intrapleural pressure starts rising and becomes equal to the atmospheric pressure or at times higher than the atmospheric pressure. This condition is known as pneumothorax and it causes collapse of the lungs.