What is the slight vacuum that exists between the two pleural layers called?

The slight vacuum that exists between the two pleural layers is called intrapleural pressure. This is the pressure within the pleural cavity, the space between the visceral pleura (which covers the lungs) and the parietal pleura (which lines the chest wall). This negative pressure is essential for lung function, as it helps keep the lungs inflated by creating a pressure gradient that allows air to be drawn into the lungs during inspiration.

Intrapleural pressure is always lower than the atmospheric pressure and intrapulmonary pressure (the pressure within the lung alveoli), which is crucial for maintaining the inflation of the lungs and preventing their collapse. This differential pressure allows for the expansion of the lungs against the elastic recoil tendency of lung tissue.

Understanding this concept is critical in physiology, as any disturbance to the intrapleural pressure can lead to respiratory problems, such as pneumothorax, where air enters the pleural cavity and disrupts this pressure balance.