Mechanics of Breathing: Pressures

The lungs actually consist of branches of millions of tiny alveoli, pockets designed for gas exchange, like oxygen and waste products from cellular respiration like Co2 (carbon dioxide). The connective tissue layer lining the lungs is called the visceral pleura. Extending a layer beyond that is the pleural cavity filled with pleural fluid. Advancing outwardly from that we have another connective tissure layer called the parietal pleura, which is the inner lining of the chest wall. During inspiration (inhalation) and expiration (exhalation), these two connective tissue layers would rub against one another causing friction, damage, and inflammation. The pleural fluid in the pleural cavity between these two layers is the protective measure against that friction.

The pressures in the respiratory system are measured in comparison to the partial pressure of oxygen in the atmosphere, which is 760 millimeters of mercury (the units of pressure). The pressure in the alveoli of the lungs is equal to that of the atmosphere, thus we can say 0 millimeters of mercury. This is called the intrapulmonary or intraalveolar pressure. The partial pressure of oxygen in the pleural cavity is called the intrapleural pressure, which is 756 millimeters of mercury or -4 we can say.

We also calculate the pressure differences across these layers, so when we measure the transpulmonary pressure, we use the formula of subtracting the intrapulmonary from the intrapleural, which would +4. Then we take the transthoracic pressure by subtracting intrapleural from the atmospheric, which totals -4. Finally, we have the intrarespiratory pressure which is intrapulmonary minus the atmospheric, which comes to 0.

The -4 of the transthoracic is important for this negative gradient is what allows the lungs to stretchable, distensible, and compliant. A law states that pressure wants to move from positive to negative, down its pressure gradient, this is why.

As a minor point, there is a difference of pressure about the pleural cavity, we stated above that it is 756 millimeters of mercury. Though, because the diaphragm is the muscle used to pull the parietal pleura downwards it creates more volume at the base of the lungs than at the apex. Boyles law states that when you increase volume (in this case thoracic cavity volume, used interchangeably with pleural cavity volume), you decrease the pressure and vice versa. Thus, at the bottom of the lungs in the pleural cavity space, it’s 753 and at the apex it’s 758 millimeters of mercury.

There is a nucleus (a cluster of neurons in the central nervous system) in the medulla oblongata right above the brainstem called the venteal respiratory group (VRG). The vrg has inspiriatory and expiration signals, for example, it’s tract (a bundle of axons sprouting from neurons) runs down to C2 to C5 regions of the spinal cord. It exits from there as the internal intercostal nerve and the phrenic nerve. The former supplies the external intercostal muscles that pull the rib below it to the one above it, in what’s called a bucket handle outward movement, and pulls out the sternum in a water pump motion (the old water pumps). These both serve to jncrease the thoracic cavity volume, thus decreasing pressure in the lungs. Now, because the pressure in the lungs decreased, the atmospheric pressure moves down its gradient supplying the lungs with oxygen until it equals that of the atmosphere.