Breathing, also known as ventilation, is how the air moves into and out of the lungs. It consists of repetitive cycles of inspiration, when air flows into the lungs; expiration, when air leaves the lungs; and a brief pause, called the rest period, between these two.
Now, the direction of airflow throughout the breathing cycle depends on the difference between the atmospheric pressure, which is the pressure of the air in the environment, and the alveolar pressure, or the pressure inside the alveoli, which are the tiny sacs of air where gas exchange happens in the lungs.
An additional parameter is the intrapleural pressure, also called the intrathoracic pressure, which is the pressure of the fluid inside the pleural cavity that surrounds the lungs.
Intrapleural pressure is usually negative compared to the alveolar or atmospheric pressure, and this is important because the alveolar pressure minus the intrapleural pressure gives the transmural pressure.
As long as the transmural pressure stays positive, the airways remain open throughout all of the phases of the breathing cycle.
Ok, now, normal, quiet breathing involves inspiration and expiration of a tidal volume, or VT for short, of about 500 mL, which includes the volume of air that fills the alveoli plus the volume of air that fills the airways.
Now, according to what is known as Boyle’s law, at a constant temperature, pressure and volume are inversely related to each other, so when the alveolar pressure decreases, more air will enter the lungs, increasing the air volume
With that in mind, let’s establish the starting point for these variables by looking at the lungs during the rest phase of the breathing cycle.
During rest, the diaphragm is at its balanced position. The alveolar pressure equals the atmospheric pressure to a value of zero centimeters H2O, so there is no pressure gradient, and no air is moving into or out of the lungs.
The intrapleural pressure is negative, approximately -5cm H2O because the lungs and the chest wall act as opposing forces, meaning the lungs have a tendency to collapse during rest, while the chest wall has a tendency to expand.
Because alveolar pressure, which is 0 cm H2O, minus the intrapleural pressure, which is -5 cm H2O, equals a transmural pressure of +5 cm H2O, that means that the airways are open during rest.
Now, inspiration, and a new breathing cycle, start when there’s a variation in the arterial pressure of oxygen, or PaO2, which is normally around 100 mmHg; the arterial pressure of carbon dioxide, or PaCO2, normally around 40 mmHg; and the arterial pH, which is normally 7.4.
Changes related to these markers activate a series of receptors, called chemoreceptors, which are specialized sensory cells that convert the concentration of a chemical substance in the blood, such as carbon dioxide or oxygen, to a biological signal for the respiratory center, located in the brainstem.
The respiratory center consists of three major respiratory groups of neurons. The dorsal respiratory group and the ventral respiratory group are found in the medulla oblongata, while the pontine respiratory group is found in the pons and consists of two areas, known as the pneumotaxic center and the apneustic center.
Of these, the dorsal respiratory group, or DRG, is the one that initiates respiration, and it also determines the basic rhythm of breathing by adjusting the frequency of inspiration so as to keep PaO2, PaCO2, and the arterial PH in the normal range.
When the DRG receives information regarding the increase of PaCO2, the DRG sends a command through the phrenic nerve to the diaphragm, which contracts to increase the vertical length of the thoracic cavity, and through the intercostal nerves to the external intercostal muscles, which contract and make the ribs move up and out, increasing the lateral size of the thoracic cavity.
Based on Boyle’s law, as lung volume increases, the alveolar pressure decreases, specifically to -1cm H2O. This is lower than the atmospheric pressure, so now there is a pressure gradient, that makes air rush into the lungs like a vacuum.
Eventually, all this air brings the alveolar pressure back up, and once it equals the atmospheric pressure, airflow into the lung stops.
At the same time, as lung volume increases, this compresses the intrapleural cavity, but the intrapleural pressure always remains lower than the alveolar pressure, reaching the value of -8cm H20 at the end of a normal inspiration.