Physiology, Boyle's Law

Issues of Concern

The lungs do not follow Boyle’s law at all volumes. In a resting state with a normal tidal volume, when the alveoli are not collapsed nor are the lungs at maximal capacity, the lungs follow proportional changes of volume and pressure per Boyle’s law. At low lung volumes, it takes a large pressure change to make small changes in the volume (low compliance of lung tissue). At high volumes within the lung, it takes a more negative pressure to expand the tissue, once again not in compliance with a direct relationship as Boyle’s law dictates. At low and high volumes, the lung has low compliance, meaning that the tissue's ability to expand or its elasticity decreases (compliance = [change in volume]/[change in pressure]).[2]

Organ Systems Involved

The primary organ system involved in using Boyle’s law is the respiratory system. The human body brings air into the lungs by negative pressure. At baseline, the thoracic cavity is in static equilibrium with an intrapleural pressure near -5 cm H2O. During inspiration, there is a contraction of inspiratory muscles (namely, the diaphragm and external intercostal muscles [additional muscles, such as the scalene and sternocleidomastoid, can take part under specific circumstances]) that increases intrathoracic volume. Due to the combined motion of the lungs and the chest wall, the lungs begin to expand as the thorax expands during inspiration. According to Boyle’s law, as the volume increases, the pressure must decrease; therefore, as the intrapleural volume increases, the intrapleural pressure decreases to about -8 cm H2O at end inspiration.[2]

At baseline (rest), the alveolar pressure is equal to the atmospheric pressure (0 cm H2O), and during inspiration, this pressure goes to -1cm H2O as the volume expands within the alveoli. When the alveolar pressure drops below the atmospheric pressure, air will flow into the lungs for gas exchange.[2]

When the inspiratory muscles relax, the volume within the thorax decreases; thus, the pressure increases and forces out alveolar air back into the atmosphere. With inspiration, lung volume increases and intrapleural pressure decreases; with expiration, lung volume decreases and intrapleural pressure increases.[2]

Function

Intrapleural pressure is the term for pressure within the intrapleural space; alveolar pressure is pressure within the alveoli. As intrapleural and alveolar pressures become increasingly negative due to the expansion of the chest cavity during inspiration, air from the atmosphere flows into the lungs, which allows the lung volume to increase and participate in gas exchange.[3]

Related Testing

Testing related to the mechanism that Boyle’s law works can be applied to the volume within the lung and equations to describe how much air is moving. The minute ventilation, calculated as the product of tidal volume and respiratory rate, is how much air is inhaled every minute. These 2 factors control ventilation, which directly depends on the thoracic cavity volume expanding and the decrease in pressure within the intrapleural space and alveoli, allowing the lungs to fill with air, producing the tidal volume. If there is an adequate tidal volume, a normal respiratory rate is ensured. If the tidal volume is insufficient, there is a compensatory increase in the respiratory rate to maintain normal minute ventilation.[4] 

Minute alveolar ventilation is an equation that also depends on Boyle’s law and the inverse relationship of pressure and thoracic cavity volume. Alveolar ventilation is the air that reaches the alveoli for gas exchange in each breath, calculated by subtracting the dead space from the tidal volume and multiplying by the ventilation frequency.[4]

Pathophysiology

With a pneumothorax or a hemothorax, there is increased pressure within the intrapleural space. This increased pressure moves the resting state of about -5 cm H2O to a higher value depending on the degree of disease. As this occurs, it would take a much more significant expansion of the thoracic cavity to create a negative pressure to bring air in from the atmosphere. In a tension pneumothorax, the pressure in the pleural space continually raises the intrapleural pressure, thus decreasing the volume in the lungs. Tension pneumothorax can generate enough pressure to cause a mediastinal shift, eventually interfering with the venous return to the right side of the heart and cardiovascular demise.[5][6][7]

Clinical Significance

At birth, newborns are born with no air within their alveoli; thus, the volume is zero. The compliance (elasticity of lung tissue) is low at birth. Therefore, the effort to create negative intrapleural pressure during the initial breaths is high; however, the lungs fill with air and become more compliant with successive breaths. As the lungs become more compliant, the newborn's lungs follow Boyle's law of the inverse relationship of pressure and volume.[2]

Pneumothorax is a clinical condition that can either be primary (typically from trauma) or secondary (the patient has a predisposing condition such as chronic obstructive pulmonary disease [COPD]). Boyle's law dictates how air is drawn into the lungs. As the intrathoracic pressure becomes increasingly negative, the intra-alveolar pressure decreases below atmospheric pressure, causing air to flow into the lungs. In a pneumothorax, there is increased pressure within the intrapleural space, thus causing an increased force to create enough negative pressure for air to come into the lungs.[5][6][7]

Boyle's law also applies when using a medical syringe. When the cylinder on the syringe is empty, it is said to be neutral as there is no air. As one pulls back on the plunger, the volume in the cylinder increases, therefore by Boyle's law, the pressure decreases. The liquid is thus drawn into the cylinder to balance the pressure within and outside the syringe.

Self-contained underwater breathing apparatus (SCUBA) divers must know Boyle's law as they descend and ascend to great depths. As a diver descends into the water, the pressure on the person's lungs increases; therefore, according to Boyle's law, the air volume inside the lungs must decrease. As the diver ascends in the water and the pressure on the thoracic cage decreases, the volume of air increases.[3] It is important to exhale steadily to release the volume of the gas; if this does not occur, the diver can experience pulmonary barotrauma, which is overexpansion and alveolar rupture. The diver may have a pneumothorax (chest pain, dyspnea, unilateral decreased breath sounds) or pneumomediastinum (neck pain, pleuritic chest pain, dyspnea, coughing; there may be subcutaneous emphysema causing a crepitation on palpation).[8]