Breathing, otherwise known as pulmonary ventilation, is the process of air movement into and out of the lungs. The main mechanisms propelling pulmonary ventilation are atmospheric pressure (P_atm), intra-pulmonary (P_pul ) or intra-alveolar pressure (P_alv) within the alveoli, and intrapleural pressure (P_ip) within the pleural cavity.

Breathing Mechanisms

Both intra-alveolar and intrapleural pressures rely on specific lung properties. The ability to breathe—allowing air to enter the lungs during inhalation and leave during expiration—depends on the air pressure inside the lungs relative to the atmospheric pressure.

Pressure Dynamics

Inhalation (inspiration) and exhalation (expiration) hinge on the pressure differences between the atmosphere and the lungs. In a gas, pressure arises from the movement of confined gas molecules. For instance, due to the reduced volume, the same amount of gas molecules will exert more pressure in a one-liter container than in a two-liter container. This relationship between volume and pressure in a gas at a constant temperature is described by Boyle's Law, which states that pressure and volume are inversely proportional (P = k/V); a volume increase results in pressure decrease and vice versa.

Three types of pressure—atmospheric, intra-alveolar, and intrapleural—dictate pulmonary ventilation. Atmospheric pressure represents the force exerted by air gases surrounding a particular surface. It can be expressed as an atmospheric unit (atm) or in millimeters of mercury (mm Hg), where 1 atm equates to 760 mm Hg, the sea-level atmospheric pressure. Other pressure values, such as atmospheric pressure, are typically discussed in respiration.

Intra-alveolar pressure (intrapulmonary pressure) denotes the air pressure within the alveoli, which fluctuates during different breathing stages. As the alveoli are connected to the atmosphere via the airways, the intrapulmonary pressure constantly equalizes with the atmospheric pressure.

Intrapleural pressure is the air pressure within the pleural cavity between the visceral and parietal pleurae. It fluctuates during different breathing stages, and due to specific lung properties, it always stays negative compared to the intra-alveolar pressure. The intrapleural pressure remains approximately –4 mm Hg throughout the breathing cycle. It is maintained by the inward recoil of the lung's elastic fibers and the outward pull of the chest wall. . The balance between these opposing forces determines the –4 mm Hg intrapleural pressure relative to the intra-alveolar pressure. Transpulmonary pressure, the difference between intrapleural and intra-alveolar pressures, determines lung size.

Factors Influencing Ventilation

Apart from pressure differences, respiration depends on the contraction and relaxation of the diaphragm and thoracic muscle fibers. The diaphragm and intercostal muscle movements primarily cause pressure changes responsible for inspiration and expiration.

Other lung properties also affect respiration. Resistance is a force that slows down motion, in this context, the flow of gases. Airway size is the main factor affecting resistance. Smaller tube diameter forces air through a smaller space, causing more collisions of air molecules with the airway walls. The formula F=ΔP/R describes the relationship between pressure changes and airway resistance.

Surface tension within the alveoli, caused by the water present in the alveoli lining, tends to restrict alveoli expansion. Pulmonary surfactant, secreted by type II pneumocytes, mixes with water, reducing this surface tension and preventing alveoli collapse during expiration.

Thoracic wall compliance, the thoracic wall's ability to stretch under pressure, affects the effort required for breathing. The thoracic cavity must expand for inspiration to occur, directly influencing lung capacity. If the thoracic wall lacks compliance (cannot expand), fetal development of the thorax and the lungs becomes challenging.