Air moves into the lungs as a fundamental process that sustains life. This continuous exchange of gases is essential for providing oxygen to the body’s cells and removing carbon dioxide, a metabolic waste product. Understanding how air enters the lungs involves examining the specific body parts involved and the physical principles that govern gas movement.
Anatomy of Inhalation
Inhalation relies on several specialized anatomical structures. The diaphragm, a large, dome-shaped muscle, forms the floor of the thoracic cavity, separating it from the abdominal cavity. This muscle is the primary driver of quiet breathing, responsible for a significant portion of the volume change within the chest.
The intercostal muscles, located between the ribs, also play a role in expanding the chest. Specifically, the external intercostal muscles contract during inhalation, pulling the ribs upward and outward. This action increases the front-to-back and side-to-side dimensions of the thoracic cavity.
Encasing the lungs are two serous membranes, the pleura, which form a pleural cavity. The visceral pleura adheres directly to the lung surface, while the parietal pleura lines the inner wall of the thoracic cavity. A thin layer of fluid within the pleural cavity allows these membranes to slide past each other, maintaining a close connection between the lungs and the chest wall.
The Role of Pressure in Breathing
Air movement into the lungs is governed by the principle of pressure gradients, where gases flow from an area of higher pressure to an area of lower pressure. Atmospheric pressure, the force exerted by the air surrounding the body, remains constant at sea level. For air to enter the lungs, the pressure inside the lungs, known as intrapulmonary pressure, must become lower than the atmospheric pressure outside the body.
This pressure difference is directly influenced by changes in the volume of the thoracic cavity and, consequently, the volume of the lungs. Boyle’s Law states that for a fixed amount of gas at constant temperature, pressure and volume are inversely proportional. As the volume of the lungs increases, the pressure within them decreases.
When the lung volume expands, the gas molecules inside spread out over a larger area, resulting in fewer collisions with the lung walls and thus lower pressure. This reduction in intrapulmonary pressure creates a gradient, drawing air from the higher atmospheric pressure into the lower-pressure lungs.
The Inhalation Process
Inhalation begins with signals from the respiratory centers of the brainstem. The diaphragm and external intercostal muscles contract. As the diaphragm contracts, it flattens and moves downward, while the external intercostals pull the rib cage upward and outward.
This coordinated muscular action significantly increases the volume of the thoracic cavity. Due to the cohesive forces of the pleural fluid, the lungs are pulled along with the expanding chest wall, causing their internal volume to increase. This expansion directly leads to a decrease in intrapulmonary pressure.
As the intrapulmonary pressure drops below atmospheric pressure, a pressure gradient is established. Air then rushes in from the outside, moving from higher atmospheric pressure to lower pressure within the lungs. This influx of air continues until the intrapulmonary pressure equalizes with the atmospheric pressure, ending the inspiratory phase.