Intrapulmonary pressure is the pressure inside the lungs’ air sacs, known as the alveoli. This pressure changes with each breath, fluctuating relative to the air pressure outside the body to drive the movement of air. This process is the fundamental mechanism of breathing. Between breaths, this internal lung pressure momentarily equals the atmospheric pressure.
The Mechanics of Breathing
The process of breathing is governed by the relationship between volume and pressure, as described by Boyle’s Law. This law states that a gas’s pressure is inversely proportional to its volume. For example, as the volume of a flexible container of air decreases, the pressure inside increases. The lungs behave similarly, with the body’s muscles creating volume changes that alter intrapulmonary pressure and cause air to flow.
Inspiration, or inhaling, is an active process caused by muscle contraction. The diaphragm, a large muscle below the lungs, contracts and moves downward. At the same time, the external intercostal muscles between the ribs contract, pulling the rib cage upward and outward. These actions expand the thoracic cavity’s volume, causing the attached lungs to stretch and increase their volume.
This increase in lung volume causes the intrapulmonary pressure to drop below atmospheric pressure, creating a pressure gradient. Since air moves from an area of higher pressure to one of lower pressure, this difference causes air to rush into the lungs. This continues until the internal and external pressures equalize.
Normal expiration, or exhaling, is a passive process that begins when the diaphragm and external intercostal muscles relax. This relaxation decreases the volume of the thoracic cavity, which compresses the lungs. The resulting decrease in lung volume raises the intrapulmonary pressure above atmospheric pressure, forcing air out of the lungs until the pressures are equal.
Key Pressures in Respiration
The primary reference point for breathing is atmospheric pressure, which is the pressure exerted by the surrounding air. At sea level, this is standardized at 760 millimeters of mercury (mm Hg). All pressures within the respiratory system are described relative to this benchmark.
Intrapleural pressure is the pressure within the pleural cavity, the fluid-filled space between the lungs and the chest wall. This pressure is always negative, meaning it remains lower than both intrapulmonary and atmospheric pressure. This negative pressure acts like a suction, holding the lungs’ surface against the chest wall and preventing them from collapsing.
The difference between the intrapulmonary and intrapleural pressures is the transpulmonary pressure. This pressure represents the force that keeps the alveoli open and the lungs expanded. A higher transpulmonary pressure corresponds to a more inflated lung, directly influencing lung size during ventilation.
Factors Influencing Airflow
Airflow is also affected by physical factors, with one of the most significant being airway resistance. The diameter of the airways affects how easily air can flow. When airways are wide, resistance is low, and air moves freely.
Small changes in the diameter of the bronchi and bronchioles can alter this resistance. When these passages narrow, resistance increases, forcing the respiratory muscles to work harder to move air.
Another factor is lung compliance, a measure of how easily the lungs can stretch. High compliance means the lungs are elastic and expand with little effort. Low compliance means the lungs are stiff and resist expansion, requiring more force to increase their volume. Conditions like pulmonary fibrosis create scar tissue that decreases compliance, impairing the ability to change lung volume and intrapulmonary pressure.
Conditions Affecting Lung Pressure
Disruptions to the pressure balance in the chest can cause respiratory problems like a pneumothorax. This condition occurs when air enters the pleural space from an external injury or a rupture within the lung. This breach allows the intrapleural pressure to equalize with the atmospheric pressure.
When the negative intrapleural pressure is lost, the suction effect holding the lung expanded is gone. Without the transpulmonary pressure holding them open, the lung’s natural elastic recoil causes it to collapse. This reduces the ability to ventilate and exchange gas on the affected side.
Obstructive lung diseases like asthma and chronic obstructive pulmonary disease (COPD) also interfere with pressure dynamics. These conditions are characterized by increased airway resistance, which impedes the flow of air out of the lungs. During exhalation, the increased resistance requires intrapulmonary pressure to be raised higher to force air out, often requiring accessory muscles. This can lead to air trapping, where air remains in the lungs after a full exhalation.