The process of breathing is fundamentally a mechanical event driven by pressure changes, not simply an act of inhaling air into the body. This process relies entirely on creating a pressure difference between the air outside the body and the air inside the lungs. Human respiration is a form of negative pressure breathing, a mechanism where the body actively expands the sealed chest cavity to drop the internal pressure below that of the atmosphere. Air then flows naturally from the higher external pressure into the lower internal pressure to balance the gradient.
How the Lungs Achieve Negative Pressure
Quiet, normal inspiration is an active process initiated by the contraction of specific respiratory muscles. The largest muscle is the diaphragm, a dome-shaped sheet of muscle situated at the base of the rib cage. When the diaphragm contracts, it flattens and moves downward into the abdominal cavity. Simultaneously, the external intercostal muscles, located between the ribs, contract to pull the rib cage upward and outward. This combined action significantly increases the volume of the thoracic cavity, the space containing the lungs.
This mechanical expansion is the first step in generating the necessary negative pressure. The relationship between volume and pressure is governed by Boyle’s Law, which states that for a fixed amount of gas at a constant temperature, pressure and volume are inversely related. As the thoracic volume increases due to muscle contraction, the intrapulmonary pressure—the pressure within the lungs—must decrease. This pressure drop is slight, often falling to about 1 millimeter of mercury (mm Hg) below the surrounding atmospheric pressure.
A sealed pleural cavity exists between the lungs and the inner wall of the chest. This space normally maintains a slightly negative pressure of about -4 mm Hg relative to the atmosphere, sustained by a thin layer of fluid that links the visceral pleura (covering the lung) to the parietal pleura (lining the chest wall). This negative intrapleural pressure acts like glue, ensuring that when the chest wall expands, the lungs are forced to follow, preventing their natural tendency to recoil. Because the air pressure outside the body is now higher than the pressure inside the lungs, air rushes in through the airways until the pressures equalize, completing the act of inhalation.
What Happens During Lung Collapse
The respiratory system depends on the integrity of the pleural cavity and its sub-atmospheric pressure. A pneumothorax, commonly known as a collapsed lung, occurs when this pressure balance is breached. This condition involves air collecting in the pleural space, which normally contains only lubricating fluid.
The air can enter the pleural space through a hole in the chest wall, often due to penetrating trauma, or via a breach in the lung surface itself, such as a ruptured air sac. When this happens, the pressure in the pleural cavity equalizes with the atmospheric pressure. The loss of the negative pressure gradient means the lung is no longer held against the chest wall.
The lung tissue possesses a natural elasticity. Without the opposing outward pull of the chest wall maintained by the negative intrapleural pressure, this elastic recoil causes the lung to shrink away from the chest wall and collapse. The degree of collapse depends on the size of the air leak into the pleural space.
Types of Pneumothorax
Pneumothorax can be caused by blunt or penetrating trauma. A spontaneous pneumothorax can occur in people without any preceding injury, often due to the rupture of small, air-filled sacs on the lung surface. In a severe form, called a tension pneumothorax, a one-way valve mechanism allows air to enter the pleural space with each breath but prevents it from escaping. This causes pressure to build rapidly, compressing the lung and shifting the structures in the chest, impairing heart function.
Negative Pressure in Medical History
The principle of negative pressure breathing was applied externally to save lives when a patient’s respiratory muscles failed. The “Iron Lung,” formally known as a tank respirator, became widespread during the polio epidemics of the mid-20th century. The patient’s entire body, except for the head, was sealed inside the airtight chamber.
A pump mechanism within the machine would rhythmically lower the air pressure inside the tank, creating a vacuum around the patient’s chest. This external negative pressure pulled the chest wall outward, mimicking the action of healthy respiratory muscles, which forced the lungs to expand and draw air in. When the negative pressure was released, the natural elastic recoil of the patient’s chest and lungs caused exhalation.
This method of external negative pressure ventilation provided life support for individuals with paralyzed respiratory muscles, such as those affected by poliomyelitis or certain poisons. The iron lung is largely obsolete in modern medicine, replaced by mechanical ventilators that use a different principle.
Modern mechanical ventilation utilizes positive pressure, a method that is the conceptual opposite of natural breathing. Instead of expanding the chest cavity to draw air in, positive pressure ventilators force air directly into the patient’s lungs through a tube in the airway. This approach is more compact and allows easier patient access, but it applies pressure to the lung tissue, unlike the external action of the negative pressure device.