Atmospheric pressure is the force exerted on every surface by the weight of the air column. The human body is adapted to withstand the pressure at sea level, which establishes equilibrium with the gases and fluids inside the body. However, the body is sensitive to rapid deviations from this baseline. When external pressure changes quickly, the physical rules governing gas behavior override the body’s internal stability, leading to predictable physiological responses.
The Mechanics of Pressure Changes and Trapped Gas
The physical effects of external pressure changes are governed by Boyle’s Law, which states that pressure and volume are inversely proportional for a fixed amount of gas. As external pressure increases, the volume of trapped gas within the body decreases; conversely, as pressure decreases, the volume expands. This mechanical stress from volume change in air-filled cavities is known as barotrauma.
The middle ear is the most common site for barotrauma because it is an air-filled space separated from the outside by the eardrum and connected to the throat by the narrow Eustachian tube. During ascent, the air inside the middle ear expands and must escape through this tube to equalize with the lower external pressure. During descent, external pressure compresses the middle ear air, requiring air to enter the tube to prevent the eardrum from bulging inward.
If the Eustachian tube is blocked, the pressure differential can cause significant pain, a condition called barotitis media, or “airplane ear.” Sinuses are also susceptible, resulting in sinus barotrauma, where expanding or contracting air causes facial pressure and pain. In rare cases, air trapped beneath a dental filling can cause a sharp pain known as barodontalgia as the gas expands or contracts.
Physiological Impact of Low Pressure Environments
When atmospheric pressure drops, such as during ascent to high altitude, the primary threat shifts from mechanical stress to oxygen availability. This is explained by Dalton’s Law of Partial Pressures, which states that total pressure is the sum of the partial pressures of individual gases. Although the percentage of oxygen remains constant at 21%, total atmospheric pressure decreases significantly with altitude, leading to a proportional drop in the partial pressure of oxygen (PO2).
At elevations above 8,000 feet, the reduced PO2 means fewer oxygen molecules are available to enter the bloodstream, resulting in hypobaric hypoxia. The body’s first defense is hyperventilation—a rapid increase in the rate and depth of breathing—which attempts to raise blood oxygen levels. This initial response is accompanied by an increased heart rate to circulate available oxygen more quickly.
If ascent occurs too quickly, these compensatory mechanisms may fail, leading to Acute Mountain Sickness (AMS). AMS manifests as headache, nausea, dizziness, and fatigue, developing within 6 to 12 hours of arrival at altitude. In severe cases, the lack of oxygen can trigger two life-threatening conditions: High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE).
HACE involves a potentially fatal swelling of the brain, caused by hypoxia-induced changes in blood flow and fluid leakage. HAPE is caused by uneven constriction of blood vessels in the lungs due to low oxygen, forcing fluid to leak into the lung tissue and severely impairing gas exchange. The body adjusts over several days and weeks through acclimatization, which involves producing more red blood cells and modifying the blood’s acid-base balance.
Physiological Impact of High Pressure Environments
Conversely, descending into a high-pressure environment, such as deep-sea diving, causes a different set of physiological challenges governed by Henry’s Law. This law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. As a diver descends, the total pressure increases, forcing more of the inhaled inert gases, primarily nitrogen, to dissolve into the body’s blood and tissues.
During a prolonged deep dive, the partial pressure of nitrogen becomes high enough to produce an anesthetic effect on the central nervous system, called nitrogen narcosis, or “raptures of the deep.” This state causes impaired judgment, poor motor control, and euphoria, similar to alcohol intoxication. The increased pressure also raises the partial pressure of oxygen, but excessive PO2 can become toxic, leading to central nervous system oxygen toxicity, which may cause seizures and loss of consciousness.
The most recognized danger of high-pressure exposure occurs during the subsequent ascent, when external pressure decreases rapidly. If the diver rises too quickly, dissolved nitrogen gas comes out of solution faster than the body can safely exhale it, forming bubbles in the tissues and bloodstream. This condition is known as Decompression Sickness (DCS), or “the bends.” Nitrogen bubbles can cause joint pain, skin rashes, or potentially life-threatening blockages in the spinal cord or brain. Safe diving procedures, including controlled ascent rates and mandatory decompression stops, allow gradual pressure reduction, giving inert gas time to diffuse out and be safely expelled.