The human body constantly interacts with pressure, a fundamental force present in every environment. Pressure variations exert significant effects on our physiology, revealing both the body’s remarkable adaptability and its inherent vulnerabilities. This exploration delves into how the human body responds to and is challenged by varying pressure conditions.
How Pressure Affects the Human Body
Pressure changes significantly impact the human body, particularly its air-filled spaces and the gases dissolved within its fluids. Boyle’s Law states that gas volume is inversely proportional to pressure. This explains how air in the body’s cavities, like lungs, sinuses, and middle ears, reacts to external pressure shifts: volume decreases as external pressure increases, and expands when pressure drops. This principle is fundamental to breathing, where lung volume changes create pressure differentials that facilitate air movement.
Henry’s Law states that the amount of gas dissolved in a liquid is directly proportional to its partial pressure above the liquid. This is especially relevant in environments like underwater diving, where increasing ambient pressure causes more gases, such as nitrogen, to dissolve into blood and tissues. While the human body is largely composed of incompressible water, its gases are highly susceptible to these pressure-volume and solubility changes, forming the basis for many pressure-related physiological concerns.
External Pressure Limits: Water and Air
The human body faces distinct challenges from extreme external pressures. In high-pressure environments, such as deep-sea submersion, the primary concern is the effect on air-filled spaces. While the body’s fluid and solid components are largely incompressible, increasing water pressure compresses gases in the lungs, sinuses, and middle ears.
Without specialized equipment, human tolerance underwater is limited, as the lungs cannot effectively expand against the external force. Even at shallow depths, breathing through a snorkel becomes impossible due to chest pressure. Professional free divers can reach depths of around 400 feet through physiological adaptations and training. With highly specialized equipment, saturation divers have reached pressures equivalent to 701 meters (1,043 PSI). For context, the Mariana Trench exerts about 15,750 PSI.
Conversely, extremely low pressure, such as in a vacuum or at very high altitudes, presents different dangers. At sea level, atmospheric pressure averages 14.7 PSI. As altitude increases, external pressure decreases, causing gases within the body to expand. At approximately 61,000 feet (0.9 PSI), ebullism can occur. At this pressure, water’s boiling point drops to body temperature, causing body fluids in open cavities and tissues to vaporize and form bubbles. This can lead to swelling, impaired circulation and respiration, and ultimately unconsciousness and death within minutes without recompression.
Internal Pressure Changes: Decompression and G-Forces
Rapid shifts in external pressure or significant acceleration forces induce substantial internal pressure changes. Rapid decompression, such as a sudden loss of aircraft cabin pressure, causes a swift drop in ambient pressure. This sudden pressure differential leads to gases dissolved in the blood and tissues forming bubbles too quickly. While human tolerance varies, aircraft cabin pressurization systems maintain a differential of around 8.3 PSI above outside air. A sudden loss of this pressure can cause internal imbalances, damaging air-filled cavities and leading to serious physiological consequences.
G-forces, or gravitational forces, create internal pressure differences due to inertia, influencing blood flow and organ displacement. Positive G-forces (+Gz), experienced when accelerating upwards or in tight turns, push blood towards the lower extremities, away from the brain. An average person without training or a G-suit may experience G-LOC at 4 to 6 Gs. Fighter pilots, using specialized anti-G suits and straining maneuvers, can tolerate sustained forces of 9 Gs or more. Conversely, negative G-forces (-Gz), which push blood towards the head, are less tolerable, typically causing issues at -2 to -3 Gs due to increased head pressure and potential “redout.”
When Pressure Exceeds Human Limits
When the body’s pressure tolerance is exceeded, specific physiological injuries can arise. Barotrauma refers to tissue damage caused by pressure differences between a gas-filled space inside the body and the surrounding fluid or gas. Common examples include ear barotrauma, causing pain and potential eardrum rupture, and sinus barotrauma, affecting air-filled sinuses. Pulmonary barotrauma, or “lung squeeze,” occurs when rapid ascent or decompression causes expanding air in the lungs to damage tissue.
Decompression Sickness (DCS), or “the bends,” results from dissolved gases, primarily nitrogen, forming bubbles in blood and tissues when external pressure decreases too rapidly. These bubbles can cause symptoms from joint pain and skin rashes to neurological impairments, paralysis, and even death. This highlights the importance of controlled ascent rates in diving and careful management of pressure changes in aerospace environments. G-LOC is another severe consequence, where sustained positive G-forces cause blood to pool in the lower body, leading to brain oxygen deprivation. This can result in temporary loss of consciousness, posing a significant risk in high-performance aviation.