Atmospheric pressure, also known as barometric pressure, is the force exerted by the weight of gas molecules in the atmosphere. This pressure decreases as elevation increases because the air becomes thinner and less dense. The human body functions optimally at standard sea-level pressure (one atmosphere), which provides an external force that balances the internal pressures within the body’s tissues and fluids. When this environmental pressure changes significantly, the body’s equilibrium is disrupted, leading to various physiological responses.
How Gas Laws Govern Bodily Response
The physical behavior of gases inside the body directly influences how humans respond to pressure changes, following fundamental principles of physics. Two gas laws are particularly relevant to understanding the effects of pressure gradients.
Boyle’s Law describes the inverse relationship between the pressure and volume of a gas when temperature remains constant. When external pressure decreases, any fixed volume of gas trapped within the body, such as in the sinuses or lungs, will expand proportionally. Conversely, an increase in external pressure causes the volume of trapped gas to compress.
Henry’s Law governs the interaction between gases and liquids, stating that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. In the body, this law dictates how gases like nitrogen and oxygen dissolve into the blood and tissues. For example, when external pressure increases significantly, more nitrogen is forced into solution within the bloodstream.
Physiological Effects of Low Pressure Environments (Ascending)
Moving from sea level to higher altitudes causes atmospheric pressure to drop exponentially, triggering challenges related to gas expansion and reduced oxygen availability. The primary consequence is hypobaric hypoxia, where lower barometric pressure results in a proportionate decrease in the partial pressure of oxygen. At altitudes above 2,100 meters (about 6,900 feet), the saturation of oxygen in the blood begins to decrease rapidly, which directly impairs oxygen delivery to tissues.
The body’s immediate short-term response to oxygen deprivation includes hyperventilation (increased breathing rate) and an elevated heart rate. While this increases oxygen intake, it can lead to respiratory alkalosis, where carbon dioxide levels in the blood become too low. Continued exposure often leads to acute mountain sickness (AMS), presenting as headache, fatigue, dizziness, insomnia, and nausea.
The second major effect of ascent is barotrauma, resulting from the expansion of gas volumes according to Boyle’s Law. Air trapped in enclosed spaces, such as the middle ear or sinuses, expands as the surrounding pressure drops. If this gas cannot equalize through the Eustachian tubes or sinus openings, it pushes against surrounding tissue, causing pain known as ear squeeze or sinus squeeze.
For individuals remaining at high altitude, the body attempts long-term acclimatization by increasing red blood cell production (polycythemia) to enhance oxygen-carrying capacity. This adaptation involves the kidneys sensing low oxygen and stimulating the release of erythropoietin. Full acclimatization requires days or weeks and is a complex biological adjustment to the persistent lack of oxygen.
Physiological Effects of High Pressure Environments (Descending)
Descending into a high-pressure environment, such as underwater during a deep dive, exposes the body to hydrostatic pressure that increases dramatically with depth. The effects are driven by gas compression and the increased dissolution of gases into bodily fluids. As a diver descends, ambient pressure forces the volume of gas in the lungs, sinuses, and middle ear to compress, potentially causing barotrauma of descent if these spaces are not actively equalized.
Due to Henry’s Law, inert gases breathed by the diver, primarily nitrogen, dissolve into the blood and tissues in greater amounts. The body becomes saturated with nitrogen proportional to the depth and duration of the dive. This excess dissolved gas becomes problematic upon ascent (reduction in ambient pressure).
If the ascent is too rapid, the dissolved nitrogen forms bubbles in the tissues and bloodstream, leading to decompression sickness (DCS), commonly known as “the bends.” These bubbles can obstruct blood vessels, causing symptoms ranging from joint pain and tingling to paralysis and unconsciousness.
The high partial pressure of nitrogen also causes nitrogen narcosis at significant depths, typically becoming noticeable below 30 meters (98 feet). This condition alters consciousness and neuro-sensory state, producing an intoxicating effect. Another risk is oxygen toxicity, which occurs when the partial pressure of oxygen becomes too high, potentially causing seizures due to toxicity to the central nervous system. Deep-diving techniques often use special gas mixtures like Trimix, which substitutes some nitrogen for helium to mitigate the narcotic effects.
How Weather-Related Pressure Shifts Affect the Body
Daily weather patterns involve minor fluctuations in barometric pressure, distinct from the extreme changes experienced during diving or high-altitude climbing. These subtle shifts, particularly a drop in atmospheric pressure preceding a storm, are linked to physical discomfort.
When barometric pressure falls, the external force lessens, allowing tissues and fluids inside the body to expand slightly. In people with chronic joint conditions, this slight expansion can put pressure on nerves surrounding the joints, leading to increased pain and discomfort, especially in joints affected by arthritis. Scientists theorize that low pressure may also affect the viscosity of the synovial fluid that lubricates the joints.
Barometric pressure changes are also associated with triggering headaches and migraines. Low pressure events can activate the autonomic nervous system and heighten pain sensitivity, which may trigger a migraine attack. Researchers speculate that these pressure shifts may influence levels of brain chemicals, such as serotonin, implicated in migraine mechanisms. Monitoring these unique patterns, sometimes called a storm signature, helps individuals understand how their body responds to these minor atmospheric instabilities.