When a person travels down, whether into the ocean or toward the bottom of a deep valley, the surrounding pressure consistently increases. Pressure is defined as the force exerted over a specific area. This force results from the collective weight of the material—be it a liquid or a gas—that exists above the point of measurement. The increase in this overlying weight is why pressure always rises as descent occurs through any medium.
The Physics of Pressure and Depth
The mechanism that governs the rise in pressure with depth is the column of material pressing down due to gravity. The total pressure at any point is directly related to the density of the surrounding fluid and the vertical distance from the surface. In any static fluid, this relationship is known as hydrostatic pressure, which accounts for the accumulation of weight from all the layers above.
The greater the density of the medium, the faster the pressure will increase with descent. Gravitational pull is the constant force acting on this mass, compressing the fluid below it. The resulting pressure is not directional; it acts equally on an object from all sides, a concept known as being isotropic.
Pressure Changes in Liquid Environments
Liquid environments, such as oceans, lakes, and rivers, demonstrate the most rapid pressure increase because they are largely incompressible and highly dense. Water maintains a constant density throughout its depth, which results in a linear and predictable rate of pressure change. This rate of increase is significantly faster than what is experienced in a gaseous environment.
In seawater, the pressure increases by approximately one standard atmosphere (atm) for every 10 meters (about 33 feet) of descent. For example, a diver who descends to 30 meters is exposed to four times the pressure felt at the surface—one atmosphere from the air above and three additional atmospheres from the water column. The high density of water causes the weight of the overlying column to accumulate quickly over a short vertical distance.
Pressure Changes in Gaseous Environments
Pressure increases when traveling down through a gaseous environment, such as descending from a high mountain to sea level, but the rate is much slower and non-linear. The atmosphere is a compressible fluid, meaning its density is greatest at the lowest altitudes, where the entire weight of the air column presses down. As altitude decreases, the pressure increases because more air molecules are stacked above the traveler.
The pressure gradient is not constant because air density decreases significantly with elevation. The highest atmospheric pressure, defined as one standard atmosphere, is found at sea level, where the column of air is longest and most dense. This contrasts sharply with liquid environments, where the density remains stable.
How Pressure Affects the Human Body
The human body is affected by increased external pressure, particularly because of the presence of air-filled spaces. As a person descends, the increased ambient pressure causes the gas in these enclosed spaces to compress. If the pressure in internal spaces, such as the middle ear or sinuses, cannot be equalized with the outside pressure, the resulting imbalance can lead to barotrauma.
The air in the middle ear and sinuses is vulnerable, and failure to equalize can cause pain and tissue damage, commonly known as an ear or sinus squeeze. To avoid this injury, divers must constantly breathe gas at a pressure equal to the external water pressure. This necessity introduces physiological problems related to the gases themselves.
When breathing compressed gas, the increased partial pressure forces more inert gas, like nitrogen, to dissolve into the blood and body tissues. Nitrogen is highly soluble in lipid-rich tissues, such as the central nervous system, and at high pressures, this can cause nitrogen narcosis. Symptoms of this effect, often described as similar to alcohol intoxication, include impaired judgment, loss of coordination, and euphoria.
The increased saturation of inert gas in the body’s tissues creates the risk for decompression sickness, commonly known as “the bends.” This condition occurs when the person ascends too quickly, causing the dissolved gas to come out of solution and form bubbles within the tissues and bloodstream. The formation of these bubbles can block blood flow and damage nerve tissue, potentially leading to severe pain, paralysis, or death.