The human body is adapted to the constant pressure of the atmosphere at sea level, approximately 14.7 pounds per square inch (psi). When a person enters the water, this balance is immediately disrupted by hydrostatic pressure, the force exerted by the weight of the water column above the body. This pressure increases rapidly and linearly with depth, adding one atmosphere (atm) of pressure for every 33 feet (about 10 meters) of descent. Since the body is mostly water, which is incompressible, the primary effects of this change occur in the air-filled spaces and through the behavior of the gases breathed under compression.
Mechanical Effects on Air Spaces
The immediate physical challenges of descending into water are governed by Boyle’s Law, which states that for a fixed amount of gas at a constant temperature, pressure and volume are inversely proportional. As a diver descends, the increasing external water pressure acts to compress any gas-filled space within or connected to the body. If the pressure inside these spaces is not actively equalized with the surrounding water pressure, the resulting pressure differential can cause physical damage known as barotrauma.
A common example is middle ear barotrauma, or “ear squeeze,” which occurs when the volume of air in the middle ear cavity shrinks and pulls the eardrum inward, leading to pain and potential rupture if not equalized via the Eustachian tube. Similarly, if the passageways to the sinuses are congested, the air within those cavities cannot equalize, causing a “sinus squeeze” that can result in internal bleeding. Another form of barotrauma is “mask squeeze,” where the volume of air inside the diving mask is compressed, creating a suction effect that can cause blood vessels in the face and eyes to burst, leading to bruising and red eyes.
Gas Saturation and Decompression Sickness
The increasing pressure at depth alters how gases behave in the body’s tissues and blood, a principle described by Henry’s Law. This law dictates that the amount of gas that will dissolve into a liquid is directly proportional to the partial pressure of that gas above the liquid. As a diver breathes compressed air at depth, the partial pressures of the constituent gases—primarily nitrogen—are much higher than at the surface.
This elevated partial pressure forces inert gases, especially nitrogen, to dissolve into the bloodstream and then into the body’s various tissues. The longer the diver remains at depth, the more saturated these tissues become with dissolved nitrogen. While this “on-gassing” is harmless at depth, the danger arises during the ascent, when the surrounding pressure decreases.
If the ascent is too rapid, the dissolved nitrogen comes out of solution much like carbon dioxide escaping from a quickly opened soda bottle, forming bubbles within the tissues and blood. This bubble formation is the basis of Decompression Sickness (DCS), commonly known as “the bends.” These bubbles can obstruct blood flow and cause damage, leading to symptoms that range from joint pain and skin rashes to paralysis or neurological impairment. To avoid DCS, divers must adhere to strict ascent rates and sometimes perform required decompression stops, allowing the excess nitrogen to be safely expelled through the lungs.
Hyperbaric Gas Toxicity
Beyond the physical effects of volumetric change and bubble formation, high pressure can turn gases into toxins that affect the central nervous system. This is a function of Dalton’s Law of Partial Pressures, which states that the total pressure exerted by a gas mixture is the sum of the partial pressures of each individual gas. At depth, the partial pressure of every gas in the breathing mixture increases, and two gases become problematic: nitrogen and oxygen.
Nitrogen, normally an inert gas, begins to exhibit a narcotic effect on the brain when its partial pressure increases sufficiently, typically around a depth of 100 feet (30 meters) when breathing standard air. This condition, called nitrogen narcosis or “rapture of the deep,” produces symptoms similar to alcohol intoxication, including impaired judgment, decreased coordination, and euphoria.
The second toxicity involves oxygen itself. While oxygen is essential for life, breathing it at high partial pressures can become toxic to the central nervous system (CNS). CNS oxygen toxicity poses a risk when the partial pressure of oxygen exceeds approximately 1.4 atmospheres absolute (ATA) and can occur without warning. The toxicity is caused by the body producing excessive reactive oxygen species, which damage the CNS and can lead to symptoms like uncontrolled muscle twitching, vision changes, and ultimately, a generalized seizure.
The Mammalian Dive Response
Despite the hazards, the human body retains protective reflexes that help mitigate the effects of pressure and oxygen consumption during short, breath-hold dives. This physiological adaptation is known as the mammalian dive response, a reflex present in all mammals triggered primarily by facial immersion in cold water while holding one’s breath.
The first component is bradycardia, an immediate slowing of the heart rate, which helps conserve the body’s limited oxygen supply. Simultaneously, peripheral vasoconstriction occurs, causing blood vessels in the extremities to constrict. This shunts oxygenated blood away from the muscles and skin, redirecting it to the body’s most oxygen-sensitive organs: the heart and the brain.
For extreme depth, a phenomenon known as blood shift takes place, where blood plasma and water move into the chest cavity and the small blood vessels surrounding the lungs. This influx of fluid helps to physically support the lung tissues against the immense external pressure, preventing the air sacs from collapsing entirely. The combined action of these three reflexes acts as a survival mechanism, significantly reducing the body’s metabolic rate and optimizing the use of stored oxygen during submersion.