The human body is adapted to atmospheric pressure at sea level. When a person descends underwater, pressure increases significantly due to the weight of the water column. For every 33 feet (10 meters) of descent in saltwater, pressure increases by another atmosphere, placing immense forces on the body. This escalating pressure profoundly influences human physiology, necessitating various adaptations and posing unique challenges.
How Pressure Affects Air-Filled Spaces
Increasing underwater pressure primarily affects the body’s air-filled spaces, governed by Boyle’s Law. This principle states that as pressure increases, the volume of a gas decreases proportionally. This compression can lead to barotraumas, injuries caused by pressure differences between an internal gas space and the surrounding environment.
Ear squeeze (middle-ear barotrauma) occurs when middle ear pressure cannot equalize with increasing external water pressure. The Eustachian tube normally allows air to flow to maintain equilibrium. If blocked, a pressure imbalance causes the eardrum to bulge inward, leading to pain, fluid leakage, or ruptured blood vessels. Sinus squeeze (barosinusitis) affects the air-filled sinus cavities. If their openings become blocked, decreasing air volume during descent creates a painful vacuum, potentially causing inflammation, bleeding, and facial discomfort.
The lungs also experience significant volume changes. During descent, lung volume decreases substantially, with a 50% reduction in the first 33 feet (10 meters) of water. In extreme free-diving, “lung squeeze” (pulmonary barotrauma of descent) can occur if lungs are compressed beyond their residual volume, potentially causing fluid or blood from ruptured capillaries to enter lung tissue.
During ascent, the air in the lungs expands. If a diver holds their breath, this expanding air can overinflate the lungs, leading to a lung overexpansion injury (pulmonary barotrauma of ascent). This can cause severe conditions like pneumothorax or arterial gas embolism.
The air space within a diving mask is also affected. Mask squeeze occurs if a diver fails to exhale small amounts of air through their nose into the mask, causing the mask to press against the face due to negative pressure, leading to bruising around the eyes or nosebleeds.
How Pressure Affects Dissolved Gases
Increasing underwater pressure also changes how gases dissolve in the body’s tissues and blood. This is explained by Henry’s Law: the amount of gas dissolved in a liquid is directly proportional to its partial pressure above the liquid. As a diver descends and ambient pressure rises, more inert gases, primarily nitrogen, dissolve into the body’s fluids and tissues.
Increased dissolved nitrogen can lead to nitrogen narcosis, often called “rapture of the deep.” At greater depths, nitrogen acts as an anesthetic, impairing cognitive function and judgment. Symptoms include euphoria, disorientation, impaired motor skills, and memory loss, similar to alcohol intoxication. Effects typically become noticeable around 100 feet (30 meters) and worsen with increasing depth.
Decompression sickness (DCS), commonly known as “the bends,” is a more serious concern. This condition arises during ascent when surrounding pressure decreases, causing dissolved nitrogen to come out of solution. If ascent is too rapid, nitrogen forms bubbles within tissues and the bloodstream rather than being exhaled. These bubbles can block blood vessels, distort tissues, and trigger inflammatory responses. Symptoms include joint pain, skin rashes, severe fatigue, or more serious neurological issues like numbness, tingling, weakness, or paralysis if bubbles affect the brain or spinal cord.
The Body’s Other Physiological Responses
The human body exhibits other physiological responses to the underwater environment. These involuntary adaptations aim to conserve oxygen and protect vital organs. The mammalian dive reflex, an innate mechanism found in all mammals including humans, is triggered by facial immersion in cold water and breath-holding.
The dive reflex initiates several physiological changes. Bradycardia, a significant slowing of the heart rate, reduces the heart’s oxygen demand and conserves oxygen reserves. Peripheral vasoconstriction narrows blood vessels in the extremities and non-vital areas like the skin and digestive organs. This redirects blood flow, prioritizing oxygen-rich blood for the brain and heart, which are highly sensitive to oxygen deprivation.
Another adaptation, particularly in deep free-diving, is the blood shift. As a diver descends and the lungs compress, blood and other bodily fluids are shunted from the extremities and abdominal regions into the chest cavity and surrounding lung blood vessels. This helps maintain lung integrity and prevents collapse under extreme pressure by filling the space left by compressed air. The spleen also contracts to release additional oxygenated red blood cells, increasing the blood’s oxygen-carrying capacity.
Finally, immersion in water can trigger pressure diuresis, an increased urine production. This response results from the blood shift increasing central blood volume, which the body perceives as fluid overload, prompting the kidneys to excrete more fluid.