Venturing into deep water introduces unique and significant challenges for the human body. The ocean’s immense pressures and frigid temperatures, fundamental physical properties of water, directly impact how the body’s systems function and respond.
Effects of Pressure
Water exerts a substantial amount of pressure that increases dramatically with depth, known as hydrostatic pressure. For every 33 feet (10 meters) of descent in seawater, the pressure increases by approximately one atmosphere (1 Atm), equivalent to the pressure at sea level. This escalating pressure directly affects the air-filled spaces within the human body, causing them to compress.
Air-filled cavities such as the lungs, sinuses, and middle ears are susceptible to these pressure changes. As a diver descends, the increasing external pressure can cause discomfort or injury if the pressure inside these spaces is not equalized with the surrounding water pressure. This imbalance leads to a condition called “squeeze” or barotrauma, which is physical damage to tissues caused by pressure differences.
Examples of barotrauma include ear squeeze, where the eardrum bulges inward and can rupture. Sinus squeeze results from pressure differences in the sinus cavities, often causing pain, headaches, or nosebleeds. Pulmonary barotrauma, or lung squeeze, occurs when the lungs are compressed below their residual volume, potentially causing fluid or blood to enter the lung tissues. Solid tissues and fluids, like blood, are largely incompressible under these pressures, meaning the primary concerns stem from the body’s air-filled compartments.
Gas Interactions
The behavior of gases within the body changes considerably under high pressure, leading to specific physiological impacts. Henry’s Law explains that as pressure increases, more gas dissolves into liquids, such as the blood and tissues.
Nitrogen narcosis, sometimes called “rapture of the deep” or “the martini effect,” occurs when increased partial pressure of nitrogen acts as an anesthetic on the brain. Divers may experience impaired judgment, disorientation, and issues with motor skills, similar to alcohol intoxication. The effects can vary significantly among individuals, with some experiencing symptoms at depths around 30 meters (100 feet).
Oxygen, while necessary for life, becomes toxic at high partial pressures. Central Nervous System (CNS) oxygen toxicity can lead to severe symptoms such as seizures and convulsions. Pulmonary oxygen toxicity, affecting the lungs, can occur with prolonged exposure to elevated oxygen levels.
Decompression Sickness (DCS), commonly known as “the bends,” is a condition where dissolved gases, primarily nitrogen, form bubbles in tissues and the bloodstream upon rapid ascent. This happens when the surrounding pressure decreases too quickly, causing the gases to come out of solution. These bubbles can block blood vessels, damage tissues, and trigger inflammatory responses. Symptoms of DCS can range from joint pain and skin rashes to more severe neurological issues, paralysis, or even death, depending on where the bubbles form.
Cold Water’s Impact
Deep water environments are typically cold, and water conducts heat away from the body much more efficiently than air, approximately 25 times faster. This rapid heat loss can quickly drop the body’s core temperature, leading to hypothermia.
Hypothermia is a reduction in core body temperature below the normal range, generally considered below 95°F (35°C). Mild hypothermia is characterized by shivering, clumsiness, and impaired motor function, while moderate hypothermia can lead to uncontrollable shivering, confusion, and reduced coordination. In severe hypothermia, shivering may cease, basic body functions slow significantly, and unconsciousness can occur, potentially leading to organ failure.
Sudden immersion in cold water can also trigger a “cold shock response,” an involuntary reaction that includes an immediate gasp for air and rapid, uncontrolled breathing (hyperventilation). This response can increase the risk of drowning, especially if the initial gasp occurs underwater. The cold shock response also causes blood vessels in the skin to constrict, elevating heart rate.
The Mammalian Dive Response
The human body possesses an innate physiological adaptation known as the mammalian dive response, a reflex present in all mammals that helps conserve oxygen during immersion. This involuntary response is primarily triggered by facial immersion in cold water. It serves as a protective mechanism to extend survival time by prioritizing oxygen delivery to essential organs.
One component of this response is bradycardia, a significant slowing of the heart rate. This reduction in heart rate decreases the heart’s oxygen consumption. Simultaneously, peripheral vasoconstriction occurs, where blood vessels in the extremities, such as arms, legs, and skin, constrict. This shunts oxygenated blood away from less critical areas and redirects it toward the brain and heart, which are the most oxygen-sensitive organs.
During extreme breath-hold diving, blood shift may occur. Blood plasma and water can move into the thoracic cavity, helping to prevent lung collapse under intense pressure. This mechanism allows the lungs to be compressed beyond their normal residual volume without immediate injury. While these adaptations have distinct limits, they are not sufficient for prolonged deep-water survival without specialized equipment and training.