Underwater diving
Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also often referred to as diving, an ambiguous term with several possible meanings, depending on context.
Immersion in water and exposure to high ambient pressure have physiological effects that limit the depths and duration possible in ambient pressure diving. Humans are not physiologically and anatomically well-adapted to the environmental conditions of diving, and various equipment has been developed to extend the depth and duration of human dives, and allow different types of work to be done.
In ambient pressure diving, the diver is directly exposed to the pressure of the surrounding water. The ambient pressure diver may dive on breath-hold or use breathing apparatus for scuba diving or surface-supplied diving, and the saturation diving technique reduces the risk of decompression sickness after long-duration deep dives. Atmospheric diving suits may be used to isolate the diver from high ambient pressure. Crewed submersibles can extend depth range to full ocean depth, and remotely controlled or robotic machines can reduce risk to humans.
The environment exposes the diver to a wide range of hazards, and though the risks are largely controlled by appropriate diving skills, training, types of equipment and breathing gases used depending on the mode, depth and purpose of diving, it remains a relatively dangerous activity. Professional diving is usually regulated by occupational health and safety legislation, while recreational diving may be entirely unregulated.
Diving activities are restricted to maximum depths of about for recreational scuba diving, for commercial saturation diving, and wearing atmospheric suits. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable can vary, and fatal incidents may occur.
Recreational diving is a popular leisure activity. Technical diving is a form of recreational diving under more challenging conditions. Professional diving involves working underwater. Public safety diving is the underwater work done by law enforcement, fire rescue, and underwater search and recovery dive teams. Military diving includes combat diving, clearance diving and ships husbandry.
is underwater diving, usually with surface-supplied equipment, and often refers to the use of standard diving dress with the traditional copper helmet. Hard hat diving is any form of diving with a helmet, including the standard copper helmet, and other forms of free-flow and lightweight demand helmets.
The history of breath-hold diving goes back at least to classical times, and there is evidence of prehistoric hunting and gathering of seafoods that may have involved underwater swimming. Technical advances allowing the provision of breathing gas to a diver underwater at ambient pressure are recent, and self-contained breathing systems developed at an accelerated rate following the Second World War.
Physiological effects of ambient pressure diving
Immersion in water and exposure to cold water and high pressure have physiological effects on the diver which limit the depths and duration possible in ambient pressure diving. Breath-hold endurance is a severe limitation, and breathing at high ambient pressure adds further complications, both directly and indirectly. Technological solutions have been developed which can greatly extend depth and duration of human ambient pressure dives, and allow useful work to be done underwater.As of 2009, there was no clear evidence that diving causes long term neurological dysfunction except by acute damage.
Immersion
Immersion of the human body in water affects the circulation, renal system, fluid balance, and breathing, because the external hydrostatic pressure of the water provides support against the internal hydrostatic pressure of the blood. This causes a blood shift from the extravascular tissues of the limbs into the chest cavity, and fluid losses known as immersion diuresis compensate for the blood shift in hydrated subjects soon after immersion. Hydrostatic pressure on the body from head-out immersion causes negative pressure breathing which contributes to the blood shift.The blood shift causes an increased respiratory and cardiac workload. Stroke volume is not greatly affected by immersion or variation in ambient pressure, but slowed heartbeat reduces the overall cardiac output, particularly because of the diving reflex in breath-hold diving. Lung volume decreases in the upright position, owing to cranial displacement of the abdomen from hydrostatic pressure, and resistance to air flow in the airways increases because of the decrease in lung volume. There appears to be a connection between pulmonary edema and increased pulmonary blood flow and pressure, which results in capillary engorgement. This may occur during higher intensity exercise while immersed or submerged.
The diving reflex is a response to immersion that overrides the basic homeostatic reflexes. It optimises respiration by preferentially distributing oxygen stores to the heart and brain, which allows extended periods underwater. It is exhibited strongly in aquatic mammals, and also exists in other mammals, including humans. Diving birds, such as penguins, have a similar diving reflex. The diving reflex is triggered by chilling the face and holding the breath. The cardiovascular system constricts peripheral blood vessels, slows the pulse rate, redirects blood to the vital organs to conserve oxygen, releases red blood cells stored in the spleen, and, in humans, causes heart rhythm irregularities. Aquatic mammals have evolved physiological adaptations to conserve oxygen during submersion, but apnea, slowed pulse rate, and vasoconstriction are shared with terrestrial mammals.
Exposure
is the physiological response of organisms to sudden cold, especially cold water, and is a common cause of death from immersion in very cold water, such as by falling through thin ice. The immediate shock of the cold causes involuntary inhalation, which if underwater can result in drowning. The cold water can also cause heart attack due to vasoconstriction; the heart has to work harder to pump the same volume of blood throughout the body, and for people with heart disease, this additional workload can cause the heart to go into arrest. A person who survives the initial minute after falling into cold water can survive for at least thirty minutes provided they do not drown. The ability to stay afloat declines substantially after about ten minutes as the chilled muscles lose strength and co-ordination.Hypothermia is reduced core body temperature that occurs when a body loses more heat than it generates. It is a major limitation to swimming or diving in cold water. The reduction in finger dexterity due to pain or numbness decreases general safety and work capacity, which in turn increases the risk of other injuries. Non-freezing cold injury can affect the extremities in cold water diving, and frostbite can occur when air temperatures are low enough to cause tissue freezing. Body heat is lost much more quickly in water than in air, so water temperatures that would be tolerable as outdoor air temperatures can lead to hypothermia, which may lead to death from other causes in inadequately protected divers.
Thermoregulation of divers is complicated by breathing gases at raised ambient pressure and by gas mixtures necessary for limiting inert gas narcosis, work of breathing, and for accelerating decompression.
Breath-hold limitations
Breath-hold diving by an air-breathing animal is limited to the physiological capacity to perform the dive on the oxygen available until it returns to a source of fresh breathing gas, usually the air at the surface. As this internal oxygen supply reduces, the animal experiences an increasing urge to breathe caused by buildup of carbon dioxide and lactate in the blood, followed by loss of consciousness due to cerebral hypoxia. If this occurs underwater, it will drown.Blackouts in freediving can occur when the breath is held long enough for metabolic activity to reduce the oxygen partial pressure sufficiently to cause loss of consciousness. This is accelerated by exertion, which uses oxygen faster, and can be exacerbated by hyperventilation directly before the dive, which reduces the carbon dioxide level in the blood. Lower carbon dioxide levels increase the oxygen-haemoglobin affinity, reducing availability of oxygen to brain tissue towards the end of the dive ; they also suppress the urge to breathe, making it easier to hold the breath to the point of blackout. This can happen at any depth.
Ascent-induced hypoxia is caused by a drop in oxygen partial pressure as ambient pressure is reduced. The partial pressure of oxygen at depth may be sufficient to maintain consciousness at that depth and not at the reduced pressures nearer the surface.
Ambient pressure changes
, a type of dysbarism, is physical damage to body tissues caused by a difference in pressure between a gas space inside, or in contact with the body, and the surrounding gas or fluid. It typically occurs when the organism is exposed to a large change in ambient pressure, such as when a diver ascends or descends. When diving, the pressure differences which cause the barotrauma are changes in hydrostatic pressure.The initial damage is usually due to over-stretching the tissues in tension or shear, either directly by expansion of the gas in the closed space, or by pressure difference hydrostatically transmitted through the tissue.
Barotrauma generally manifests as sinus or middle ear effects, decompression sickness, lung over-expansion injuries, and injuries resulting from external squeezes. Barotraumas of descent are caused by preventing the free change of volume of the gas in a closed space in contact with the diver, resulting in a pressure difference between the tissues and the gas space, and the unbalanced force due to this pressure difference causes deformation of the tissues resulting in cell rupture. Barotraumas of ascent are also caused when the free change of volume of the gas in a closed space in contact with the diver is prevented. In this case the pressure difference causes a resultant tension in the surrounding tissues which exceeds their tensile strength. Besides tissue rupture, the overpressure may cause ingress of gases into the adjoining tissues and further afield by bubble transport through the circulatory system. This can cause blockage of circulation at distant sites, or interfere with the normal function of an organ by its presence.