Human physiology of underwater diving
It, therefore, includes the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus.
Metabolically inactive gases are absorbed by the tissues and may have narcotic or other undesirable effects, and must be released slowly to avoid the formation of bubbles during decompression.
However, the ability to perform useful work like staying afloat declines substantially after ten minutes as the body protectively cuts off blood flow to "non-essential" muscles.
[6][7] It optimizes respiration by preferentially distributing oxygen stores to the heart and brain which allows staying underwater for extended periods of time.
[8] The reduction in finger dexterity due to pain or numbness decreases general safety and work capacity, which consequently increases the risk of other injuries.
Ingassing while warm is faster than when cold, as is outgassing, due to differences in perfusion in response to temperature perception, which is mostly sensed in superficial tissues.
When this internal oxygen supply is depleted, the animal suffers an increasing urge to breathe caused by a buildup of carbon dioxide in the circulation, followed by loss of consciousness due to central nervous system hypoxia.
[35] Onset commonly occurs around 60 msw (meters of sea water), and symptoms are variable depending on depth, compression rate and personal susceptibility.
Provision of breathing gas at ambient pressure can greatly prolong the duration of a dive, but there are other problems that may result from this technological solution.
[44] The physiology of decompression involves a complex interaction of gas solubility, partial pressures and concentration gradients, diffusion, bulk transport and bubble mechanics in living tissues.
Under equilibrium conditions, the total concentration of dissolved gases will be less than the ambient pressure, as oxygen is metabolised in the tissues, and the carbon dioxide produced is much more soluble.
Thus narcosis while diving in open water rarely develops into a serious problem as long as the divers are aware of its symptoms, and are able to ascend to manage it.
[47] High-pressure nervous syndrome (HPNS) is a neurological and physiological diving disorder that results when a diver descends below about 500 feet (150 m) using a breathing gas containing helium.
[38] Symptoms of HPNS include tremors, myoclonic jerking, somnolence, EEG changes,[64] visual disturbance, nausea, dizziness, and decreased mental performance.
The compression effects may occur when descending below 500 feet (150 m) at rates greater than a few metres per minute, but reduce within a few hours once the pressure has stabilised.
[71][72][73] In addition, many external factors, such as underwater immersion, exposure to cold, and exercise will decrease the time to onset of central nervous system symptoms.
Symptoms and signs of early hypercapnia include flushed skin, full pulse, tachypnea, dyspnea, muscle twitches, reduced neural activity, headache, confusion and lethargy, increased cardiac output, an elevation in arterial blood pressure, and a propensity toward arrhythmias.
The consequence is that breathing gases for hyperbaric use must have proportionately lower acceptable limits for toxic contaminants compared to normal surface pressure use.
[2] A high work of breathing may be partially compensated by a higher tolerance for carbon dioxide, and can eventually result in respiratory acidosis.
Free-flow systems inherently operate under a positive pressure relative to the head, to allow controlled exhaust flow, but not necessarily to the lungs in the upright diver.
Mechanical dead space can be reduced by design features such as: Underwater, things are less visible because of lower levels of natural illumination caused by rapid attenuation of light with distance passed through the water.
The visual acuity of the air-optimised eye is severely adversely affected by the difference in refractive index between air and water when immersed in direct contact.
[98] Stereoscopic acuity, the ability to judge relative distances of different objects, is considerably reduced underwater, and this is affected by the field of vision.
A narrow field of vision caused by a small viewport in a helmet results in greatly reduced stereoacuity, and associated loss of hand-eye coordination.
[98] The optical distortion effects of the diver's mask or helmet faceplate also produce an apparent movement of a stationary object when the head is moved.
Sound from an underwater source can propagate relatively freely through body tissues where there is contact with the water as the acoustic properties are similar.
The combination of instability, equipment, neutral buoyancy and resistance to movement by the inertial and viscous effects of the water encumbers the diver.
Cold causes losses in sensory and motor function and distracts from and disrupts cognitive activity The ability to exert large and precise force is reduced.
[103] Air-breathing marine vertebrates that dive to feed must deal with the effects of pressure at depth and the need to find and capture their food.
Burst and glide locomotion is also often used to minimise energy consumption, and may involve using positive or negative buoyancy to power part of the ascent or descent.
