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Questions and Answers on
Physiology and Medical Aspects of Scuba Diving


Lawrence Martin, M.D. Copyright 1997


Buy the book
Scuba quiz
Myths & Misconceptions
Disclaimer & Invitation

Brief History of Diving
Recreational  Diving
The Respiratory System
Explanation of Pressure

Water & Physical Laws
Unequal Air Pressures
Decompression Sickness
Oxygen Therapy
Gas Pressure at Depth

Dive Tables & Computers
Stress & Diving
Non-air Gas Mixtures
Women & Diving
Medical Fitness for Diving
Asthma & Diving
The Great Debate

All About DAN
Scuba Training Agencies
Magazines & Newsletters
Books & Videos

Diving Odds N' Ends

Internet Links



air - a gas mixture containing 21% oxygen, 78% nitrogen, and 1% other gases (mainly argon); compressed air is used for recreational scuba diving.

air compressor - a machine that compresses or pressurizes air; for scuba purposes, air is compressed from the atmospheric level (14.7 psi at sea level) to the capacity of the tank, usually between 2500-3000 psi.

air embolism - see arterial gas embolism

air pressure - the force per unit area exerted by the weight of air; at sea level the air pressure is 14.7 psi. Air pressure decreases with altitude.

algorithm - a set of equations incorporated into diving computers in order to compute nitrogen uptake and elimination from changes in depth and elapsed time.

ambient pressure - the surrounding pressure; on land, comes from the weight of the atmosphere (see air pressure); at depth, comes from the weight of the water plus the weight of the atmosphere.

argon - an inert gas that makes up less than one percent of air.

arterial gas embolism - the condition characterized by bubble(s) of air from a ruptured lung segment under pressure; the bubbles enter the pulmonary circulation and travel to the arterial circulation, where they may cause a stroke.

asthma - a common condition manifested by narrowing of air passages within the lungs (the bronchi); one reason for the narrowing is excess mucous in the airways.

ata - atmosphere absolute; 1 ata is the atmospheric pressure at sea level; is measured with a barometer.

atmosphere - the blanket of air surrounding the earth, from sea level to outer space. Also, a unit of pressure; "one atmosphere" is pressure of the atmosphere at sea level, i.e., 760 mm Hg. Two atmospheres is twice this pressure, 1520 mm Hg, etc. Abbreviated atm.

atmospheric pressure - pressure of the atmosphere at a given altitude or location.

barometric pressure - same as atmospheric pressure.

barotrauma - any disease or injury due to unequal pressures between a space inside the body and the ambient pressure, or between two spaces within the body; examples include arterial gas embolism, pneumomediastinum, and pneumothorax.

bathyscaphe - literally, deep boat; a vessel that can descend and ascend under its own power, but is not designed for horizontal travel. The bathyscaphe Trieste set the world depth record in 1960.

bathysphere - a round, heavy walled vessel that can be lowered to great depths in the ocean via a tether from the surface ship; does not ascend or descend under its own power.

BC - see buoyancy compensator.

bends - a form of decompression sickness caused by dissolved nitrogen leaving the tissues too quickly on ascent; is manifested by pain, usually in the limbs and joints; "the bends" is sometimes used to signify any manifestation of decompression sickness.

bleb - an abnormal pocket of air in the lungs, usually under the lining of a lung, that can rupture with ascent and lead to barotrauma.

bottom time - variable definition; in square wave diving, the time between descending below the surface to the beginning of ascent. In multi-level diving, the time between descending below the surface and beginning the safety stop. (Other definitions may apply depending on the specific type of diving.)

Boyle's law - at a fixed temperature and for a fixed mass of gas, pressure times volume is a constant value.

breath-hold diving - diving without life support apparatus, while holding one's breath.

bubble - a collection of air or gas surrounded by a permeable membrane through which gases can enter or exit.

bulla - similar to bleb; an abnormal pocket of air or fluid; sometimes found in the lungs of patients with emphysema.

buoyancy - tendency of object to float or sink when placed in a liquid; objects that float are positively buoyant, those that sink are negatively buoyant and those that stay where placed are neutrally buoyant.

buoyancy compensator - an inflatable vest worn by the diver that can be automatically or orally inflated to help control buoyancy; abbreviated BC.

carbon dioxide - CO2; an odorless, tasteless gas that is a byproduct of metabolism; is excreted by the lungs in exhaled air.

carbon dioxide toxicity - problems resulting from buildup of CO2 in the blood; they may range from headache and shortness of breath, all the way to sudden blackout.

carbon monoxide - CO; odorless, tasteless, highly poisonous gas given off by incomplete combustion of hydrocarbon fuels.

carbon monoxide toxicity - illness from inhaling excess CO; problems may range from headache to unconsciousness and death.

Charles's Law - at a constant volume, the pressure of a gas varies inversely with absolute temperature.

chokes - a form of decompression sickness caused by enough bubbles entering the lungs to interfere with gas exchange; manifested by shortness of breath and can be fatal.

closed circuit scuba - apparatus designed to allow divers to re-breathe exhaled air after removal of CO2 and addition of supplemental O2. In contrast to "open circuit," closed circuit scuba is noiseless and produces no bubbles.

compartment - a theoretical division of the body with an arbitrarily assigned half time for nitrogen uptake and elimination. In designing decompression tables the body is divided into a finite number of compartments for purposes of making calculations, e.g., five, six or more.

computer - see dive computer

Dalton's Law - the total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume.

decompression - any change from one ambient pressure to a lower ambient pressure; always results in a reduction of gas pressures within the body.

decompression dive - any dive where the diver is exposed to a higher pressure than when the dive began; the decompression occurs as the diver ascends.

decompression stop - on ascent from a dive, a specified time spent at a specific depth, for purposes of nitrogen off-gassing; when not mandatory it is called a safety stop.

DCI - see decompression illness.

decompression illness - DCI; a relatively new term to encompass all bubble-related problems arising from decompression, including both decompression sickness and arterial gas embolism.

DCS - see decompression sickness

decompression sickness - DCS; a general term for all problems resulting from nitrogen leaving the body when ambient pressure is lowered. Can be divided into Type I (musculoskeletal and/or skin manifestations only) or the more serious Type II (neurologic, cardiac, and/or pulmonary manifestations).

dive computer - a small computer, carried by the diver, that constantly measures water pressure (and hence depth), and time; based on a pre-programmed algorithm, the computer calculates tissue nitrogen uptake and elimination in several theoretical compartments and provides a continuous readout of the dive profile, including: depth, elapsed time of dive, duration at current depth before decompression becomes mandatory; and a warning if the rate of ascent is too fast.

dive tables - a printed collection of dive times for specific depths, by which the diver can avoid contracting DCS. Most tables are based on Haldanian theory for nitrogen uptake and elimination.

dry suit - a water-tight garment that keeps the diver's body warm by providing insulation with a layer of gas, such as air; for diving in waters that are too cold for comfortable wetsuit protection, usually below 65�F.

EAN - enriched air nitrogen; nitrox.

eustachian tube - a short, muco-cartilaginous tube connecting the back of the nose to the middle ear. The anatomy of this tube is such that it tends to close naturally when ambient pressure is higher than middle ear pressure (as on descent in a dive), and tends to open naturally when ambient pressure is lower than middle ear pressure (on ascent).

first stage regulator - regulator attached to the scuba tank that lowers the tank pressure to ambient pressure + a pre-determined pressure (e.g., ambient + 140 psi).

free diving - variably defined; in some usage, diving without any scuba or other equipment and synonymous with breath-hold diving; in other usage, diving with-out any attachment to the surface, and therefore includes scuba diving.

fsw - feet of sea water; used to indicate either an actual depth, or just a pressure equal to that depth (e.g., in a hyperbaric chamber).

gas embolism - see arterial gas embolism

gas laws - laws that predict how gases will behave with changes in pressure, temperature and volume.

gauge pressure - pressure exclusive of atmospheric pressure; when diving, gauge pressure is due solely to the water pressure.

half time - half the time it takes for a dissolved gas in a tissue (such as nitrogen) to equilibrate to a new pressure, or to reach full saturation at a new pressure. Theoretical tissue half times are used in designing dive tables and algorithms for dive computers.

Haldanian - related to Haldane's theory that nitrogen is taken up and given off in exponential fashion during a dive, and that there is some safe ratio of pressure change for ascent (originally, 2:1).

heliox - mixture of helium and oxygen, used for very deep diving.

helium - second lightest gas; does not cause problems of narcosis seen with nitrogen, and is therefore used for very deep diving.

Henry's Law - the amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas in contact with the liquid and the solubility coefficient of the gas in the liquid.

high pressure nervous syndrome - convulsions or seizure-like activity arising from high gas pressure at depth, especially with helium.

hookah - a surface-supplied compressed air apparatus, for use in shallow diving in calm waters. The air is delivered to one or more divers through a long hose.

hydrogen - an inert gas, and lightest of all the elements; has been used in experimental diving situations.

hyperbaric chamber - air-tight chamber that can simulate the ambient pressure at altitude or at depth; is used for treating decompression illness.

hypercapnia - a higher than normal PCO2 level in the blood.

hyperthermia - a body temperature warmer than normal; less common in diving than hypothermia, but can occur from overheating in a wet suit.

hyperventilation - over breathing to the extent that the blood carbon dioxide level is lowered; may lead to tingling in fingers and dizziness.

hypothermia - a body temperature colder than normal (37�C/98.6�F); severe problems start to manifest when body temperature reaches about 35�C (95�F).

hypoventilation - under breathing to the extent that the blood carbon dioxide level is elevated; may be manifested by carbon dioxide narcosis.

hypoxemia - lower than normal PO2 level in the blood; insufficient oxygen in the blood.

hypoxia - same as hypoxemia; terms are often used interchangeably.

liveaboard - a dive boat with sleeping and eating accommodations. Commercial liveaboards are usually between 50 and 130 feet long, and can carry anywhere from 10 to 30+ divers for a week or more.

middle ear - air-containing space of the ear bordered on one side by the tympanic membrane, which is exposed to any change in ambient pressure. Air pressure in the middle ear space can only be equalized through the eustachian tube, which connects the middle ear to the back of the nose.

mixed gas - variously defined; basically, any non-air mixture (e.g., nitrox), although some authors use the term only for mixes that contain a gas in addition to (or in place of) nitrogen (e.g., helium).

nitrogen - inert gas that makes up 78% of air. Nitrogen is inert in that it does not enter into any chemical reaction in the body, but it can cause problems under pressure (see nitrogen narcosis, decompression sickness).

narcosis - depressed mental state, anywhere from confusion or drowsiness to coma.

nitrogen narcosis - depressed mental state from high nitrogen pressure; usually does not begin to manifest on compressed air until below 80 fsw.

nitrox - any mixture of nitrogen and oxygen that contains less than the 78% nitrogen as found in ordinary air.

NOAA - National Oceanic and Atmospheric Association.

OEA - Oxygen enriched air; nitrox.

open circuit scuba - apparatus used in recreational diving; exhaled air is expelled into the water as bubbles; no part is rebreathed by the diver. Compare to closed circuit scuba.

otitis - inflammation or infection of any part of the ear; otitis media involves the middle ear, otitis externa the outer ear (ear canal).

oxygen - O2; gas vital for all life on this planet; makes up 21% of air by volume.

oxygen therapy - administration of any gas, for medical purpose, that contains more than 21% oxygen.

oxygen toxicity - damage or injury from inhaling too much oxygen; can arise from either too high an oxygen concentration or oxygen pressure. The first manifestation of oxygen toxicity while diving can be seizures.

oxygen window - difference between total gas pressures in arterial and venous blood; exists because oxygen is partly metabolized by the tissues, so venous oxygen pressure is lower than arterial oxygen pressure.

partial pressure - pressure exerted by a single component of a gas within a gas mixture, or dissolved in a liquid.

partial pressure, carbon dioxide - PCO2; pressure exerted by carbon dioxide in any mixture of gases, or dissolved in a liquid.

partial pressure, nitrogen - PN2; pressure exerted by nitrogen component in any mixture of gases, or dissolved in a liquid.

partial pressure, oxygen - PO2; pressure exerted by oxygen in any mixture of gases, or dissolved in a liquid.

pneumomediastinum - abnormal collection of air in the middle part of the chest, between the two lungs (mediastinum); often a consequence of barotrauma.

pneumothorax - abnormal collection of air outside the lining of the lung, between the lung and the chest wall; often a consequence of barotrauma.

pressure - any force exerted over an area; see atmospheric pr., ambient pr.

psi - pounds per square inch; a common measurement of air pressure.

recreational scuba diving - diving to prescribed limits, including a depth no greater than 130 fsw, using only compressed air, and never requiring a decompression stop; abbreviated RSD.

regulator - in scuba, any device which changes air pressure from one level to a lower level. See first and second stage regulator.

repetitive dive - any dive done within a certain time frame after a previous dive; variable definition exists as to what time frame constitutes "repetitive." With some tables any dive within 12 hours of a previous dive is considered repetitive; when using a computer, any dive whose profile is affected by a previous dive is considered repetitive.

residual nitrogen time - the time it would take to off-gas any extra nitrogen remaining after a dive; in dive tables, RNT is designated by a letter A through Z. Residual nitrogen time is always taken into consideration in determining the safe duration for any repetitive dive.

reverse squeeze - pain or discomfort in enclosed space (e.g., sinuses, middle ear, inside face mask) on ascent from a dive.

RSD - see recreational scuba diving.

safety stop - on ascent from a dive, a specified time spent at a specific depth, for purposes of nitrogen off-gassing; by definition it is not mandatory for safe ascent from the dive. Compare with decompression stop.

saturation - the degree to which a gas is dissolved in the blood or tissues; full saturation occurs when the pressure of gas dissolved in the blood or tissues is the same as the ambient (surrounding) pressure of that gas.

saturation diving - diving performed after the body is fully saturated with nitrogen; to become fully saturated the diver must stay under water for a much longer period than is allowed in recreational scuba diving tables.

scuba - self-contained underwater breathing apparatus.

sea level - the level of the world's oceans; all oceans are at sea level.

second stage regulator - the regulator that follows, in line, the first stage regulator, and delivers compressed air to the diver.

shallow water blackout - a sudden unconsciousness, from hypoxia, that occurs among some breath hold divers. Often occurs near the surface after a deeper dive, hence "shallow water."

sinuses - air spaces within the skull that are in contact with ambient pressure through openings into the back of the nasal passages.

sinusitis - inflammation or infection of the sinuses in the head.

skin diving - another term for breath-hold diving; diving without the use of scuba equipment.

squeeze - pain or discomfort in an enclosed space (sinuses, middle ears, inside a face mask) caused by shrinkage of that space; occurs on descent. See reverse squeeze.

submarine - heavy walled vessel that can withstand pressures under water and allow occupants to breathe air at sea level pressure; can travel under its own power.

surface interval - length of time on the surface, usually out of the water, between two consecutive dives.

surface-supplied compressed air diving - diving with the air continuously supplied by a compressor on the surface; can be used for both sport and professional diving.

supersaturation - an unstable situation where the pressure of a gas dissolved in the blood or tissues is higher than the ambient pressure of that gas. Supersaturation is always present to some degree with every decompression.

thermocline - intersection between two layers of water of that are of distinctly different temperatures; usually the colder layer is deeper. A diver can easily feel a thermocline.

tissue - a part of the body characterized by specific characteristics, such as muscle, bone, or cartilage. The term is also used to refer to any part of the body with a specific half time for loading and unloading nitrogen; in this latter context a tissue may be contiguous or non-contiguous, or even a theoretical compartment.

trimix - mixture of helium, nitrogen and oxygen, used for very deep diving.

water pressure - force per unit area exerted by the weight of water; each 33 feet of sea water exerts a pressure equivalent to one atmosphere, or 14.7 psi.

wet suit - any suit that provides thermal protection in or under water by trapping a layer of water between the diver's skin and the suit; see dry suit.

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