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Glossary - H


H Hydrogen
Halocline A Halocline is a change in the salt content of the water, but is not often apparent to divers unless severe enough to cause buoyancy changes.
Halyard Line for hoisting sails or flags.
HDS  Historical Diving Society
He  Helium
Head A boat's toilet.
Heading The direction in which the longitudinal axis of a craft is aligned, measured clockwise from a given reference point. Not necessarily the direction the craft is travelling.
Heave to To bring a vessel to a position where she will maintain little or no headway, usually with the bow into the wind or nearly so.
Heel The leaning of a vessel to one side such as caused by the wind.
Helm The wheel or tiller controlling the rudder
Heliox A breathing gas consisting of a mixture of Helium and Oxygen.
Helium A gas which is easily eliminated from body in long dives and builds up fast in short dives. It is the least narcotic of standard gases and causes that "micky mouse" voice effect. It is a poor insulator and moderately costly.
Helm The apparatus (tiller or wheel) by which a vessel is steered, including the rudder.
Haemoglobin The most important constituent of the red blood cell is the molecule of haemoglobin, an iron containing protein that has the ability to unite with oxygen and transport it to cells throughout the body it also combines with CO2 and adds in the transport and removal of CO2 from the body.
Henrys Law The amount of gas that will dissolve into a liquid at a given temperature is almost directly proportional to the partial pressure
of that gas.
Hitch A hitch join lines to ring, post, bollards or very thick rope.
Hold A compartment below deck in a large vessel, used solely for carrying cargo.
HP High Pressure.
HPNS High Pressure Nervous Syndrome - a nervous disorder caused by breathing air under high pressure, as in underwater habitats.
HSA  Handicapped Scuba Association
HSC  Health and Safety Commission (UK)
HSE  Health & Safety Executive
Hull The basic structure and shell of a boat.
Hydrogen A gas that is easily eliminated from the body in long dives and builds up fast in short dives. It has a slight narcosis affect at depth and cause a large voice distortion: 
Hydrostatic Pressure The pressure exerted underwater by the surrounding water column.
Hydrostatic Test A test that is required every five years on SCUBA cylinders. This test involves pressurising the cylinder to test pressure.
Hydrox Breathing gas consisting of a mixture of Hydrogen and Oxygen.
Hyperbaric Dealing with pressures greater than 1 bar.
Hyperbaric Chamber A recompression chamber.
Hypercapnea An undue amount of CO2 in the blood caused by improper breathing habits and incomplete scrubbing of breathing gases in closed circuit re-breathers or excessive amounts in the breathing mixture. As concentrations of CO2 approach a PPCO2 of 0.10ATM symptoms begin to appear. Confusion & drowsiness, then muscle spasms, rigidity and unconsciousness. High PPO2, high gas density and high breathing resistance add to the severity of Hypercapnea. Treatment is a reduction of the PPCO2 and administration of oxygen.
Hyperoxia Elevated oxygen levels in the tissues. See oxygen toxicity, CNS oxygen toxicity.
Hyperthermia Inadequate loss of heat from the body resulting in a rise in core temperature. This is most commonly seen in areas with high ambient temperatures and divers with low body fluid levels.
Hyperventilation The process of rapidly inhaling and exhaling to purge the body of carbon dioxide, thus decreasing the natural urge to breath. Hyperventilation is a potentially dangerous practice and can lead to, hypocapnia, shallow water blackout, unconsciousness and drowning.
Hypobaric Dealing with pressures less than 1 Bar
hypocapnia An insufficient amount of CO2 in the blood (defined as pCO2 levels less than 35 mmHg), often the result of hyperventilation. Hypocapnia can cause weakness, faintness, headache, blurred vision and if untreated possibly unconsciousness. The body's breathing stimulus results from the monitoring of the CO2 and oxygen (the oxygen drive is only active at extremely low oxygen levels, CO2 is the MAJOR controlling factor) levels in the blood stream. An increase in CO2 and a decrease in oxygen trigger the body to breathe. As the CO2 levels drop, so does the body's natural stimulus to breathe. This is one of the primary causes of shallow water blackout.
Hypothermia A condition in which the deep tissue or core temperature of the body falls below the normal physiological range, approximately 36.9C. Heat loss is the result of exposure to cold, and most divers are exposed to water temperatures below that of their body. The rate of the heat loss is dependent on body area, the difference in the temperature between the body and the surrounding environment, body fat, external insulation and the level of physical exertion. As the core temperature begins to drop signs of hypothermia will begin to appear. This will trigger the body to begin producing extra heat, usually in the form of shivering. If the cooling is allowed to continue the core temperature will begin to drop and all bodily processes will slowly grind to a halt as pulse and respiration's slow. Death will follow if the diver is not treated promptly.
Hypoxaemia Insufficient oxygen in the bloodstream
Hypoxia A condition brought on due to an insufficient Partial Pressure of Oxygen (PPO2) in a breathing gas. A failure of the tissues to receive sufficient Oxygen.









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Page last updated on October 16, 2007
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