Terminology

• Air embolism: Any bubbles of air that obstruct blood flow.
• Ambient Pressure: Surrounding pressure. Under water, this is made up of the combination of air plus water pressure.
• Antidiuretic Hormone: Vasopressin. ADH works to limit the output of fluid from the kidneys, constrict blood vessels, and raise blood pressure. When ADH is suppressed through immersion, fluid output increases, blood vessels relax, and blood pressure lowers.
• Apnea: A temporary cessation of breathing. This can include voluntary breath holding during breath-hold diving, and involuntary lapses from sleep disorders.
• Absolute Atmospheric Pressure: Atmospheric pressure plus water pressure. Usually used with the SI units Pascal (Pa).
• Atrial Natriuretic Factor (ANF): A substance secreted in the atria of the heart. ANF inhibits renin, a kidney enzyme, which through the angiotensin pathway causes increased fluid excretion from the body.
• Barodontalgia: Pain in the teeth usually associated with tooth squeeze
• Baroreceptors: Sensory centers in blood vessel walls, which respond to changes in blood pressure. These are found in greater quantity in the heart and neck region.
• Boyle's Law: One of the principle gas laws. At a constant temperature, the volume of a confined ideal gas is inversely proportional to pressure. In other words, as the pressure increases, the volume decreases. This is the concept that explains why a balloon taken to depth under water will decrease in size.
• Central Nervous System Oxygen Toxicity: Acute reaction to breathing high partial pressure of oxygen above a range that is seven to ten times that of normal. May include sudden convulsions, which is potentially dangerous underwater.
• Charles' Law: One of the principal gas laws. States that at constant pressure, the volume of a fixed quantity of ideal gas varies directly with temperature. That is, as the temperature of the gas increases, so to will the volume.
• Conduction: Heat transfer through a medium, such as water, without movement of the medium itself.
• Convection: Heat transfer through a medium involving absorption of heat at one point, movement of the medium, and then release of the heat at another point.
• Counter-Current Heat Exchange: The process of exchanging heat between vessels in the extremities. This allows for the heat to be conserved in the animal's core, rather than being allowed to circulate near the outer surfaces where the heat can be lost to the environment.
• Dalton's Law: This law describes the relationship of partial gas pressure to the total gas pressure. Each gas in a mixture exerts its own "partial pressure" which is the pressure that gas would exert if it were alone in the container. The sum of the partial pressures equals the total pressure of the mixture.
• Decompression Sickness: A diving injury that leads to pain, numbness and possibly paralysis. This is a result of a too rapid ascent, which doesn't allow the gases that were absorbed by body tissues at depth to return to solution. In doing so, bubbles form in the body, which can cause mechanical and chemical damage. Also referred to as the bends.
• Diuresis: Increased urine output.
• Dive Reflex: Decreased heart rate and limb blood flow due to immersion.
• Free radical: An atom, molecule or macromolecule that exists with an unpaired electron in the outer orbit. The state of being unpaired makes the atom or molecule particularly reactive.
• Gay-Lussac's Law: A physical principle that states that at a constant volume, the pressure of a confined ideal gas is proportional to temperature. In essence, as the temperature increases, the pressure will also increase.
• Hemoglobin: Specialized molecule found in red blood cells that carry oxygen and carbon dioxide. Additionally, these molecules have a very great affinity for carbon monoxide, which they carry at the loss of carrying oxygen. This is what makes carbon monoxide poisonous, because in large concentrations, not enough hemoglobin molecules will be carrying oxygen to support life.
• Henry's Law: A physical principle that states that at a constant temperature, the solubility of a gas in a liquid varies directly with the partial pressure of the gas on the liquid. Stated differently, as the pressure increases, the solubility will also increase.
• Hydrostatic pressure: The pressure exerted by the weight of fluids. As depth increases in water, the pressure also increases due to an increased column of water above the point in question.
• Hyperbaric: Pressure higher than atmospheric pressure. This occurs most frequently underwater, but can be simulated in a hyperbaric chamber.
• Hyperbaric Chamber: A room or enclosed capsule that is capable of sustaining air pressures greater than one atmosphere. This simulates depth and is often used in the treatment of pressure related diving injuries.
• Hypercapnia: Higher than normal levels of carbon dioxide in the blood.
• Hypobaric: Pressure lower than normal atmospheric pressure. This is experienced at altitude.
• Hypothermia: A physical condition in which the body's core temperature drops below 95° F.
• Hypoxia: A condition in which there is insufficient oxygen reaching body tissues.
• Ideal gas: An imaginary gas that obeys all of the gas laws exactly. Ideal gases do not really exist; however, most gases behave very close to an ideal gas when at O° C and standard pressure.
• Ingassing: The process of transferring dissolved gases into the body.
• Mediastinal Emphysema: A condition that results from air being forced into the tissues about the heart, the major blood vessels and the trachea in the middle of the chest. As the individual decreases their depth, the trapped gases expand and place increasing pressure on the heart, thereby impairing venous return to the heart.
• Narcosis: A state of stupor brought on by a drug or chemical that depresses nerve excitability. Most often associated in diving with nitrogen. As the partial pressure of nitrogen increases with depth, an increased amount of nitrogen becomes dissolved in body solutions. Nitrogen is known to have an anesthetic effect. Nitrogen narcosis is usually reversible by a decrease in depth.
• Negative Pressure Breathing: A situation where the air pressure that is being breathed, and therefore the pressure of the air in one's lungs is lower than the ambient pressure. This is often encountered in snorkel breathing.
• Offgassing: The transferring of dissolved gases out of the body.
• Pneumothorax: An injury in which air enters the normally airtight pleural cavity surrounding the lungs. When this occurs, air occupies lung space and prevents the expansion of the lungs associated with inspiration.
• Pulmonary Oxygen Toxicity: Lung and airway damage due to long exposure to oxygen at elevated partial pressures. Also called oxygen poisoning.
• Shallow Water Blackout: The term that was coined by Barlow and MacIntoch in 1944 to refer to a blackout resulting from elevated carbon dioxide levels. Today the term most often refers to unconsciousness resulting from low oxygen content on ascent from breath hold dives.
• Subcutaneous emphysema: A condition caused by air being forced into the tissues beneath the skin of the neck. Typically associated with feelings of fullness in the neck tissues, although in extreme cases a crackling of the skin when touched can be felt.
• Swimmer's Ear: A painful infection or inflammation of the outer ear usually due to bacteria growth.
• Venous Gas Emboli: In diving, small gas bubbles in the veins.
• Venous Pooling: Blood collecting in the veins of the lower extremities when stationary on land. Blood pools because of gravity's pull and because veins are able to stretch and distend.