Boyle's law (sometimes referred to as the Boyle–Mariotte law, or Mariotte's law) is an experimental gas law that describes how the pressure of a gas tends to increase as the volume of the container decreases. A modern statement of Boyle's law is

The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.

Mathematically, Boyle's law can be stated as

or

where P is the pressure of the gas, V is the volume of the gas, and k is a constant.

The equation states that the product of pressure and volume is a constant for a given mass of confined gas and this holds as long as the temperature is constant. For comparing the same substance under two different sets of conditions, the law can be usefully expressed as

:P1V1=P2V2.{displaystyle P_{1}V_{1}=P_{2}V_{2}.}

The equation shows that, as volume increases, the pressure of the gas decreases in proportion. Similarly, as volume decreases, the pressure of the gas increases. The law was named after chemist and physicist Robert Boyle, who published the original law in 1662. Read more...

Decompression theory is the study and modelling of the transfer of the inert gas component of breathing gases from the gas in the lungs to the tissues and back during exposure to variations in ambient pressure. In the case of underwater diving and compressed air work, this mostly involves ambient pressures greater than the local surface pressure, but astronauts, high altitude mountaineers, and travellers in aircraft which are not pressurised to sea level pressure, are generally exposed to ambient pressures less than standard sea level atmospheric pressure. In all cases, the symptoms caused by decompression occur during or within a relatively short period of hours, or occasionally days, after a significant pressure reduction.

The term "decompression" derives from the reduction in ambient pressure experienced by the organism and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during and after this reduction in pressure. The uptake of gas by the tissues is in the dissolved state, and elimination also requires the gas to be dissolved, however a sufficient reduction in ambient pressure may cause bubble formation in the tissues, which can lead to tissue damage and the symptoms known as decompression sickness, and also delays the elimination of the gas.

Decompression modeling attempts to explain and predict the mechanism of gas elimination and bubble formation within the organism during and after changes in ambient pressure, and provides mathematical models which attempt to predict acceptably low risk and reasonably practicable procedures for decompression in the field.
Both deterministic and probabilistic models have been used, and are still in use. Read more...

A patent foramen ovale (PFO) is a remnant opening of the fetal foramen ovale, which normally closes after a person's birth. In medical use, the term "patent" means open or unobstructed. In about 25% of people, the foramen ovale fails to close properly, leaving them with a PFO or at least with what some physicians classify as a "pro-PFO", which is a PFO that is normally closed, but can open under increased blood pressure. On echocardiography, shunting of blood may not be noted except when the patient coughs.

Clinically, PFO is linked to stroke, sleep apnea, migraine with aura, and decompression sickness. No cause is established for a foramen ovale to remain open instead of closing naturally, but heredity and genetics may play a role. PFO is not treated in the absence of other symptoms.

The mechanism by which a PFO may play a role in stroke is called paradoxical embolism. In the case of PFO, a blood clot from the venous circulatory system is able to pass from the right atrium directly into the left atrium via the PFO, rather than being filtered by the lungs, and thereupon into systemic circulation toward the brain. PFO is common in patients with an atrial septal aneurysm (ASA), a much-more rare condition, which is also linked to cryptogenic (i.e., of unknown cause) stroke.

PFO is more prevalent in patients with cryptogenic stroke than in patients with a stroke of known cause. While PFO is present in only 25% in the general population, the probability of someone having a PFO increases to about 40 to 50% in patients who have had a cryptogenic stroke. Statistically speaking, this is particularly true for patients who have a stroke before the age of 55.

Some data suggest that PFOs may be involved in the pathogenesis of some migraine headaches. Several clinical trials are currently underway to investigate the role of PFO in these clinical situations. Read more...

Artificial gills are unproven conceptualised devices to allow a human to be able to take in oxygen from surrounding water. This is speculative technology that has not been demonstrated in a documented fashion. Natural gills work because nearly all animals with gills are thermoconformers (cold-blooded), so they need much less oxygen than a thermoregulator (warm-blood) of the same size. As a practical matter, therefore, it is unclear that a usable artificial gill could be created because of the large amount of oxygen a human would need extracted from the water. Read more...

Ekman transport, part of Ekman motion theory first investigated in 1902 by Vagn Walfrid Ekman, refers to the wind-driven net transport of the surface layer of a fluid that, due to the Coriolis effect, occurs at 90° to the direction of the surface wind. This phenomenon was first noted by Fridtjof Nansen, who recorded that ice transport appeared to occur at an angle to the wind direction during his Arctic expedition during the 1890s. The direction of transport is dependent on the hemisphere: in the northern hemisphere, transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at a 90° counterclockwise. Read more...

Neutral buoyancy is a condition in which a physical body's average density is equal to the density of the fluid in which it is immersed. The buoyancy offsets the force of gravity that would otherwise cause the object to sink (if the body's density is greater than the density of the fluid in which it is immersed) or rise (if it's less). An object that has neutral buoyancy will neither sink nor rise.
In scuba diving, the ability to maintain neutral buoyancy through controlled breathing, accurate weighting, and management of the buoyancy compensator is an important skill. A scuba diver maintains neutral buoyancy by continuous correction, usually by controlled breathing, as neutral buoyancy is an unstable condition for a compressible object in a liquid. Read more...

Cold shock response is the physiological response of organisms to sudden cold, especially cold water.

In humans, cold shock response is perhaps the most 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. For people with heart disease, this additional workload can cause the heart to go into arrest. Inhalation of water (and thus drowning) may result from hyperventilation. Some people are much better able to survive swimming in very cold water due to body or mental conditioning.

Hypothermia from exposure to cold water is not as sudden as is often believed. A person who survives the initial minute of trauma (after falling into icy water), can survive for at least thirty minutes provided they don't drown. However, the ability to perform useful work (for example to save oneself) declines substantially after ten minutes (as the body protectively cuts off blood flow to "non-essential" muscles). Read more...

The combined gas law is a gas law that combines Charles's law, Boyle's law, and Gay-Lussac's law. There is no official founder for this law; it is merely an amalgamation of the three previously discovered laws. These laws each relate one thermodynamic variable to another mathematically while holding everything else constant. Charles's law states that volume and temperature are directly proportional to each other as long as pressure is held constant. Boyle's law asserts that pressure and volume are inversely proportional to each other at fixed temperature. Finally, Gay-Lussac's law introduces a direct proportionality between temperature and pressure as long as it is at a constant volume. The inter-dependence of these variables is shown in the combined gas law, which clearly states that:

This can be stated mathematically as:

:PVT=k{displaystyle qquad {frac {PV}{T}}=k}

where:

:P is the pressure,
:V is the volume,
:T is the temperature measured in kelvins,
:k is a constant (with units of energy divided by temperature).

For comparing the same substance under two different sets of conditions, the law can be written as:

:P1V1T1=P2V2T2{displaystyle qquad {frac {P_{1}V_{1}}{T_{1}}}={frac {P_{2}V_{2}}{T_{2}}}}

The addition of Avogadro's law to the combined gas law yields the ideal gas law. Read more...

Tidal race or tidal rapid is a natural occurrence whereby a fast-moving tide passes through a constriction, resulting in the formation of waves, eddies and hazardous currents. The constriction can be a passage where the sides narrow, for example the Gulf of Corryvreckan and the Saltstraumen maelstrom, or an underwater obstruction (a reef or rising seabed), such as is found at the Portland Race.

In extreme cases, such as Skookumchuck Narrows in British Columbia, through which tides can travel at more than 17 knots, very large whirlpools develop, which can be extremely hazardous to navigation. Read more...

In underwater diving activities such as saturation diving, technical diving and nitrox diving, the maximum operating depth (MOD) of a breathing gas is the depth below which the partial pressure of oxygen (pO₂) of the gas mix exceeds an acceptable limit. This limit is based on risk of central nervous system oxygen toxicity, and is somewhat arbitrary, and varies depending on the diver training agency or Code of Practice, the level of underwater exertion planned and the planned duration of the dive, but is normally in the range of 1.2 to 1.6 bar.

The MOD is significant when planning dives using gases such as heliox, nitrox and trimix because the proportion of oxygen in the mix determines a maximum depth for breathing that gas at an acceptable risk. There is a risk of acute oxygen toxicity if the MOD is exceeded. The tables below show MODs for a selection of oxygen mixes. Atmospheric air contains approximately 21% oxygen, and has an MOD calculated by the same method. Read more...

A diving regulator is a device that reduces the high pressure in a diving cylinder to the same pressure as the scuba diver's surroundings. The breathing performance of regulators is a factor in choosing a suitable regulator for the type of diving to be undertaken, as it indicates the ability of a regulator to meet the demands placed on it at depth and under high breathing loads. It is desirable that breathing from a regulator requires low effort even when supplying large amounts of air. It is also recommended that it breathe smoothly without any sudden changes in resistance while inhaling or exhaling. Although these factors may be judged subjectively, it is convenient to have a standard by which the many different types and manufactures of regulators may be compared.

Various breathing machines have been developed and used for assessment of breathing apparatus performance. Ansti Test Systems developed a turnkey system that measures the inhalation and exhalation effort in using a regulator. Publishing results of the performance of regulators in the ANSTI test machine has resulted in performance improvements. Read more...

In chemistry, Henry's law is a gas law that states that the amount of dissolved gas is proportional to its partial pressure in the gas phase. The proportionality factor is called the Henry's law constant. It was formulated by the English chemist William Henry, who studied the topic in the early 19th century. In his publication about the quantity of gases absorbed by water, he described the results of his experiments:

:..."water takes up, of gas condensed by one, two, or more additional atmospheres, a quantity which, ordinarily compressed, would be equal to twice, thrice, &c. the volume absorbed under the common pressure of the atmosphere."

An example where Henry's law is at play is in the depth-dependent dissolution of oxygen and nitrogen in the blood of underwater divers that changes during decompression, leading to decompression sickness. An everyday example is given by one's experience with carbonated soft drinks, which contain dissolved carbon dioxide. Before opening, the gas above the drink in its container is almost pure carbon dioxide, at a pressure higher than atmospheric pressure. After the bottle is opened, this gas escapes, moving the partial pressure of carbon dioxide above the liquid to be much lower, resulting in degassing as the dissolved carbon dioxide comes out of solution. Read more...

The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, hormones, and blood cells to and from the cells in the body to provide nourishment and help in fighting diseases, stabilize temperature and pH, and maintain homeostasis.

The circulatory system includes the lymphatic system, which circulates lymph. The passage of lymph for example takes much longer than that of blood. Blood is a fluid consisting of plasma, red blood cells, white blood cells, and platelets that is circulated by the heart through the vertebrate vascular system, carrying oxygen and nutrients to and waste materials away from all body tissues. Lymph is essentially recycled excess blood plasma after it has been filtered from the interstitial fluid (between cells) and returned to the lymphatic system. The cardiovascular (from Latin words meaning "heart" and "vessel") system comprises the blood, heart, and blood vessels. The lymph, lymph nodes, and lymph vessels form the lymphatic system, which returns filtered blood plasma from the interstitial fluid (between cells) as lymph.

The circulatory system of the blood is seen as having two components, a systemic circulation and a pulmonary circulation.

While humans, as well as other vertebrates, have a closed cardiovascular system (meaning that the blood never leaves the network of arteries, veins and capillaries), some invertebrate groups have an open cardiovascular system. The lymphatic system, on the other hand, is an open system providing an accessory route for excess interstitial fluid to be returned to the blood. The more primitive, diploblastic animal phyla lack circulatory systems.

Many diseases affect the circulatory system. This includes cardiovascular disease, affecting the cardiovascular system, and lymphatic disease affecting the lymphatic system. Cardiologists are medical professionals which specialise in the heart, and cardiothoracic surgeons specialise in operating on the heart and its surrounding areas. Vascular surgeons focus on other parts of the circulatory system. Read more...

Deep diving is underwater diving to a depth beyond the norm accepted by the associated community. In some cases this is a prescribed limit established by an authority, and in others it is associated with a level of certification or training, and it may vary depending on whether the diving is recreational, technical or commercial. Nitrogen narcosis becomes a hazard below 30 metres (98 ft) and hypoxic breathing gas is required below 60 metres (200 ft) to lessen the risk of oxygen toxicity. Professional divers may use an atmospheric diving suit that allows very deep dives of up to 610 metres (2,000 ft).

For some recreational diving agencies, Deep diving, or Deep diver may be a certification awarded to divers that have been trained to dive to a specified depth range, generally deeper than 30 metres (98 ft). However, the Professional Association of Diving Instructors (PADI) defines anything from 18 metres (60 ft) to 30 metres (100 ft) as a "deep dive" in the context of recreational diving (other diving organisations vary), and considers deep diving a form of technical diving.

In technical diving, a depth below about 60 metres (200 ft) where hypoxic breathing gas becomes necessary to avoid oxygen toxicity may be considered a "deep dive".

In professional diving, a depth that requires special equipment, procedures, or advanced training may be considered a deep dive.

Deep diving can mean something else in the commercial diving field. For instance early experiments carried out by Comex S.A. (Compagnie maritime d'expertises) using hydrox and trimix attained far greater depths than any recreational technical diving. One example being the Comex Janus IV open-sea dive to 501 metres (1,644 ft) in 1977. The open-sea diving depth record was achieved in 1988 by a team of Comex divers who performed pipeline connection exercises at a depth of 534 metres (1,752 ft) in the Mediterranean Sea as part of the Hydra 8 programme. These divers needed to breathe special gas mixtures because they were exposed to very high ambient pressure (more than 50 times atmospheric pressure).

An atmospheric diving suit allows very deep dives of up to 2,000 feet (610 m). These suits are capable of withstanding the pressure at great depth permitting the diver to remain at normal atmospheric pressure. This eliminates the problems associated with breathing high-pressure gases. Read more...

In physics, buoyancy (/ˈbɔɪ.ənsi, ˈbuːjən-/) or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. This pressure difference results in a net upwards force on the object. The magnitude of that force exerted is proportional to that pressure difference, and (as explained by Archimedes' principle) is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.

For this reason, an object whose density is greater than that of the fluid in which it is submerged tends to sink. If the object is either less dense than the liquid or is shaped appropriately (as in a boat), the force can keep the object afloat. This can occur only in a non-inertial reference frame, which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction. In a situation of fluid statics, the net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body.

The center of buoyancy of an object is the centroid of the displaced volume of fluid. Read more...

: See the Glossary of underwater diving terminology for definitions of technical terms, jargon, diver slang and acronyms used in underwater diving
: See the Outline of underwater diving for a hierararchical listing of underwater diving related articles
: See the Outline of underwater divers for a hierararchical listing of biographical articles on underwater divers
: See the Index of underwater diving for an alphabetical listing of underwater diving related articles
: See the Index of recreational dive sites for an alphabetical listing of underwater dive site related articles

The following index is provided as an overview of and topical guide to underwater divers:

Underwater divers are people who take part in underwater diving activities – Underwater diving is practiced as part of an occupation, or for recreation, where the practitioner submerges below the surface of the water or other liquid for a period which may range between seconds to order of a day at a time, either exposed to the ambient pressure or isolated by a pressure resistant suit, to interact with the underwater environment for pleasure, competitive sport, or as a means to reach a work site for profit or in the pursuit of knowledge, and may use no equipment at all, or a wide range of equipment which may include breathing apparatus, environmental protective clothing, aids to vision, communication, propulsion, maneuverability, buoyancy and safety equipment, and tools for the task at hand. Read more...

Fraction of inspired oxygen (FiO₂) is the fraction of oxygen in the volume being measured. Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmospheric FiO₂. Natural air includes 21% oxygen, which is equivalent to FiO₂ of 0.21. Oxygen-enriched air has a higher FiO₂ than 0.21; up to 1.00 which means 100% oxygen. FiO₂ is typically maintained below 0.5 even with mechanical ventilation, to avoid oxygen toxicity.

Often used in medicine, the FiO₂ is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, the FiO₂ may remain constant while the partial pressure of oxygen changes with the change in barometric pressure. Read more...

Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants. Read more...

Altitude diving is underwater diving using scuba or surface supplied diving equipment where the surface is 300 metres (980 ft) or more above sea level (for example, a mountain lake). The U.S. Navy tables recommend that no alteration be made for dives at altitudes lower than 91 metres (299 ft) and dives between 91 and 300 metres correction is required for dives over 44 metres (144 ft) of sea water. Altitude is significant in diving because the depths and decompression used for dives at altitude are different from those used for the same dive profile at sea level. Read more...

In physical oceanography, undertow is the under-current which is moving offshore when waves are approaching the shore. Undertow is a necessary and universal feature for almost any large body of water: it is a return flow compensating for the onshore-directed average transport of water by the waves in the zone above the wave troughs. The undertow's flow velocities are generally strongest in the surf zone, where the water is shallow and the waves are high due to shoaling.

In popular usage, the word "undertow" is often misapplied to rip currents. An undertow occurs everywhere underneath shore-approaching waves, whereas rip currents are localized narrow offshore currents occurring at certain locations along the coast. Unlike undertow, rip currents are strong at the surface. Read more...

Solubility is the property of a solid, liquid or gaseous chemical substance called solute to dissolve in a solid, liquid or gaseous solvent. The solubility of a substance fundamentally depends on the physical and chemical properties of the solute and solvent as well as on temperature, pressure and presence of other chemicals (including changes to the pH) of the solution. The extent of the solubility of a substance in a specific solvent is measured as the saturation concentration, where adding more solute does not increase the concentration of the solution and begins to precipitate the excess amount of solute.

Insolubility is the inability to dissolve in a solid, liquid or gaseous solvent.

Most often, the solvent is a liquid, which can be a pure substance or a mixture. One may also speak of solid solution, but rarely of solution in a gas (see vapor–liquid equilibrium instead).

Under certain conditions, the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is metastable. Metastability of crystals can also lead to apparent differences in the amount of a chemical that dissolves depending on its crystalline form or particle size. A supersaturated solution generally crystallises when 'seed' crystals are introduced and rapid equilibration occurs. Phenylsalicylate is one such simple observable substance when fully melted and then cooled below its fusion point.

Solubility is not to be confused with the ability to 'dissolve' a substance, because the solution might also occur because of a chemical reaction. For example, zinc 'dissolves' (with effervescence) in hydrochloric acid as a result of a chemical reaction releasing hydrogen gas in a displacement reaction. The zinc ions are soluble in the acid.

The solubility of a substance is an entirely different property from the rate of solution, which is how fast it dissolves. The smaller a particle is, the faster it dissolves although there are many factors to add to this generalization.

Crucially solubility applies to all areas of chemistry, geochemistry, inorganic, physical, organic and biochemistry. In all cases it will depend on the physical conditions (temperature, pressure and concentration) and the enthalpy and entropy directly relating to the solvents and solutes concerned.
By far the most common solvent in chemistry is water which is a solvent for most ionic compounds as well as a wide range of organic substances. This is a crucial factor in acidity/alkalinity and much environmental and geochemical work. Read more...

A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below.

Depending largely on season, latitude, and turbulent mixing by wind, thermoclines may be a semi-permanent feature of the body of water in which they occur, or they may form temporarily in response to phenomena such as the radiative heating/cooling of surface water during the day/night. Factors that affect the depth and thickness of a thermocline include seasonal weather variations, latitude, and local environmental conditions, such as tides and currents. Read more...

Supersaturation is a state of a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapor of a compound that has a higher (partial) pressure than the vapor pressure of that compound. Read more...

In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. The term aqueous solution is when one of the solvents is water. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. The mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation. The solution assumes the phase of the solvent when the solvent is the larger fraction of the mixture, as is commonly the case. The concentration of a solute in a solution is the mass of that solute expressed as a percentage of the mass of the whole solution. Read more...

In biology and biochemistry, a lipid is a biomolecule that is soluble in nonpolar solvents. Non-polar solvents are typically hydrocarbons used to dissolve other naturally occurring hydrocarbon lipid molecules that do not (or do not easily) dissolve in water, including fatty acids, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, and phospholipids.

The functions of lipids include storing energy, signaling, and acting as structural components of cell membranes. Lipids have applications in the cosmetic and food industries as well as in nanotechnology.

Scientists sometimes broadly define lipids as hydrophobic or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, multilamellar/unilamellar liposomes, or membranes in an aqueous environment. Biological lipids originate entirely or in part from two distinct types of biochemical subunits or "building-blocks": ketoacyl and isoprene groups. Using this approach, lipids may be divided into eight categories: fatty acids, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, and polyketides (derived from condensation of ketoacyl subunits); and sterol lipids and prenol lipids (derived from condensation of isoprene subunits).

Although the term "lipid" is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides. Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, monoglycerides, and phospholipids), as well as other sterol-containing metabolites such as cholesterol. Although humans and other mammals use various biosynthetic pathways both to break down and to synthesize lipids, some essential lipids can't be made this way and must be obtained from the diet. Read more...

Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.

Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle rapidly to the bottom of the container if a liquid sample is left to stand (the settable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid.

Turbidity (or haze) is also applied to transparent solids such as glass or plastic. In plastic production, haze is defined as the percentage of light that is deflected more than 2.5° from the incoming light direction. Read more...

: See the Glossary of underwater diving terminology for definitions of technical terms, jargon, diver slang and acronyms used in underwater diving
: See the Outline of underwater diving for a hierararchical listing of underwater diving related articles
: See the Index of underwater divers for an alphabetical listing of articles about underwater divers
: See the Index of recreational dive sites for an alphabetical listing of articles about places which are recreational dive sites

The following index is provided as an overview of and topical guide to underwater diving:

Underwater diving can be described as all of the following:

• A human activity – intentional, purposive, conscious and subjectively meaningful sequence of actions. Underwater diving is practiced as part of an occupation, or for recreation, where the practitioner submerges below the surface of the water or other liquid for a period which may range between seconds to order of a day at a time, either exposed to the ambient pressure or isolated by a pressure resistant suit, to interact with the underwater environment for pleasure, competitive sport, or as a means to reach a work site for profit or in the pursuit of knowledge, and may use no equipment at all, or a wide range of equipment which may include breathing apparatus, environmental protective clothing, aids to vision, communication, propulsion, maneuverability, buoyancy and safety equipment, and tools for the task at hand.

Read more...

The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, hormones, and blood cells to and from the cells in the body to provide nourishment and help in fighting diseases, stabilize temperature and pH, and maintain homeostasis.

The circulatory system includes the lymphatic system, which circulates lymph. The passage of lymph for example takes much longer than that of blood. Blood is a fluid consisting of plasma, red blood cells, white blood cells, and platelets that is circulated by the heart through the vertebrate vascular system, carrying oxygen and nutrients to and waste materials away from all body tissues. Lymph is essentially recycled excess blood plasma after it has been filtered from the interstitial fluid (between cells) and returned to the lymphatic system. The cardiovascular (from Latin words meaning "heart" and "vessel") system comprises the blood, heart, and blood vessels. The lymph, lymph nodes, and lymph vessels form the lymphatic system, which returns filtered blood plasma from the interstitial fluid (between cells) as lymph.

The circulatory system of the blood is seen as having two components, a systemic circulation and a pulmonary circulation.

While humans, as well as other vertebrates, have a closed cardiovascular system (meaning that the blood never leaves the network of arteries, veins and capillaries), some invertebrate groups have an open cardiovascular system. The lymphatic system, on the other hand, is an open system providing an accessory route for excess interstitial fluid to be returned to the blood. The more primitive, diploblastic animal phyla lack circulatory systems.

Many diseases affect the circulatory system. This includes cardiovascular disease, affecting the cardiovascular system, and lymphatic disease affecting the lymphatic system. Cardiologists are medical professionals which specialise in the heart, and cardiothoracic surgeons specialise in operating on the heart and its surrounding areas. Vascular surgeons focus on other parts of the circulatory system. Read more...

The respiratory exchange ratio (RER) is the ratio between the amount of carbon dioxide (CO₂) produced in metabolism and oxygen (O₂) used.

The ratio is determined by comparing exhaled gases to room air. Measuring this ratio can be used for estimating the respiratory quotient (RQ), an indicator of which fuel (carbohydrate or fat) is being metabolized to supply the body with energy. This estimation is only valid if metabolism is in a steady state.

RER is about 0.8 at rest with a modern diet. This value, however, can exceed 1 during intense exercise, as CO₂ production by the working muscles becomes greater and more of the inhaled O₂ gets used rather than being expelled. During moderate or higher intensity aerobic exercise and anaerobic exercise, using RER for estimating RQ loses accuracy because of factors including bicarbonate buffering of hydrogen ions, which affects the CO₂ levels being expelled by the respiratory system.

Calculation of RER is commonly done in conjunction with exercise tests such as the VO₂ max test and can be used as an indicator that the participants are nearing exhaustion and the limits of their cardio-respiratory system. An RER greater than or equal to 1.15 is often used as a secondary endpoint criterion of a VO₂ max test.

An RER of 0.70 indicates that fat is the predominant fuel source, RER of 0.85 suggests a mix of fat and carbohydrates, and a value of 1.00 or above is indicative of carbohydrate being the predominant fuel source.

Oxidation of a carbohydrate molecule
: 6 O2+C6H12O6→6 CO2+6 H2O+38 ATP{displaystyle 6 mathrm {O} _{2}+mathrm {C} _{6}mathrm {H} _{12}mathrm {O} _{6}to 6 mathrm {CO} _{2}+6 mathrm {H} _{2}mathrm {O} +38 mathrm {ATP} }
: RER=VCO2VO2=6 CO26 O2=1.0{displaystyle mathrm {RER} ={frac {mathrm {VCO} _{2}}{mathrm {VO} _{2}}}={frac {6 mathrm {CO} _{2}}{6 mathrm {O} _{2}}}=1.0}

Oxidation of a fatty acid molecule
: 23 O2+C16H32O2→16 CO2+16 H2O+129 ATP{displaystyle 23 mathrm {O} _{2}+mathrm {C} _{16}mathrm {H} _{32}mathrm {O} _{2}to 16 mathrm {CO} _{2}+16 mathrm {H} _{2}mathrm {O} +129 mathrm {ATP} }
: RER=VCO2VO2=16 CO223 O2=0.7{displaystyle mathrm {RER} ={frac {mathrm {VCO} _{2}}{mathrm {VO} _{2}}}={frac {16 mathrm {CO} _{2}}{23 mathrm {O} _{2}}}=0.7} Read more...

Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment.
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 (DCS) after long-duration deep dives. Atmospheric diving suits (ADS) may be used to isolate the diver from high ambient pressure. Crewed submersibles can extend depth range, 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.
Diving activities are restricted to maximum depths of about 40 metres (130 ft) for recreational scuba diving, 530 metres (1,740 ft) for commercial saturation diving, and 610 metres (2,000 ft) wearing atmospheric suits. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable can vary.

Recreational diving (sometimes called sport diving or subaquatics) is a popular leisure activity. Technical diving is a form of recreational diving under especially challenging conditions. Professional diving (commercial diving, diving for research purposes, or for financial gain) 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.
Deep sea diving 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. Read more...

A rip current, often simply called a rip (or by the misnomers rip tide and undertow), is a specific kind of water current which can occur near beaches with breaking waves. A rip is a strong, localized, and narrow current of water which moves directly away from the shore, cutting through the lines of breaking waves like a river running out to sea, and is strongest near the surface of the water.

Rip currents can be hazardous to people in the water. Swimmers who are caught in a rip current and who do not understand what is going on, and who may not have the necessary water skills, may panic, or exhaust themselves by trying to swim directly against the flow of water. Because of these factors, rips are the leading cause of rescues by lifeguards at beaches, and rips are the cause of an average of 46 deaths by drowning per year in the United States.

A rip current is not the same thing as undertow, although some people use the term incorrectly when they often mean a rip current. Contrary to popular belief, neither rip nor undertow can pull a person down and hold them under the water. A rip simply carries floating objects, including people, out beyond the zone of the breaking waves. Read more...

Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment.
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 (DCS) after long-duration deep dives. Atmospheric diving suits (ADS) may be used to isolate the diver from high ambient pressure. Crewed submersibles can extend depth range, 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.
Diving activities are restricted to maximum depths of about 40 metres (130 ft) for recreational scuba diving, 530 metres (1,740 ft) for commercial saturation diving, and 610 metres (2,000 ft) wearing atmospheric suits. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable can vary.

Recreational diving (sometimes called sport diving or subaquatics) is a popular leisure activity. Technical diving is a form of recreational diving under especially challenging conditions. Professional diving (commercial diving, diving for research purposes, or for financial gain) 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.
Deep sea diving 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. Read more...

Recreational dive sites include specific places that recreational scuba divers go to enjoy the underwater environment. This includes publicly accessible recreational diver training sites and technical diving sites beyond the range generally accepted for recreational diving. In this context all diving done for recreational purposes is included. Professional diving tends to be done where the job is, and with the exception of the recreational diving service industry, does not generally occur at specific sites chosen for their easy access, pleasant conditions or interesting features.

Recreational dive sites may be found in a wide range of bodies of water, and may be popular for various reasons, including accessibility, biodiversity, spectacular topography, historical interest and artifacts (such as shipwrecks), and water clarity. Tropical waters of high biodiversity and colourful sea life are popular recreational diving vacation destinations. Indonesia, the Caribbean islands, the Red Sea and the Great Barrier Reef of Australia are regions where the clear, warm, waters and colourful and diverse sea life have made recreational diving an economically important tourist industry.

Recreational divers may accept a relatively high level of risk to dive at a site perceived to be of special interest. Wreck and cave diving have their adherents, and enthusiasts will endure considerable hardship, risk and expense to visit caves and wrecks where few have been before. Some sites are popular almost exclusively for their convenience for training and practice of skills, such as flooded quarries. They are generally found where more interesting and pleasant diving is not locally available, or may only be accessible when weather or water conditions permit. Read more...

Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment.
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 (DCS) after long-duration deep dives. Atmospheric diving suits (ADS) may be used to isolate the diver from high ambient pressure. Crewed submersibles can extend depth range, 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.
Diving activities are restricted to maximum depths of about 40 metres (130 ft) for recreational scuba diving, 530 metres (1,740 ft) for commercial saturation diving, and 610 metres (2,000 ft) wearing atmospheric suits. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable can vary.

Recreational diving (sometimes called sport diving or subaquatics) is a popular leisure activity. Technical diving is a form of recreational diving under especially challenging conditions. Professional diving (commercial diving, diving for research purposes, or for financial gain) 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.
Deep sea diving 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. Read more...

Gay-Lussac's law can refer to several discoveries made by French chemist Joseph Louis Gay-Lussac (1778–1850) and other scientists in the late 18th and early 19th centuries pertaining to thermal expansion of gases and the relationship between temperature, volume, and pressure.

He is most often recognized for the Pressure Law which established that the pressure of an enclosed gas is directly proportional to its temperature and which he was the first to formulate (c. 1808). He is also sometimes credited, rightfully according to many modern scholars, with being the first to publish convincing evidence that, in Gay-Lussac’s shows the relationship between the pressure and temperature of a fixed mass of gas kept at a constant volume.'

These laws are also known variously as the Pressure Law or Amontons's law and Dalton's law respectively. Read more...

Work of breathing (WOB) is the energy expended to inhale and exhale a breathing gas. It is usually expressed as work per unit volume, for example, joules/litre, or as a work rate (power), such as joules/min or equivalent units, as it is not particularly useful without a reference to volume or time. It can be calculated in terms of the pulmonary pressure multiplied by the change in pulmonary volume, or in terms of the oxygen consumption attributable to breathing.
In a normal resting state the work of breathing constitutes about 5% of the total body oxygen consumption. It can increase considerably due to illness or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition. Read more...

Water stratification is when water masses with different properties - salinity (halocline), oxygenation (chemocline), density (pycnocline), temperature (thermocline) - form layers that act as barriers to water mixing which could lead to anoxia or euxinia. These layers are normally arranged according to density, with the least dense water masses sitting above the more dense layers.

Water stratification also creates barriers to nutrient mixing between layers. This can affect the primary production in an area by limiting photosynthetic processes. When nutrients from the benthos cannot travel up into the photic zone, phytoplankton may be limited by nutrient availability. Lower primary production also leads to lower net productivity in waters. Read more...

: See the Glossary of underwater diving terminology for definitions of technical terms, jargon, diver slang and acronyms used in underwater diving
: See the Outline of underwater diving for a hierararchical listing of underwater diving related articles
: See the Index of underwater divers for an alphabetical listing of articles about underwater divers
: See the Index of underwater diving for an alphabetical listing of underwater diving related articles

The following index is provided as an overview of and topical guide to recreational dive sites:

• Recreational dive sites – specific places that recreational divers go to enjoy the underwater environment or are used for training purposes.

Read more...

In a mixture of gases, each gas has a partial pressure which is the notional pressure of that gas if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of the gases in the mixture.

The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases dissolve, diffuse, and react according to their partial pressures, and not according to their concentrations in gas mixtures or liquids. This general property of gases is also true in chemical reactions of gases in biology. For example, the necessary amount of oxygen for human respiration, and the amount that is toxic, is set by the partial pressure of oxygen alone. This is true across a very wide range of different concentrations of oxygen present in various inhaled breathing gases or dissolved in blood. The partial pressures of oxygen and carbon dioxide are important parameters in tests of arterial blood gases, but can also be measured in, for example, cerebrospinal fluid. Read more...

The blood–air barrier (alveolar–capillary barrier or membrane) exists in the gas exchanging region of the lungs. It exists to prevent air bubbles from forming in the blood, and from blood entering the alveoli. It is formed by the type 1 pneumocytes of the alveolar wall, the endothelial cells of the capillaries and the basement membrane between the two cells. The barrier is permeable to molecular oxygen, carbon dioxide, carbon monoxide and many other gases. Read more...

In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F.

The original form of Newton's second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.

Concepts related to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque, which produces changes in rotational speed of an object. In an extended body, each part usually applies forces on the adjacent parts; the distribution of such forces through the body is the internal mechanical stress. Such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate. Stress usually causes deformation of solid materials, or flow in fluids. Read more...

Wreck diving is recreational diving where the wreckage of ships, aircraft and other artificial structures are explored. Although most wreck dive sites are at shipwrecks, there is an increasing trend to scuttle retired ships to create artificial reef sites. Diving to crashed aircraft can also be considered wreck diving. The recreation of wreck diving makes no distinction as to how the vessel ended up on the bottom.

Some wreck diving involves penetration of the wreckage, making a direct ascent to the surface impossible for a part of the dive. Read more...

In biology, tissue is a cellular organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.

The English word is derived from the French tissu, meaning something that is woven, from the verb tisser, "to weave".

The study of human and animal tissues is known as histology or, in connection with disease, histopathology. For plants, the discipline is called plant anatomy. The classical tools for studying tissues are the paraffin block in which tissue is embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of medical diagnosis and prognosis. Read more...