about 758 mb (569 mmhg) in air at normal atmospheric pressure. at a steady state, when the tissues have been saturated by the inert gases of the breathing mixture, metabolic processes reduce the partial pressure of the less soluble oxygen and replace it with carbon dioxide, which is considerably more soluble in water. in the cells of a typical tissue, the partial pressure of oxygen will drop to around 13 mbar (10 mmhg), while the partial pressure of carbon dioxide will be about 65 mbar (49 mmhg). the sum of these partial pressures (water, oxygen, carbon dioxide and nitrogen) comes to roughly 900 mbar (675 mmhg), which is some 113 mbar (85 mmhg) less than the total pressure of the respiratory gas. this is a significant saturation deficit, and it provides a buffer against supersaturation and a driving force for dissolving bubbles. experiments suggest that the degree of unsaturation increases linearly with pressure for a breathing mixture of fixed composition, and decreases linearly with fraction of inert gas in the breathing mixture. as a consequence, the conditions for maximising the degree of unsaturation are a breathing gas with the lowest possible fraction of inert gas – i.e. pure oxygen, at the maximum permissible partial pressure. this saturation deficit is also referred to as the "oxygen window". or partial pressure vacancy. residual inert gas tissue loading when the diver surfaces after decompression there is a residual inert gas content distributed among the tissues. there is the unknown actual gas content and the modelled gas content according to the decompression algorithm. residual gas imbalance will continue to equilibrate towards the breathing gas, and for computational purposes is assumed to continue to equilibrate in accordance with the algorithm, normally assuming atmospheric air as the breathing gas. the residual gas loading is computed and the model tissue compartments updated so that it can be used as the baseline for repetitive dives. it would also be the baseline for further decompression if the diver were to ascend to a higher altitude. post dive oxygen or nitrox breathing will flush inert gases out of the tissues faster than air, but this is not normally calculated by dive computers. reduced inert gas tissue loading reduces risk of developing dcs when flying or in any other way being exposed to a lower ambient pressure after diving.: ch9 factors influencing uptake and elimination of dissolved gases the exchange of dissolved gases between the blood and tissues is controlled by perfusion and to a lesser extent by diffusion, particularly in heterogeneous tissues. the distribution of blood flow to the tissues is variable and subject to a variety of influences. when the flow is locally high, that area is dominated by perfusion, and by diffusion when the flow is low. the distribution of flow is controlled by the mean arterial pressure and the local vascular resistance, and the arterial pressure depends on cardiac output and the total vascular resistance. basic vascular resistance is controlled by the sympathetic nervous system, and metabolites, temperature, and local and systemic hormones have secondary and often localised effects, which can vary considerably with circumstances. peripheral vasoconstriction in cold water decreases overall heat loss without increasing oxygen consumption until shivering begins, at which point oxygen consumption will rise, though the vasoconstriction can persist. breathing gas composition the composition of the breathing gas during pressure exposure and decompression is significant in inert gas uptake and elimination for a given pressure exposure profile. breathing gas mixtures for diving will typically have a different gas fraction of nitrogen to that of air. the partial pressure of each component gas will differ to that of nitrogen in air at any given depth, and uptake and elimination of each inert gas component is proportional to the actual partial pressure over time. the two foremost reasons for use of mixed breathing gases are the reduction of nitrogen partial pressure by dilution with oxygen, to make nitrox mixtures, to reduce nitrogen uptake during pressure exposure and accelerate nitrogen elimination during decompression, and the substitution of helium (and occasionally other gases) for the nitrogen to reduce the narcotic effects and work of breathing under high pressure exposure. depending on the proportions of helium and nitrogen, these gases are called heliox if there is no nitrogen, or trimix if there is nitrogen and helium along with the essential oxygen. the inert gases used as substitutes for nitrogen have different solubility and diffusion characteristics in living tissues to the nitrogen they replace. for example, the most common inert gas diluent substitute for nitrogen is helium, which is significantly less soluble in living tissue, but also diffuses faster due to the relatively small size and mass of the helium atom in comparison with the nitrogen molecule. breathing gas composition is measurable, quantifiable and is used in current decompression algorithms. for open circuit diving it is usually provided as user input, including user input of gas switches. in closed circuit rebreathers the gas composition is often calculated in real-time, using user-input diluent composition which defines the ratio of nitrogen to helium, and the measured instantaneous oxygen partial pressure. body temperature and exercise blood flow to skin and fat are affected by skin and core temperature, and resting muscle perfusion is controlled by the temperature of the muscle itself. during exercise increased flow to the working muscles is often balanced by reduced flow to other tissues, such as kidneys, spleen, and liver. blood flow to the muscles is lower in cold water, but exercise keeps the muscle warm and flow elevated even when the skin is chilled. blood flow to fat normally increases during exercise, but this is inhibited by immersion in cold water. adaptation to cold reduces the extreme vasoconstriction which usually occurs with cold water immersion. exercise that increases heart rate increases overall perfusion, which will increase the rate of transport of inert gases to and from the more perfused tissues, and higher temperature of tissues will increase the rate of diffusion through those tissues. there is a tradeoff during decompression between mild exercise enhancing inert gas elimination and strenuous exercise triggering bubble formation and growth. variations in perfusion distribution do not necessarily affect respiratory inert gas exchange, though some gas may be locally constrained by changes in perfusion. rest in a cold environment will reduce inert gas exchange from skin, fat and muscle, whereas exercise will increase gas exchange where perfusion is increased. exercise during decompression can reduce decompression time and risk, providing bubbles are not present, but can increase risk if bubbles are present. inert gas exchange is least favourable for the diver who is warm and exercises at depth during the ingassing phase, and rests and is cold during decompression, and most favourable for the diver who is cool and relaxed at depth during ingassing, and warm with mild exercise during decompression. isobaric counterdiffusion main article: isobaric counterdiffusion isobaric counterdiffusion (icd) is the diffusion of gases in opposite directions caused by a change in the composition of the external ambient gas or breathing gas without change in the ambient pressure. during decompression after a dive this can occur when a change is made to the breathing gas, or when the diver moves into a gas filled environment which differs from the breathing gas. while not strictly speaking a phenomenon of decompression, it is a complication that can occur during decompression, and that can result in the formation or growth of bubbles without changes in the environmental pressure. two forms of this phenomenon have been described by lambertsen: superficial icd (also known as steady state isobaric counterdiffusion) occurs when the inert gas breathed by the diver diffuses more slowly into the body than the inert gas surrounding the body. an example of this would be breathing air in an heliox environment. the helium in the heliox diffuses into the skin quickly, while the nitrogen diffuses more slowly from the capillaries to the skin and out of the body. the resulting effect generates supersaturation in certain sites of the superficial tissues and the formation of inert gas bubbles. deep tissue icd (also known as transient isobaric counterdiffusion) occurs when different inert gases are breathed by the diver in sequence. the rapidly diffusing gas is transported into the tissue faster than the slower diffusing gas is transported out of the tissue. this can occur as divers switch from a nitrogen mixture to a helium mixture (diffusivity of helium is 2.65 times faster than nitrogen), or when saturation divers breathing hydreliox switch to a heliox mixture. there is another effect which can manifest as a result of the disparity in solubility between inert breathing gas diluents, which occurs in isobaric gas switches near the decompression ceiling between a low solubility gas, typically helium, and a higher solubility gas, typically nitrogen. an inner ear decompression model by doolette and mitchell suggests that a transient increase in gas tension after a switch from helium to nitrogen in breathing gas may result from the difference in gas transfer between compartments. if the transport of nitrogen into the vascular compartment by perfusion exceeds removal of helium by perfusion, while transfer of helium into the vascular compartment by diffusion from the perilymph and endolymph exceeds the counterdiffusion of nitrogen, this may result in a temporary increase in total gas tension, as the input of nitrogen exceeds the removal of helium, which can result in bubble formation and growth. this model suggests that diffusion of gases from the middle ear across the round window is negligible. the model is not necessarily applicable to all tissue types. lambertsen made suggestions to help avoid icd problems while diving: if the diver is surrounded by or saturated with nitrogen, they should not breathe helium rich gases. gas switches that involve going from helium rich mixtures to nitrogen rich mixtures would be acceptable, but changes from nitrogen to helium should include recompression. however doolette and mitchell's more recent study of inner ear decompression sickness (iedcs) shows that the inner ear may not be well-modelled by common (e.g. bühlmann) algorithms. doolette and mitchell propose that a switch from a helium-rich mix to a nitrogen-rich mix, as is common in technical diving when switching from trimix to nitrox on ascent, may cause a transient supersaturation of inert gas within the inner ear and result in iedcs. they suggest that breathing-gas switches from helium-rich to nitrogen-rich mixtures should be carefully scheduled either deep (with due consideration to nitrogen narcosis) or shallow to avoid the period of maximum supersaturation resulting from the decompression. switches should also be made during breathing of the largest inspired oxygen partial pressure that can be safely tolerated with due consideration to oxygen toxicity. a similar hypothesis to explain the incidence of iedcs when switching from trimix to nitrox was proposed by steve burton, who considered the effect of the much greater solubility of nitrogen than helium in producing transient increases in total inert gas pressure, which could lead to dcs under isobaric conditions. burton argues that effect of switching to nitrox from trimix with a large increase of nitrogen fraction at constant pressure has the effect of increasing the overall gas loading within particularly the faster tissues, since the loss of helium is more than compensated by the increase in nitrogen. this could cause immediate bubble formation and growth in the fast tissues. a simple rule for avoidance of icd problems when gas switching at a decompression ceiling is suggested: any increase in gas fraction of nitrogen in the decompression gas should be limited to 1/5 of the decrease in gas fraction of helium. this rule has been found to successfully avoid icd problems on hundreds of deep trimix dives. bubble formation, growth and elimination the location of micronuclei or where bubbles initially form is not known. heterogeneous nucleation and tribonucleation are considered the most likely mechanism for bubble formation. homogeneous nucleation requires much greater pressure differences than experienced in decompression. the spontaneous formation of nanobubbles on hydrophobic surfaces is a possible source of micronuclei, but it is not yet clear if these can grow to symptomatic dimensions as they are very stable. the incorporation of bubble formation and growth mechanisms in decompression models may make the models more biophysical and allow better extrapolation. flow conditions and perfusion rates are dominant parameters in competition between tissue