Fluid is not a natural medium for sustaining human life after the fetal stage; human respiration requires ventilation with air. Nevertheless, all vertebrates, including humans, exhibit a set of responses that may be called a “diving reflex,” which involves cardiovascular and metabolic adaptations to conserve oxygen during diving into water. Other physiological changes are also observed, either artificially induced (as by hyperventilation) or resulting from pressure changes in the environment at the same time that a diver is breathing from an independent gas supply.

Hyperventilation, a form of overbreathing that increases the amount of air entering the pulmonary alveoli, may be used intentionally by swimmers to prolong the time they are able to hold their breath under water. Hyperventilation can be dangerous, and this danger is greatly increased if the swimmer descends to depth, as sometimes happens in snorkeling. The increased ventilation prolongs the duration of the breath-hold by reducing the carbon dioxide pressure in the blood, but it cannot provide an equivalent increase in oxygen. Thus the carbon dioxide that accumulates with exercise takes longer to reach the threshold at which the swimmer is forced to take another breath, but concurrently the oxygen content of the blood falls to unusually low levels. The increased environmental pressure of the water around the breath-holding diver increases the partial pressures of the pulmonary gases. This allows an adequate oxygen partial pressure to be maintained in the setting of reduced oxygen content, and consciousness remains unimpaired. When the accumulated carbon dioxide at last forces the swimmer to return to the surface, however, the progressively diminishing pressure of the water on his ascent reduces the partial pressure of the remaining oxygen. Unconsciousness may then occur in or under the water.

Divers who breathe from an apparatus that delivers gas at the same pressure as that of the surrounding water need not return to the surface to breathe and can remain at depth for prolonged periods. But this apparent advantage introduces additional hazards, many of them unique in human physiology. Most of the hazards result from the environmental pressure of water. Two factors are involved. At the depth of a diver, the absolute pressure, which is approximately one additional atmosphere for each 10-metre (32.8-foot) increment of depth, is one factor. The other factor, acting at any depth, is the vertical hydrostatic pressure gradient across the body. The effects of pressure, seen in many processes at the molecular and cellular levels, include the physiological effects of the increased partial pressures of the respiratory gases, the increased density of the respiratory gases, the effect of changes of pressure upon the volumes of the gas-containing spaces in the body, and the consequences of the uptake of respiratory gases into, and their subsequent elimination from, the blood and tissues of the diver, often with the formation of bubbles. The multiple effects of submersion upon respiration are not easily separated from one another or clearly distinguishable from related effects of pressure upon other bodily systems.

The increased work of breathing, rather than cardiac or muscular performance, is the limiting factor for hard physical work underwater. Although the increased work of breathing may be largely due to the effects of increased respiratory gas density upon pulmonary function, the use of underwater breathing apparatus adds significant external breathing resistance to the diver’s respiratory burden.

Arterial carbon dioxide pressure should remain unchanged during changes of ambient pressure, but the impaired alveolar ventilation at depth leads to some carbon dioxide retention (hypercapnia). This may be compounded by an increased inspiratory content of carbon dioxide, especially if the diver uses closed-circuit and semiclosed-circuit rebreathing equipment or wears an inadequately ventilated helmet. Alveolar oxygen levels can also be disturbed in diving. Hypoxia may result from failure of the gas supply and may occur without warning. More commonly, the levels of inspired oxygen are increased. Oxygen in excess can be a poison; at a partial pressure greater than 1.5 bar (“surface equivalent value” = 150 percent), it may cause the rapid onset of convulsions, and after prolonged exposures at somewhat lower partial pressures it may cause pulmonary oxygen toxicity with reduced vital capacity and later pulmonary edema. In mixed-gas diving, inspired oxygen is therefore maintained at a partial pressure somewhere between 0.2 and 0.5 bar, but at great depths the inhomogeneity of alveolar ventilation and the limitations of gas diffusion appear to require oxygen provision at greater than normal levels.

The maximum breathing capacity and the maximum voluntary ventilation of a diver breathing compressed air diminish rapidly with depth, approximately in proportion to the reciprocal of the square root of the increasing gas density. Thus the practice of using an inert gas such as helium as the oxygen diluent at depths where nitrogen becomes narcotic, like an anesthetic, has the additional advantage of providing a breathing gas of lesser density. The use of hydrogen, which in a mixture with less than 4 percent oxygen is noncombustible, provides a greater respiratory advantage for deep diving.

At the extreme depths now attainable by humans—including records of some 330 metres (1,083 feet) for scuba diving and 214 metres (702 feet) for free-diving—direct effects of pressure upon the respiratory centre may be part of the “high-pressure neurological syndrome” and may account for some of the anomalies of breathlessness (dyspnea) and respiratory control that occur with exercise at depth.

The term carbon dioxide retainer is commonly applied to a diver who fails to eliminate carbon dioxide in the normal manner. An ability to tolerate carbon dioxide may increase the work capacity of a diver at depth but also may predispose him or her to other consequences that are less desirable. High values of end-tidal carbon dioxide (the maximal carbon dioxide concentration at the end of exhalation) with only moderate exertion may be associated with a diminished tolerance to oxygen neurotoxicity, a condition that, if it occurs underwater, places the diver at great risk. Nitrogen narcosis is enhanced by the presence of excess carbon dioxide, and the physical properties of carbon dioxide facilitate the nucleation and growth of bubbles on decompression.

Independent of the depth of the dive are the effects of the local hydrostatic pressure gradient upon respiration. The supporting effect of the surrounding water pressure upon the soft tissues promotes venous return from vessels no longer solely influenced by gravity; and, whatever the orientation of the diver in the water, this approximates the effects of recumbency (i.e., lying down) upon the cardiovascular and respiratory systems. Also, the uniform distribution of gas pressure within the thorax contrasts with the hydrostatic pressure gradient that exists outside the chest. Intrathoracic pressure may be effectively lower than the pressure of the surrounding water, in which case more blood will be shifted into the thorax, or it may be effectively greater, resulting in less intrathoracic blood volume. The concept of a hydrostatic balance point within the chest, which represents the net effect of the external pressures and the effects of chest buoyancy, has proved useful in designing underwater breathing apparatuses.

Intrapulmonary gas expands exponentially during the steady return of a diver toward the surface. Unless vented, the expanding gas may rupture alveolar septa and escape into interstitial spaces. The extra-alveolar gas may cause a “burst lung” (pneumothorax) or the tracking of gas into the tissues of the chest (mediastinal emphysema), possibly extending into the pericardium or into the neck. More seriously, the escaped alveolar gas may be carried by the blood circulation to the brain (arterial gas embolism). This is a major cause of death among divers. Failure to exhale during ascent causes such accidents and is likely to occur if the diver makes a rapid emergency ascent, even from depths as shallow as 2 metres (6.6 feet). Other possible causes of pulmonary barotrauma include retention of gas by a diseased portion of lung and gas trapping due to dynamic airway collapse during forced expiration at low lung volumes.

Decompression sickness may be defined as the illness, following a reduction of pressure, that is caused by the formation of bubbles from gases that were dissolved in the tissues while the diver was at an increased environmental pressure. The causes are related to the inadequacy of the diver’s decompression, perhaps failure to follow a correct decompression protocol, or occasionally a diver’s idiosyncratic response to an apparently safe decompression procedure. The pathogenesis begins both with the mechanical effects of bubbles and their expansion in the tissues and blood vessels and with the surface effects of the bubbles upon the various components of the blood at the blood–gas interface. The lung plays a significant role in the pathogenesis and natural history of this illness and may contribute to the clinical picture. Shallow, rapid respiration, often associated with a sharp retrosternal pain on deep inspiration, signals the onset of pulmonary decompression sickness, the “chokes.” Whether occurring alone or as part of a more complex case of decompression sickness, this respiratory pattern constitutes an acute emergency. It usually responds rapidly to treatment by recompression in a compression chamber.