gases in the body

gases in the body Man evolved from animals living in water, but occupies an oxygen-containing gaseous environment. The body uses food as fuel for what can be loosely termed a combustion reaction, in which the fuel combines with oxygen and is oxidized, producing carbon dioxide, water, and energy. The necessary oxygen is taken into the body by breathing air into the lungs, where it is absorbed into the blood which is then pumped round the body by the heart. The same blood then transports the carbon dioxide formed in the tissues back to the lungs, where it diffuses into the alveoli and is breathed out.

Air is only about 21% oxygen; its other components are nitrogen (about 78%), argon (about 0.9%), carbon dioxide (about 0.03%), and other rarer gases, including carbon monoxide, neon, helium, krypton, hydrogen, xenon, and radon, which together generally form about 0.03%. Of course human activities can markedly alter the concentration of some of these agents in air, and carbon monoxide and carbon dioxide levels can be locally much higher.

Whatever is breathed into the lungs dissolves to some extent into the blood and passes via the blood to the tissues — so the body contains all the same gases as the surrounding air. Some gases are more soluble than others and some, notably oxygen, combine chemically with blood. Gases can also be absorbed into the body, to a limited extent, across the skin and mucous membranes. This latter route allows gases formed in the gut, such as methane, also to be present in blood and body tissues.

The effect that a gas has on the body depends on the number of gas molecules that are available to act on receptor molecules in the body. The number of molecules in a given volume of gas at a fixed pressure and temperature is the same no matter what the gas is (Avogadro's principle). Since body temperature is more or less constant, the number of molecules of a gas is dependent only on its partial pressure. This is calculated from Dalton's law, which states that the pressure exerted by a mixture of gases is the sum of the pressures exerted by the individual gases occupying the same volume alone. So air, approximately 79% nitrogen and 21% oxygen, has a partial pressure of these gases of 79% and 21% of the prevailing atmospheric pressure. Atmospheric pressure varies but, at sea level, it can be taken to measure approximately 1 bar, 1,000 millibar or 100 kiloPascals (kPa). The partial pressures of nitrogen and oxygen in air at atmospheric pressure at sea level are therefore 79 kPa and 21 kPa respectively. As atmospheric pressure is reduced, for example with increased altitude, then, although the percentage composition of air remains the same, the partial pressure of the gases falls. The main effect of this is to cause oxygen lack — hypoxia. If atmospheric pressure is increased, for instance during diving, the partial pressure of oxygen and nitrogen rise. The most obvious effect of this is that nitrogen is breathed in sufficient quantities to cause partial anaesthesia or intoxication. The partial pressure of oxygen in the body can be raised easily at atmospheric pressure by adding it to the inhaled air. This technique is used to reverse the hypoxia of altitude in aviators and climbers, and of course is of great benefit in diseases that lower blood oxygen levels. Abnormally high partial pressures of oxygen in the blood, however, are toxic.

The amount or mass of a gas contained in the body depends directly on its partial pressure in the gas breathed, its solubility in the blood and various tissues, and whether it combines chemically with other substances. The solubility of a gas in another substance is expressed as the volume of gas which dissolves in the substance under stated conditions of temperature and pressure. Since the temperature of the body is more or less constant this factor does not normally influence gas uptake. The mass of gas dissolving is directly related to its partial pressure so if this is doubled the amount of gas dissolving is also doubled. This is important in conditions where decompression occurs (divers returning to the surface and astronauts during space walks, for example), since time must be allowed for excess gas to be cleared from the tissues and blood without the formation of bubbles of gas leading to decompression illness.

In addition to being simply dissolved in blood and tissues, some gases actually also form chemical compounds. For example, carbon dioxide forms bicarbonate and also combines with haemoglobin in the blood. Carbon monoxide combines with haemoglobin in the blood and many other iron-containing compounds in the tissues. Chemical combination allows very much larger amounts of gas to be held in the body than by simple solution. The amount of nitrogen gas, which is held only in solution in the body, is about 1 litre for someone weighing about 70 kg, while chemical combination allows the same person to contain about 35 litres of carbon dioxide.

Gases are not only taken up into the blood and tissues of the body but may also be contained, as gases, in body cavities. The lungs and upper and lower airways, of course, contain gas but so too do the bony sinuses of the skull and the middle ear. All these compartments are open to the atmosphere. The Eustachian tube connects the middle ear to the nasal cavity, and all the skull sinuses also open into the nasal cavity. This ensures that the pressure in these spaces remains at atmospheric pressure. If for some reason these connections become blocked, most commonly by a upper respiratory tract infection, and atmospheric pressure changes (for instance during flying or diving) then the tissues around the gas spaces can become damaged due to barotrauma (pressure-induced tissue damage); this is usually associated with a great deal of pain. Small pockets of gas may also exist under improperly maintained dental fillings and, if there is no connection to atmosphere, the expansion or contraction induced by changing pressure can cause toothache — barodontalgia. Extreme effects in teeth can cause the gas pocket to rupture or explode (dontopraxia). Free gas can also be present in the joints; this is commoner with increasing age and is of unknown significance. There is, however, some suggestion that such gas spaces are important in the causation of decompression illness. Free gas also exists in the intestinal tract but, since this is highly distensible and open to atmosphere (at both ends!), changes in atmospheric pressure rarely cause problems except under the most severe instances of sudden or explosive decompression. However, if the gut wall becomes perforated, such as occasionally happens due to a perforated ulcer, the appearance or free gas under the diaphragm on an X-ray picture is typical.

Some metabolic processes actually produce gases. Nitric oxide is formed in the lining of blood vessels; it regulates their diameter and is very important in controlling blood pressure. Nitric oxide is also formed in the brain and is an important neuromodulating agent. Carbon monoxide is formed from the metabolism of the haem in haemoglobin. This gas is also a vasodilator — like nitric oxide, but less potent. Recently carbon monoxide has been identified, like nitric oxide, as a possible neurotransmitter in the brain.

John A. S. Ross

See also carbon dioxide; oxygen; nitrogen.