carbon dioxide When the body ‘burns’ food the end products are mainly water and carbon dioxide, together with some nitrogenous chemicals such as urea. The carbon dioxide enters the bloodstream, is carried to the
lungs, and is excreted in the expired air of
breathing. The atmospheric air we inhale contains virtually no carbon dioxide, whereas there is about 5% in the air we breathe out.
Carbon dioxide reacts in the blood to form carbonic acid and bicarbonate and, if it were allowed to accumulate, would cause acidosis. This condition is particularly harmful to the cells of the brain. Carbon dioxide diffuses into the liquid in the brain, the
cerebrospinal fluid (CSF); any excess makes it more acid, and this in turn stimulates neural receptors in the
brain stem that increase breathing. The result is that the carbon dioxide is blown off in the lungs and the acidity of the blood and brain are kept close to normal levels. Carbon dioxide is the main chemical stimulus to breathing, which is regulated primarily to keep blood and brain acidity at healthy values. If the carbon dioxide in the lungs increases by only 0.2%, from a normal level of about 5%, then breathing is doubled. Breathholding accumulates carbon dioxide in the body, which leads to an irrepressible desire to breathe (lack of oxygen is also a stimulus, but far weaker than carbon dioxide). Conversely, if we voluntarily hyperventilate, the level of carbon dioxide in the blood will decrease, and breathing may be inhibited until more carbon dioxide accumulates.
Hyperventilation can have harmful effects because of the pronounced reduction in blood and CSF acidity. Since decreases in carbon dioxide and acidity constrict
blood vessels, particularly in the brain, one effect is to reduce the blood supply to the brain.
Carbon dioxide was identified, but not understood chemically, in about 1600 ad by van Helmont, who called it ‘gas sylvestre’, the gas produced by combustion. He showed that it would not support life. Later Joseph Black, who had a lifelong interest in chemistry and was Professor of Medicine in Glasgow from 1757 to 1766, called it ‘fixed acid’, because it was absorbed by lime solution, and he showed that it was produced in
respiration. The story goes that in 1764 Black climbed to the ceiling of a church in Glasgow, occupied for 10 hours of religious devotions by a congregation of 1500, and measured the ‘fixed acid’ that was exhaled by the diligent and sleepy congregation. But it was Lavoisier (1743–94) who definitely established the excretion of carbon dioxide after its formation in metabolism, although he erroneously believed that it was formed in the lungs. Lavoisier was guillotined, and it was said that ‘it took but a second to cut off his head; a hundred years will not suffice to produce one like it.’ Lavoisier concluded that any series of lectures in an auditorium extending over 3 hours would leave the audience in a soporific state due to the accumulation of carbon dioxide. In theory he was right. Carbon dioxide in excess can act as an anaesthetic and, in animals, major surgery has been performed under its influence alone. Some human lung diseases such as chronic bronchitis may leave the patient drowsy or even comatose because of the build up of carbon dioxide in the body. It is claimed, probably incorrectly, that in social environments yawning and weariness are due to an accumulation of carbon dioxide. Van Helmont investigated a Grotto del Cane (cave of dogs) in Italy in which it was claimed, rather implausibly, that a tall dog owner would survive while his lowly dog would perish, due to the depressant effect of carbon dioxide, held to the ground because of its greater density than air. Perhaps Black's Glasgow congregation was fortunate.
John Widdicombe
See also
acid–base homeostasis;
blood;
respiration.
