Respiration a Combustion

Antoine Laurent Lavoisier

Respiration is a combustion, very slow, indeed, but otherwise precisely similar to that of carbon; it takes place in the lungs, without producing sensible light, because the fire-matter set free is at once absorbed by the humidity of these organs; the heat developed in this combustion is communicated to the blood which traverses the lungs, and from there is diffused throughout the entire animal system. Hence the air we breathe serves two purposes equally necessary for our conservation; it removes from the blood the base of fixed air whose overabundance would be very injurious; and the heat which this combination produces in the lungs repairs the constant loss of heat which we experience through the atmosphere and surrounding objects.

Animal heat is about the same in different parts of the body; this effect seems to depend on the three following causes: first, the rapidity of the circulation of the blood, which promptly transmits even to the extremities of the body the heat which it receives in the lungs; the second cause is the evaporation which the heat produces in these organs, and which diminishes the degree of their temperature; finally, the third is connected with the increase observed in the specific heat of the blood, when, by contact with pure air, it rids itself of the base of fixed air which it contains; a part of the specific heat developed in the formation of fixed air [carbonic acid gas] is thus absorbed by the blood, its temperature remaining always the same; but when, in the circulation, the blood takes up again the base of fixed air, its specific heat diminishes, and heat is developed; and, as this combination takes place in all parts of the body, the heat which it produces contributes to maintaining the temperature of parts distant from the lungs at about the same degree as that of these organs. Furthermore, whatever may be the manner in which the animal heat is kept up, that which is produced by the formation of fixed air is its first cause; hence we may establish the following proposition: When an animal is in a permanent and tranquil state; when it can live for a considerable time, without suffering, in the environment which surrounds it; in general, when the circumstances in which it finds itself do not sensibly impair its blood or its humors, so that after several hours the animal system experiences no sensible variation; the conservation of the animal heat is due, at least in large part, to the heat which the combination of the pure air respired by the animal with the base of fixed air which the blood supplies to it, produces.