As has been said, the exact income of heat is uncertain and variable, because the data upon which the absolute amount can be calculated are not scientifically free from error. According to the most careful estimates, an adult weighing 82 kilo, produces 2,700,000 units of heat in the twenty-four hours, which are expended in the following way: -
In warming ingesta,......
70,157 units of heat.
In warming tidal air ...................................
140,064 " "
By the evaporation of 656 grm of water from the air passages, . .
By surface loss,........
From this it appears that more than three-quarters of our heat is lost by the skin (77.5 per cent.); by pulmonary evaporation, 14.7 per cent.; in heating the air breathed, 5.2 per cent.; in heating ingesta, 2.6 per cent.
In order that the vital processes of man and the other homceo-thermic animals should go on in a normal manner, it is necessary that their mean temperature remain nearly the same, and we have seen that under ordinary circumstances it varies only about one degree below or above the standard 370 C, notwithstanding the changes taking place in the temperature around us. Thus we can live in any climate, however cold or warm, and if our body temperature remains unaltered, we suffer no immediate injury.
There is a limit, however, to this power of maintaining a uniform standard temperature. If a mammal be kept for some time in a moist medium, where evaporation cannot take place, at a temperature but little higher than its body, say over 45 ° C, its temperature soon begins to rise, and it dies with the signs of dyspnoea and convulsions (probably from the nervous centres being affected) when its temperature arrives at 43°-45°. If placed in water at freezing point an animal loses its heat quickly, and when its body temperature has fallen to about 200 C. it dies in a condition resembling somnolence, the circulation and respiration gradually failing.
Since a variation of more than one or two degrees in the temperature of our bodies interferes with the vital activities of the controlling tissue in the nervous centres, it is, of course, of the utmost importance that adequate means for the regulation of the mean temperature of our bodies should exist.
The temperature of an animal's body must depend on the relations existing between the amount of heat generated in the tissues and organs and the amount allowed to escape at the surface, and these must closely correspond in order that the heat of the body may remain uniform. Both these factors are found to be very variable. Every increase in the activity of the muscles, liver, etc., causes a greater production of heat, while a fall in external temperature or increase in the moisture of cool air causes a greater escape of heat from the surface.
The maintenance of uniform temperature may be accomplished by (1) variations in the heat income, so arranged as to make up for the irregularities of expenditure, or (2) variations in the loss to compensate for the differences of heat generated.
Since the temperature and moisture of our surroundings are constantly varying between tolerably wide limits, the amount of heat given off by our bodies must also vary. In cold, damp weather a great quantity of heat is lost in comparison with that which escapes from the body when the air is dry or warm. If the heat generated had to make up for the changes in the heat lost, we should expect to find a correspondingly great difference in the amount of heat generated at different times of the year. No doubt we have some evidence in the keener appetite or use of more fuel, and the natural tendency to active muscular exertion during cold weather, to show that a greater amount of combustion takes place in winter than in summer. Further, if the preservation of a uniform body temperature depend solely upon the variations in the amount of income keeping pace with the variation in the expenditure, we should find it inconvenient to set our muscular or glandular tissues in action except when the external temperature was such as would enable us easily to get rid of the increased heat following their activity. It no doubt appears that the general tissue combustion, as measured by the amount of C02 given off, increases when we are placed in colder surroundings - such as a cold bath; still, it is probable that the variations in heat income have but secondary regulating influence on the body temperature. If the rate of income have any regulating influence, we are ignorant of the manner in which such influence is exerted, for it must act more slowly and cannot follow so closely, as the variations in expenditure do, extrinsic changes of temperature.
On the other hand, we know that the amount of heat expenditure may be varied by mechanisms which are almost self regulating. It has already been stated that the great majority of the heat is lost by the parts in contact with the air, namely, the skin and air passages. In these places the warm blood is exposed to the cool air, and loses much of its heat by radiation, conduction and evaporation. It is obvious that the greater the quantity of blood thus exposed for cooling, the greater will be the amount of heat lost in a given time by the body as a whole.
If we review the circumstances which interfere with the uniformity of the temperature of the body, we shall see that each one is accompanied by certain physiological actions which tend to compensate for the disturbing influences.
The chief common events tending to make our temperature exceed or fall short of its normal standard may be enumerated as follows, and the explanation of their modes of compensation will at the same time be given: -
A casual increase in the heat income may be induced by any increased chemical activity in the tissues, notably the action of the muscles and large glands. When this increased heat is communicated to it, the warm blood, by the help of certain nerve centres, brings about the following results: (a) An acceleration of respiratory movement, increasing the amount of cold air to be warmed and saturated with moisture by the air passages, and facilitating the escape of the surplus caloric. (b) Relaxation of the cutaneous arterioles, exposing a greater quantity of blood to the cooling influence of the air. (V) Greater rapidity of the heart beat, supplying a greater quantity of blood to the air passages and to the surface vessels, (d) An increase in the amount of sweat secreted, affording opportunity for greater surface evaporation.
As examples of these points may be mentioned active muscular exercise, which daily experience shows us is always accompanied by quick breathing, rapid heart's action, and a moist skin. The increased production of heat in fever gives rise to the same results, with the exception of the secretion of the sweat, which want (probably owing to the toxic inhibition of the special nerve mechanisms of the glands) is an important deficiency in the heat-regulating arrangements, and has much to do with the abnormally high temperature of the disease.
When a lesser quantity of heat is produced, owing to inactivity of the heat-producing tissues, the reverse takes place, namely, the respiration and heart's action are slow, the skin is pale and dry, so that little heat can escape.