Very hot baths cause a momentary constriction of the blood vessels of the skin, giving rise to pallor, the sensation of cold with shivering, and in some instances “goose flesh.” The pallor and roughness of the skin last for only a few seconds and quickly give way to a dusky redness. A moderately hot bath is not accompanied by these phenomena there is no initial pallor or shivering.
The general application of heat causes a rise in temperature if it lasts long enough and if the evaporation of sweat is interfered with. The less the application of heat impedes the evaporation of sweat, the less the body temperature is apt to rise. Generally speaking, however, in a full immersion bath, since the evaporation of sweat is retarded, heat accumulates in the body and there is a rise of body temperature over skin temperature after a few minutes. A plain water bath of 104 degrees F will raise the internal temperature about five degrees, i.e., from about 98 degrees to 103 degrees F.
On the other hand, a quick hot bath will lower temperature. The reason for this lies in the fact that the heat production is diminished and its elimination encouraged by increased perspiration, relaxation of the surface vessels and increased activity of the heart. You yourself have experienced the delightful and refreshing coolness following a hot bath on a stifling summer’s day or after a hard muscular exertion. It is the same principle that underlies the cooling effects of a cup of hot tea or hot soup in contrast to iced tea or ice cream.
Heat also tends to increase metabolism, provided it is intense enough to raise the body temperature. The mechanism is similar to that in the case of cold water.
The contractile elements of both the blood vessels and the skin relax under heat. With the first shock of immersion and the transitory spasm of the cutaneous blood vessels, the tonus of the heart is increased by the raising of the pressure in the systemic vessels. The pulse increases in frequency and the tone of the heart muscles is probably lowered. Within a short time, with the dilation of the peripheral vessels, the blood pressure falls. The higher the temperature and the longer the duration, the more pronounced are the changes. As in the case of cold water, mechanical stimuli enhance the effects.
Likewise, as in the case of cold water, the frequency of respiration is increased, but to a less marked degree. The breathing is usually shallow. The sweat, sebaceous glands and the kidneys react readily to heat,so that the hot bath has a tendency to hasten the elimination of toxic substances.
As to the nervous system, warm baths of short duration have the same effect as cold baths, that is, they increase the sensibility of the nerves. Heat, how-ever, is usually more effective in lessening pain than cold. Generally speaking, baths at a high temperature are at first exciting, but later cause exhaustion, muscular weakness and a disinclination to muscular work. If prolonged, vertigo and nausea may be provoked. On the other hand, hot baths of a short duration have a reviving effect in cases of exhaustion following severe muscular exertion. This may be due to the elimination of the waste products of muscular activity.
Hot baths of increasing temperature followed by a cold shower (if permissible) are excellent for those sensitive to cold. They also are good for diffused pain such as associated with diseases of the viscera, dysmenorrhea, bronchitis, gallstone colic, renal colic and many rheumatic conditions. They have a wonderfully relaxing effect after a hard day’s work or after a severe muscular exercise and are superior to cold baths when one wishes “to cool off.” However, they should not be taken in cases of organic diseases of the central nervous system, myocardial weakness, cardiac hypertrophy or arteriosclerosis.