Body temperature as a biomarker

Animals that are able to maintain their temperature within narrow limits regardless of the temperature of the external environment are called warm-blooded, or homoeothermic. Warm-blooded animals include mammals and birds. Animals lacking this ability are called cold-blooded, or poikilothermic. The maintenance of body temperature by an organism is called thermoregulation.

In cold-blooded animals, the body temperature does not differ much from the ambient temperature, and only during intensive muscular activity in some species it can significantly exceed the ambient temperature.

Physiological fluctuations of body temperature during the day – diurnal rhythms – are known: the difference between early morning and evening body temperature in humans reaches 0.5-1.0 °C. Temperature differences between internal organs reach several tenths of a degree. The difference between the temperature of internal organs, muscles and skin can be up to 5-10 °C, which makes it difficult to determine the average body temperature needed to determine the thermal state of the organism as a whole.

Lowering (hypothermia) or raising (hyperthermia) body temperature by a few degrees disrupts vital processes and can lead to cooling or overheating of the organism and even to its death. In many diseases, body temperature rises to certain limits and is regulated by the body at a new level, for example, in fever or cold.

Temperatures can rise as a result of stress, fear, nightmares, intense mental work, sex and infection.

Body temperature is measured with a maximal thermometer usually in the axillary (axillary) region, in the rectum, in the mouth or in the external ear canal, determining the intensity of IR radiation from the tympanic membrane. For this purpose, the initial reading of the maximum thermometer should be known to be lower than the measured temperature.

The body temperature of each person fluctuates within small limits during the day, remaining between 36.0 and 36.9 °C. for a healthy person. In case of overwork, the temperature ranges from 30 to 35 degrees Celsius. Following the diurnal rhythm, the lowest body temperature is noted in the morning, around 6 o’clock, and the maximum value is reached in the evening. Like many other biorhythms, temperature follows the daily cycle of the sun rather than our activity level. People who work at night and sleep during the day exhibit the same temperature cycle as the rest of us. A temperature level below 35 °C indicates a serious illness (usually the result of radiation exposure). Victims of hypothermia go into a stupor if their body temperature drops to 32.2 °C, most lose consciousness at 29.5 °C, and die at temperatures below 26.5 °C. The survival record for hypothermia is 14.2 °C, and in experimental studies it is 8.8 °C. Temperature is affected by sex and age. In girls, body temperature stabilises at 13-14 years of age and in boys at about 18 years of age. The average body temperature of men is about 0.5-0.7 °C lower than that of women.

Human body temperature is controlled by thyroid hormones and the hypothalamus. Nerve cells in the hypothalamus have receptors that respond directly to body temperature by increasing or decreasing the secretion of TTH, which in turn regulates the activity of the thyroid gland, whose hormones (T3 and T4) are responsible for the intensity of metabolism. To a lesser extent, the hormone estradiol is involved in temperature regulation (it plays a major role in the thermoregulation of the body in women during the menstrual cycle), an increase in its level leads to a decrease in basal temperature.

Many diseases of the endocrine system and brain tumours affecting the hypothalamic region cause pronounced and often persistent disturbances in thermoregulation. For example, thyrotoxic crisis (accompanied by a sudden release of hormones T3 and T4 into the blood) leads to a sharp rise in body temperature, often exceeding the critical mark and causing the death of the patient.

The normal temperature depends on where it is measured. Typical temperature measurements in a healthy person are as follows:

• temperature in the anus (rectal), vagina or ear: 37.5 °C;
• temperature in the mouth (orally): 37,0-37,3 °C;
• temperature in the axilla (axillary): 36,6 C.

• Normal human body temperature is close to that at which water (which makes up a large part of the human body) has the lowest heat capacity.
• Body temperature can rise for various reasons. For example, microbes when entering the body release toxins (poisons), which are protein substances, foreign protein causes a reaction in the body, affecting the production of specific substances that act on the temperature centre of the brain, which, in turn, changes the temperature in the direction of its increase.
• Psychosomatic spike in body temperature is a very common phenomenon. A person convinces himself that the body temperature is rising, and after a while the body temperature is really overestimated. Cases of the opposite effect are also known.
• The world’s lowest body temperature of 14.2 °C was recorded on 23 February 1994 in a 2-year-old Canadian girl who spent 6 hours in the cold.