"Temperature is the degree of sensible heat or cold, expressed in terms of a specific scale."

Temperature is a physical property of matter that quantitatively expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of energy, when a body is in contact with another that is colder.

Body temperature can be measured by placing the clinical thermometer in different parts of the body such as-

  • Mouth (oral temperature)
  • Axilla (axillary temperature)
  • Rectum (rectal temperature)

Normal body Temperature- Normal body temperature in human is 37°C (98.6°F), when measured by placing the clinical thermometer in the mouth (oral temperature). It varies between 35.8°C and 37.3°C (96.4°F and 99.1°F).

Temperature at Different parts of the Body-

  • Axillary temperature is 0.3°C to 0.6°C (0.5°F to 1°F) lower than the oral temperature.
  • The rectal temperature is 0.3°C to 0.6°C (0.5°F to 1°F) higher than oral temperature.

Core Temperature-

 Core temperature is the average temperature of structures present in deeper part of the body. The core temperature is always more than oral or rectal temperature. It is about 37.8°C (100°F)

Variation of Body Temperature-

A.Physiological Variations

  • Age

 In infants, the body temperature varies in accordance to environmental temperature for the first few days after birth. It is because the temperature regulating system does not function properly during infancy. In children, the temperature is slightly (0.5°C) more than in adults because of more physical activities. In old age, since the heat production is less, the body temperature decreases slightly.

  • Sex

 In females, the body temperature is less because of low basal metabolic rate, when compared to that of males. During menstrual phase it decreases slightly.

  • Sleep

 During sleep, the body temperature decreases by 0.5°C.

  • Emotion

 During emotional conditions, the body temperature increases.

  • Menstrual cycle

 In females, immediately after ovulation, the temperature rises (0.5°C to 1°C) sharply. It decreases (0.5°C) during menstrual phase.

  • Diurnal variation

 In early morning, the temperature is 1°C less. In the afternoon, it reaches the maximum (about 1°C more than normal).

  • After meals

 The body temperature rises slightly (0.5°C) after meals.

  • Exercise

During exercise, the temperature raises due to production of heat in muscles.

B. Pathological Variations-

Abnormal increase in body temperature is called hyperthermia or fever and decreased body temperature is called hypothermia.

Heat Gain (Production)-

  •  Metabolic Activities

Major portion of heat produced in the body is due to the metabolism of foodstuffs. It is called heat of metabolism. Heat production is more during metabolism of fat. About 9 calories of heat is produced during metabolism of fats, when 1 L of oxygen is utilized. For the same amount of oxygen, carbohydrate metabolism produces 4.7 calories of heat. Protein metabolism produces 4.5 calories/L. Liver is the organ where maximum heat is produced due to metabolic activities.

  • Role of Hormones

Thyroxine and adrenaline increase the heat production by accelerating the metabolic activities.

  • Radiation of Heat from the Environment

Body gains heat by radiation. It occurs when the environmental temperature is higher than the body temperature.

  •  Muscular Activity

Heat is produced in the muscle both at rest and during activities. During rest, heat is produced by muscle tone. Heat produced during muscular activity is called heat of activity. About 80% of heat of activity is produced by skeletal muscles.

  • Shivering

Shivering refers to shaking of the body caused by rapid involuntary contraction or twitching of the muscles as during exposure to cold. Shivering is a compensatory physiological mechanism in the body, during which enormous heat is produced.

  • Brown Fat Tissue

Brown adipose tissue is one of the two types of adipose tissues, the other being white adipose tissue.

Heat Loos from the Body-

Maximum heat is lost from the body through skin and small amount of heat is lost through respiratory system, kidney and GI tract. 

1. Conduction

Three percent of heat is lost from the surface of the body to other objects such as chair or bed, by means of conduction.

2. Convection

Fifteen percent of heat is lost from body to the air by convection. First the heat is conducted to the air surrounding the body and then carried away by air currents, i.e. convection.

3 Evaporation 

Insensible Perspiration When water evaporates, heat is lost. Twenty two percent of heat is lost through evaporation of water. Normally, a small quantity of water is continuously evaporated from skin and lungs. We are not aware of it. So it is called the insensible perspiration or insensible water loss.

4 . Radiation

Sixty percent of heat is lost by means of radiation, i.e. transfer of heat by infrared electromagnetic radiation from body to other objects through the surrounding air.

5. . Panting

Panting is the rapid shallow breathing, associated with dribbling of more saliva. In some animals like dogs which do not have sweat glands, heat is lost by evaporation of water from lungs and saliva by means of panting.