Body temperature can be measured by placing clinical thermometer in different parts of the body as given below:

1. Oral temperature: Mouth.
2. Axillary temperature: Axilla.
3. Rectal temperature: Rectum.
4. Surface temperature: Over skin.

Normal Body Temperature

Temperature at different parts of the body is given below.

Physiological Variations

1. Age

In infants, body temperature varies in accordance to the environmental temperature for the first few days after birth. It is because the temperature regulating system does not function properly during infancy.

In children, temperature is slightly 0.5°C more than in adults because of more physical activities.

In elders, since heat production is less, the body temperature decreases slightly.

2. Sex

In females, body temperature is less because of low basal metabolic rate when compared to that of males.

3. Menstrual Cycle

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

4. Diurnal Variation

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

5. After Meals

Body temperature rises slightly (0.5°C) after meals.

6. Exercise

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

7. Sleep

During sleep, body temperature decreases by 0.5°C.

8. Emotion

During emotional conditions, body temperature increases.

Pathological Variations

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

Homeothermic and Poikilothermic Animals

Living organisms are classified into two groups depending upon maintenance (regulation) of body temperature:

1. Homeothermic animals.
2. Poikilothermic animals.

Homeothermic Animals

Homeothermic animals are the animals whose body temperature is maintained at a constant level irrespective of environmental temperature.

Birds and mammals including man belong to this category. They are also called warm-blooded animals.

Poikilothermic Animals

Poikilothermic animals are the animals whose body temperature is not constant. It varies according to environment.

Amphibians and reptiles are the poikilothermic animals. Such animals are also called cold-blooded animals.

Heat Balance

Regulation of body temperature depends upon balance between heat produced in body and heat lost from the body.

Various mechanisms are involved in production of heat in the body.

1. Metabolic Activities

Major portion of heat produced in the body is due to metabolism of foodstuffs.

Heat production is more during metabolism of fat. About 9 calories of heat is produced during metabolism of fats when one liter of oxygen is utilized.

For the same amount of oxygen, carbohydrate produces 4.7 calories of heat. Protein metabolism produces 4.5 Cal/L.

Liver is the organ in which maximum heat is produced due to metabolic activity.

2. Muscular Activity

Heat is produced in the muscle both at rest and during activities.

During rest, heat is produced by muscle tone. About 80% of heat of activity is produced by the activity of skeletal muscles.

3. Role of Hormones

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

4. Radiation of Heat from Environment

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

5. Shivering

Shivering is the shaking of body caused by rapid involuntary contraction or twitching of muscles during exposure to cold.

It is a compensatory physiological mechanism in the body, during which enormous heat is produced.

6. Brown Fat Tissue

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

Brown fat produces enormous body heat, particularly in infants.

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

When environmental temperature is less than body temperature, heat is lost from the body.

Heat loss occurs by the following methods:

1. Conduction

Heat is lost from the surface of the body to other objects such as chair or bed by means of conduction.

2. 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.

3. Convection

Heat is conducted to the air surrounding the body and then carried away by air currents, i.e., convection.

4. Evaporation: Insensible Perspiration

When body temperature increases sweating occurs. During sweating, water evaporates from sweat and water is lost from the body.

About 22% of heat is lost through evaporation of water.

Perspiration

Perspiration means formation of sweat from which water is lost by means of evaporation.

Insensible Perspiration

Insensible perspiration is the perspiration which occurs before it is being perceived or sensed, i.e., we are not aware of perspiration.

In addition to loss of water through sweat, insensible perspiration includes evaporation of water through lungs also.

Usually, insensible perspiration is about 400 to 600 mL per day.

5. Panting

Panting is the rapid shallow breathing associated with dribbling of more saliva.

In some animals such as dogs, which do not have sweat glands, heat is lost by evaporation of water from lungs and saliva by means of panting.

Set Point for Body Temperature

In homeothermic animals, body temperature is regulated by hypothalamus, which sets the normal range of body temperature.

The set point for body temperature under normal physiological conditions is 37°C.

Hypothalamus has two centers to regulate body temperature:

1. Heat loss center.
2. Heat gain center.

Heat Loss Center

Heat loss center is situated in preoptic nucleus of anterior hypothalamus.

Neurons in preoptic nucleus are heat-sensitive nerve cells which are called thermoreceptors.

Stimulation of preoptic nucleus results in cutaneous vasodilatation and sweating.

Removal or lesion of this nucleus increases the body temperature.

Heat Gain Center

Heat gain center is otherwise known as heat production center.

This center is situated in posterior hypothalamic nucleus.

Stimulation of posterior hypothalamic nucleus causes shivering. Removal or lesion of this nucleus leads to fall in body temperature.

When Body Temperature Increases

When body temperature increases, blood temperature also increases.

When blood with increased temperature passes through hypothalamus, it stimulates the thermoreceptors present in the heat loss center in preoptic nucleus.

Now, heat loss center brings temperature back to normal by two mechanisms:

1. Promotion of heat loss.
2. Prevention of heat production.

Promotion of Heat Loss

Heat loss center promotes heat loss from body by:

1. Increasing the secretion of sweat. When sweat secretion increases, more water is lost from skin along with heat.
2. Inhibiting the sympathetic centers in posterior hypothalamus. This causes cutaneous vasodilatation. Blood flow through skin increases causing excess sweating. It increases the heat loss through sweat leading to decrease in body temperature.

Prevention of Heat Production

Heat loss center prevents heat production in the body by inhibiting mechanisms involved in heat production such as shivering and chemical (metabolic) reactions.

When Body Temperature Decreases

When body temperature decreases it is brought back to normal by two mechanisms:

1. Prevention of heat loss.
2. Promotion of heat production.

Prevention of Heat Loss

When body temperature decreases, the preoptic thermoreceptors are not activated. So, posterior hypothalamus is not inhibited.

This causes cutaneous vasoconstriction. Blood flow to skin decreases and so the heat loss is prevented.

Promotion of Heat Production

Heat production is promoted by two ways:

i. Shivering

Primary motor center for shivering is situated in posterior hypothalamus near the wall of the III ventricle.

When body temperature is low, this center is activated by heat gain center and shivering occurs.

Enormous heat is produced during shivering due to severe muscular activities.

ii. Increased Metabolic Reactions

Sympathetic centers, which are activated by heat gain center, stimulate secretion of adrenaline and noradrenaline.

These hormones, particularly adrenaline, increase heat production by accelerating cellular metabolic activities.

Simultaneously, hypothalamus secretes thyrotropin releasing hormone. It causes release of thyroid stimulating hormone from pituitary. It in turn increases release of thyroxine from thyroid.

Thyroxine accelerates the metabolic activities in the body and increases heat production.

Chemical Thermogenesis

It is the process in which heat is produced in the body by metabolic activities induced by hormones.

Hyperthermia: Fever

Elevation of body temperature above the set point is called hyperthermia, fever or pyrexia.

Fever itself is not an illness. But it is an important sign of something going wrong in the body. It is the part of body’s response to disease.

Fever may be beneficial to body because it plays an important role in helping the body fight diseases, particularly the infections.

Classification of Fever

Fever is classified into three categories:

1. Low-grade fever: When the body temperature rises to 100.4 to 102.2°F (38 to 39°C).
2. Moderate-grade fever: When the temperature rises to 102.2 to 104°F (39 to 40°C).
3. High-grade fever: When the temperature rises above 104 to 106.7°F (40.7 to 42°C).

Hyperpyrexia

Hyperpyrexia is the rise in body temperature beyond 107.6°F (42°C).

Hyperpyrexia results in damage of body tissues. Further increase in temperature becomes life threatening.

Causes of Fever

1. Infection: Certain substances (pyrogens) released from bacteria or parasites affect the heat regulating system in hypothalamus, resulting in production of excess heat.
2. Hyperthyroidism: Increased basal metabolic rate during hyperthyroidism causes fever.
3. Brain lesions: If lesion involves temperature regulating centers, fever occurs.
4. Diabetes insipidus: In this condition, fever occurs without any apparent cause.

Pyrogens

Pyrogens are the substances that induce fever by affecting the heat loss center in preoptic nucleus of hypothalamus.

Types of Pyrogens

1. Exogenous pyrogens.
2. Endogenous pyrogens.

Exogenous Pyrogens

Exogenous pyrogens are fever-inducing substances of external origin.

Examples:

1. Substances produced by infectious agents such as bacteria, viruses and parasites.
2. Other invaders like fungi.
3. Incompatible blood.

Endogenous Pyrogens

Endogenous pyrogens are fever-inducing substances of internal origin produced mainly by immune cells that are activated by the presence of pathogens.

Immune cells which produce the pyrogens are neutrophils, monocytes, lymphocytes, macrophages, NK cells and mast cells.

Endogenous pyrogens are usually cytokines such as tumor necrosis factor, interleukin-1, interleukin-6 and interferon-α.

Signs and Symptoms of Fever

Signs and symptoms depend upon cause of fever.

1. Headache.
2. Sweating.
3. Shivering.
4. Muscle pain.
5. Dehydration.
6. Loss of appetite.
7. General weakness.

Hyperpyrexia May Result In

1. Confusion.
2. Hallucinations.
3. Irritability.
4. Convulsions.

Decrease in body temperature below 95°F (35°C) is called hypothermia.

It is considered as a critical state of subnormal body temperature, when the body fails to produce enough heat to maintain the normal activities.

Major setback of this condition is the impairment of metabolic activities of the body.

When the body temperature drops below 87.8°F (31°C), it becomes fatal.

Elderly persons are more susceptible for hypothermia.

Classification of Hypothermia

Hypothermia is classified into three categories:

1. Mild hypothermia: When the body temperature falls between 95 and 91.4°F (35 and 33°C).
2. Moderate hypothermia: When the body temperature falls between 91.4 and 87.8°F (33 and 31°C).
3. Severe hypothermia: When the body temperature falls below 87.8°F (31°C).

Causes of Hypothermia

1. Exposure to cold temperatures.
2. Immersion in cold water.
3. Drug abuse.
4. Hypothyroidism.
5. Hypopituitarism.
6. Lesion in hypothalamus.
7. Hemorrhage in certain parts of the brainstem, particularly pons.

Signs and Symptoms of Hypothermia

1. Mild Hypothermia

Uncontrolled intense shivering occurs.

Affected person can manage by self. But the movements become less coordinated. Chills cause pain and discomfort.

2. Moderate Hypothermia

Shivering slows down or stops but the muscles become stiff.

Mental confusion and apathy (lack of feeling or emotions) occurs.

Respiration becomes shallow, followed by drowsiness.

Pulse becomes weak and blood pressure drops.

Sometimes a strange behavior develops.

3. Severe Hypothermia

The person feels very weak and exhausted with incoordination and physical disability.

Skin becomes chill and its color changes to bluish gray.

Eyes are dilated.

The person loses consciousness gradually.

Breathing slows down, followed by stiffness of arms and legs.

Pulse becomes very weak and blood pressure decreases very much, resulting in unconsciousness.

Further drop in body temperature leads to death.