Body Temperature Regulation

Body Temperature Introduction

The living organisms are classified into two groups depending upon the maintenance (regulation) of body temperature

1. Homeothermic animals
2. Poikilothermic animals.

Homeothermic Animals

• Homeothermic animals are animals in which the body temperature is maintained at a constant level irrespective of the environmental temperature.
• Birds and mammals including man belong to this category. They are also called warm-blooded animals.

Read And Learn More: Medical Physiology Notes

Poikilothermic Animals

• Poikilothermic animals are animals in which the body temperature is not constant. It varies according to environmental temperature.
• Amphibians and reptiles are poikilothermic animals. These animals are also called cold-blooded animals.

Body Temperature

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

1. Mouth (oral temperature)
2. Axilla (axillary temperature)
3. Rectum (rectal temperature)
4. Over the skin (surface temperature).

Normal Body Temperature: The normal body temperature in humans 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° and 99.1°F).

Temperature At Different Parts Of The Body

• The axillary temperature is 0.3-0.6°C (0.5 to 1°F) lower than the oral temperature. And, the rectal temperature is 0.3-0.6°C (0.5 to 1°F) higher than the oral temperature.
• The superficial temperature (skin or surface temperature) varies between 29.5° and 33.9°C (85.1° and 93°F).

Core Temperature

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

Variations Of Body Temperature

Physiological Variations

1. Age

• In infants, the body temperature varies in accordance with 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.

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

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

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

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

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

7. Emotion: During emotional conditions, the body temperature increases.

8. Menstrual cycle:In females, immediately after ovulation, the temperature rises (0.5°- 1°C) sharply. It decreases (0.5°C) during the menstrual phase.

Pathological Variations:An abnormal increase in body temperature is called hyperthermia or fever and decreased body temperature is called hypothermia.

Heat Balance

• Heat balance is the balance between heat produced in the body and the heat lost from the body.
• Normally, the temperature is maintained at a constant level by some mechanisms in the body.
• It is by adjusting the heat production in accordance with heat loss.
• Heat Gain Or Heat Production In The Body

The various mechanisms involved in the production of heat in the body are:

1. Metabolic Activities

• The major portion of heat produced in the body is due to the metabolism of foodstuffs.
• It is called heat of the metabolic reactions that occur in the liver. Heat is more during the metabolism of fat.
• About 9 catenae of heat are produced during the metabolism of fats when 1 liter of oxygen is utilized.
• For the same amount of oxygen, carbohydrate metabolism produces 4.7 calories of heat.
• Protein metabolism produces 4.5 calories/liter.

2. Muscular Activity

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

3. Role of Hormones: Thyroxine and adrenaline increase heat production by accelerating metabolic activities.

4. Radiation of Heat from the Environment: The body gains heat from radiation. It occurs when the environmental temperature is higher than the body temperature.

5. 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.

Heat Loss From The Body

• Maximum heat is lost from the body through the skin and a small amount of heat is lost through the respiratory system, kidney, and tract.
• When environmental temperature is less than body temperature, heat is lost from the body.
• Heat loss occurs by the following methods:

1. Conduction: Three percent of heat is lost from the surface of the body to other objects such as chairs or beds 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 the body to other objects through the surrounding air

3. Convection

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

4. Evaporation – Insensible Perspiration

• Twenty-two percent of heat is lost through evaporation of water. When water evaporates, heat is lost.
• Normally, a small quantity of water is continuously evaporated from the skin and lungs.
• We are not aware of it. So it is called insensible perspiration or insensible water loss.
• It is about 50 mL/hour. When body temperature increases, sweat secretion is increased. And, water evaporation is more with more heat loss.

5. Panting

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

Regulation Of Body Temperature

The body temperature is regulated by the hypothalamus which sets the normal range of body temperature.The set point under normal physiological conditions is 37°C.

Hypothalamus has two centers that regulate the body temperature:

1. Heat loss center
2. Heat gain center.

1. Heat Loss Center

• This center is situated in the preoptic nucleus of the anterior hypothalamus. Neurons in the preoptic nucleus are heat-sensitive nerve cells which are called thermoreceptors.
• Stimulation of the preoptic nucleus results in cutaneous vasodilatation and sweating.
• Removal or lesion of this nucleus increases the body temperature.

2. Heat Gain Center

• It is otherwise known as the heat production center. It is situated in the posterior hypothalamic nucleus.
• Stimulation of the posterior hypothalamic nucleus causes shivering. The removal or lesion of this nucleus leads to a fall in body temperature.

Mechanism Of Temperature Regulation

When Body Temperature Increases

• When body temperature increases, blood temperature also increases.
• When blood with increased temperature passes through the hypothalamus, it stimulates the thermo receptors present in the heat loss center in the preoptic nucleus Now, the heat loss center brings the temperature back to normal by two mechanisms:

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

1. Promotion of Heat Loss

When body temperature increases, the heat loss center promotes heat loss from the body in two ways:

• It stimulates sweat glands and increases the secretion of sweat. When sweat secretion increases, more water is lost from the skin. Along with water, heat is lost from the body
• It sends inhibitory impulses to the sympathetic centers in the posterior hypothalamus.
• Normally, the sympathetic centers cause constriction of blood vessels in the skin.
• When, these centers are inhibited, there is cutaneous vasodilatation. Now, blood flow through the skin increases causing excess sweating.
• It increases the heat loss through sweat leading to a decrease in body temperature.

2. Prevention of Heat Production

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

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

1. Prevention of heat loss

• When body temperature decreases, the preoptic thermoreceptors are not activated.
• So, the posterior hypothalamus is not inhibited. Now the sympathetic centers cause constriction of cutaneous blood vessels by increasing vasomotor tone.
• The blood flow to the skin decreases, and so the heat loss is prevented.

2. Promotion of heat production

Heat production is promoted in two ways:

Shivering:

• The primary motor center for shivering is situated in the posterior hypothalamus near the wall of the III ventricle.
• This center is activated by the heat gain center when body temperature is low. And, shivering occurs.
• Enormous heat is produced during shivering due to severe muscular activities and body temperature rises.

Increased metabolic reactions:

• The sympathetic centers, which are activated by the heat gain center, stimulate the secretion of adrenaline and noradrenaline.
• These hormones, particularly adrenaline increase heat production by accelerating cellular metabolic activities.
• Simultaneously, the hypothalamus secretes thyroid-stimulating hormone-releasing hormone.
• It causes the release of thyroid-stimulating hormone from pituitary. It in turn increases the release of thyroxine from the thyroid.
• Thyroxine accelerates the metabolic activities in the body and this increases heat production.

Chemical thermogenesis:

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

Applied Physiology

Hyperthermia-Fever

• Elevation of body temperature above a 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 and on many occasions, it plays an important role in helping the body fight diseases, particularly infections.

Classification of Fever

Fever is classified into three categories:

1. Low-grade fever: When the body temperature rises to 38-39°C, (100.4-102.2°F)
2. Moderate grade fever: When the temperature rises to 39-40°C(102.2-104°F)
3. High-grade fever: When the temperature rises above 40-42°C (104-107.6°F).

Hyperpyrexia

• Hyperpyrexia is the rise in body temperature beyond 42°C (107.6°F).
• Hyperpyrexia results in damage to body tissues. Further increase in temperature becomes life-threatening.

Causes of Fever

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

Fever Signs and Symptoms

The signs and symptoms depend upon the cause of the fever

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

Hyperpyrexia may result in:

• Confusion
• Hallucinations
• Irritability
• Convulsions.
• Hypothermia
• A decrease in body temperature below 35°C (95°F) is called hypothermia. It is considered the clinical state of subnormal body temperature when the body fails to produce enough heat to maintain normal activities.
• The major setback of this condition is the impairment of metabolic activities of the body.
• When the temperature drops below 31 °C (87.8°F) it becomes fatal. Elderly persons are more susceptible to hypothermia.

Classification of Hypothermia

Hypothermia is classified into three categories:

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

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 the pons.

Hypothermia Signs and Symptoms

1. Mild hypothermia

• Uncontrolled intense shivering occurs. The affected person can manage by themself.
• But the movements become less coordinated. The chillness causes 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) occur. Respiration becomes shallow followed by drowsiness.
• The pulse becomes weak and blood pressure drops. Sometimes strange behavior develops.

3. Severe hypothermia

• The person feels very weak and exhausted with incoordination and physical disability. The 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.
• The pulse becomes very weak and blood pressure decreases very much resulting in unconsciousness. A further drop in body temperature leads to death.