Thermoregulation, master of science

1. UNIT 6; THERMOREGULATION
Normal Body Temperature
The animal kingdom can be broadly classified into two depending upon their body
temperature. The animals which can maintain their body temperature relatively constant in the face of
wide variations of environmental temperature are known as “Warm-blooded animals” or
“homeotherms”. Whereas, those animals whose body temperature fluctuate with variation in
environmental temperature are called “cold-blooded animals” or “Poikilotherms”.
Even in homeotherms, who are capable of maintaining constant body temperature,different
parts of the body have different temperatures. The
average temperature measured has been found to lie
within a constant range.
The body is hypothetically divided into core
and shell.
Oral temperature
Average- 37degree Celsius(98.6degree F). Oral
temperature is affected by many factors.Eg;Cold or hot
drinks, chewing gum, smoking, mouth breathing etc.
When there is a rise in oral temperature there will be a
rise in subclavian arterial temperature.
The body extremities have less temperature compared to others body parts. The scrotum is
regulated at 32 degree Celsius.
Skin temperature
The normal temperature of skin is about 33 °C or 91 °F. The flow of energy to and from the
skin determines our sense of hot and cold. Heat flows from higher to lower temperature. The most
common method of skin temperature measurement is the use of a mercury-in-glass thermometer.
The measurement of temperature from the skin surface of the forehead can be measured by using
liquid crystal thermometers
The core temperature or Internal body temperature

2. The core temperature(Temperature of intra-abdominal, intra-thoracic and intra-cranial
content) is maintained at a constant temperature. Rectal and oesophageal temperatures represent Core
temperature. It is the accurate index of the temperature of blood. The hypothalamic
thermoregulatory receptors are stimulated based upon the variations in core
temperature. The core temperature is 0.5-1 degree Celsius more than the oral
temperature. It shows poor reflection of rapid temperature changes in blood.
The sites for recording core temperature; - Rectum, Vagina, Oesophagus and
Tympanic membrane.
The shell temperature
Shell temperature, the temperature of limbs and the surface layer of
trunks are included in shell temperature. It exhibits wide variation of the
temperature. Although rectal and oesophageal temperatures are most reliable,
for practical advantages, oral temperature is taken for routine clinical purposes.
Comfort Zone
The optimum range of temperature in which the organisms live
comfortably is called Comfort Zone.
FACTORS AFFECTING NORMAL BODY TEMPERATURE
1.Age ;- Infants show wide range of variations in body temperature. It is due to overall less activity
in infants (a) irregular activities (b) brown fat and (c) thermoregulatory mechanisms are not fully
developed. Generally, 0.5°C above that of the adults. In infants regulation is imperfect. Hence, range
of variation is wider. A fit of crying may raise and a cold bath may lower the body temperature
In old age, temperature is 'subnormal' due to: (a) drcease in activity (b) feeble circulation
(because BMR is low) (c) thermoregulatory mechanisms are weak, that is why they are intolerant to
extremes of external temperature.
2.Sex;-In females the body temperature may be a little lower. This is due to relatively low BMR and
thick layer of subcutaneous fat (non-conductor). During menstruation, temperature slightly falls
(0.3°F or O.l7°C). Then it gradually rises and becomes maximum 24 to 48 hours after the ovulation.
This rise is due to progesterone level of blood which is secreted by the corpus luteum. Regular record
of oral temperature in the early morning is sometimes used to detect the exact date of ovulation in a
woman, in clinical practice.
3. Constitutional hyperthermia;- Normal healthy adults with body temperature above normal
upto 100°F.
4.Diurnal variation(Circadian fluctuation); The variation of 1.5°C may occur in normal
person.It is highest in the evening ( after the day's labour-between 5 and 7 pm) and lowest in the early
hours of the morning (after the night's rest). In the night-workers, the rhythm is reversed. The average
range of variation is 1 °F (0.55°C) to l.5°F (0.83°C). This diurnal variation is related to exercise and
specific dynamic action (SDA) of food. Fasting and absolute bed rest abolishes this variation.
5.Diseases;- Hyperthyroidism - Increased BMR - body temperature is chronically elevated by 0.5°C.
(ii) Hypothyroidism (Myxoedema) opposite occurs.

3. 6.Exercises;- May cause increase in temperature - upto 40-41°C (104-106°F) due to: (i) Inability of
"heat dissipating mechanisms" to handle the greatly increased amount of heat produced.
7.Emotional factors;- Can increase the body temperature by as much as 2°C. Temperature,
humidity and movement of air are directly concerned with the amount of heat loss from the surface
and thus affect body temperature.
8. Cold and warm baths: -These have a far greater influence than air at the same temperature, but
since the duration of exposure to these baths are short, they have a little effect on the normal body
temperature. However, body temperature may remain elevated for a considerable time after a
prolonged hot water bath.
9. Sleep:- Because of muscular inactivity, sleep results in a slight fall of body temperature.
10.General anaesthetics or chlorpromazine;- Reduces the body temperature by depressing the
activity of the ascending reticular system. • Tubocurarine paralyses the skeletal muscle, and reduces
heat production and results in fall of temperature. • Antipyretic drugs, e.g. sodium salicylate and
acetylsalicylic acid (aspirin) antagonise the action of pyrogen on the hypothalamus, induce cutaneous
vasodilatation and sweating and thereby help in reduction of temperature in febrile states.
11. Posture, piloerection and clothing ;- They are also important factors which affect the body
temperature. All animals and even man may conserve heat or may prevent heat loss by curling them
up during exposure to cold.
Heat production and Heat loss
Mechanism of Heat production ( Thermogenesis)
1. By metabolic activities of the body specially in:
(i) Liver (ii) Heart cause relatively constant heat production.
(iii) Skeletal muscle generates variable amount of heat, but a major source of heat production in the
body: At rest, very little heat production.
Heat production under basal {resting) conditions, called basal metabolic rate (BMR) is 1 kcal/
kg/hour. This output works out at about 1500 kcal/day in females and 1700 kcal/ day in males.
2.By assimilation of food (SDA-Specific Dynamic Action) of food
The SDA of a food is the obligatory energy expenditure that occurs during its assimilation
into the body. Maximum heat production is seen after ingestion of proteins. Heat production takes
place through physiological oxidation of food materials in the body-by combustion of carbohydrates,
proteins and fats. Higher fat intake increases heat production. During digestion the peristaltic action of
intestines and the activity of various digestive glands produce heat.
3.Heat gained by body from environment or from objects hotter than itself
It occurs through direct radiation from the sun or heated ground or by reflected radiation from the
sky.

4. 4. By endocrine mechanisms
(i) Epinephrine and nor-epinephrine produce a rapid but short lived increase in heat production
(ii)Thyroid hormones produce a slowly developing but prolonged increase in heat production.
5.A source of considerable heat production is a special type of fat, called “Brown fat”
It makes up a small percentage of total body fat. It is more abundant in infants but is present
in adults also. As brown fat cells contain several small droplets of fat mitochondria, increased fatty
acids oxidation in mitochondria. Increased heat production(i.e. has a high rate of metabolism).
Shivering increases heat production although not as high as voluntary muscular exercise.
Vasoconstriction and Piloerection also help in heat gaining.Ingestion of hot foods or drinks
contributes to heat production in negligible amounts. Of the organs, the liver contributes the highest
amount. Heat produced by liver and heart is relatively constant. The action of some internal secretion
and enzymes, e.g. thyroxine and epinephrine (possibly) also helps to heat production.
Increased heat production which occurs by increasing the BMR and metabolic activity is termed
Chemical thermogenesis, while the heat production by increased muscular activity is termed
Physical thermogenesis.
Mechanism of Heat loss(Thermolysis)
Heat is lost from the body by three channels,
(1) Skin
(2) Lungs
(3) Rectum
(4) Urethra
The processes include radiation, conduction, convection, perspiration evaporation and
excretion. The bodily changes that regulate the exchange of heat between the body and the
environment are referred to as physical heat regulation.
The processes by which heat is lost from the body when the environmental temperature is
below body temperature. It takes place by means of
1.Skin-As environmental temperature approaches body temperature, radiation losses decline and
vaporization loss increases. The rate at which heat is transferred from the deep tissues to the skin is
called the “Tissue Conductance”.
In humans, the other major process transferring heat from the body is by the way of
Vaporization of Water from the skin and mucous membrane and respiratory passage. From the skin
(proportional to the total surface area): 1. Radiation: Due to the difference of temperature existing
between the body and the cooler environment heat is lost from the body by radiation (loss by
electromagnetic waves). The body however does not radiate to the surrounding air, but through the air
to the solid objects in the vicinity. When a number of people are present in a room, they radiate
towards one another as well as to the surrounding objects. The amount of radiation from an object is
determined by several factors. It is proportional to the surface area of the body, to its emissive power,
and to the difference in temperature between the radiating body and the surrounding. The color of
clothing's may play a part, white and pastel shades being suitable for tropical climate.

5. (2) Lungs- On average the water loss from the lungs is approx. 30ml/day. Some heat is also lost via
the lungs by raising the temperature of inspired air to body temperature. (iii) Through skin- It consists
of the passage of water through epidermis. Eccrine glands are responsible for Thermal Sweating.
Apocrine glands help in heat loss through skin surface with salt water, called “Non-thermal
sweating”.
(3)Rectum – Heat loss takes place as the excretion is expelled out.
(4)Urethra – As the process of micturition takes through urethra, the heat loss also will be taking
place.
Vasoconstriction of the cutaneous vessels reduces blood flow and thereby
less heat is lost from the body, whereas vasodilatation produces opposite effect. Wearing woollen
clothes, which are bad conductors of heat, also decrease the heat loss through conduction. Insensible
perspiration occurs due to continuous diffusion of fluid from the capillaries of the deeper layer of skin
to the dry surface of skin. From lungs • Evaporation of water in expired air is the main pathway
through which heat is lost in dogs and sheep. • Heat lost for warming the inspired air is about 2% in
man. By excreta: Urine, faeces, etc. about 2%. Protrusion of the tongue facilitates heat loss through
salivation in dogs.
Acclimatization
Introduction:
● Acclimatization is also known as acclimatation. In this process, an individual gives their best
to adjust in the surrounding as per changes occurring in the environment.
● Some of the common factors with which organisms adjust themself are:
● Change in altitude, temperature, humidity, photoperiod, or pH.
● By adapting to these conditions, organisms make themself capable of surviving in adverse
conditions.
● Acclimatization process does not take too much of time, and these changes occur in a short
period, i.e. in hours or weeks.
● “Acclimatization” means adjustment i.e. a condition where an organism adjusts themselves
with respect to their surroundings like temperature, altitude, humidity, pH, light, salinity,
pressure and presence of certain chemicals.
Define Acclimatization:
● It is referred to as an adjustment which organisms do by changing their behaviour and
physiology to survive with changes which are occurring in the environment.
● We call these changes as phenotypic changes as they occur in a short duration of time. And
mostly these are reversible changes.
● Acclimatization is restricted by individuals' genomes, where this same statement can’t be true
or correct for the process happening over multiple generations, thus facilitating the
recombination of genetic traits which enhances the chances of better survival in a newly formed
environment.

6. Example for better understanding:
1. As tomatoes grow in temperate climates. In that case, they can also survive in freezing
temperatures if the temperature drops slowly and gradually rather than occurring suddenly. So,
short time adjustment is the adjustment shown by the tomato in such an adverse climate.
2. A few plants which are found in desert areas bloom only at night. This adaptation is made by
plants to ensure that the plant does not dehydrate in the extreme desert heat. Apart from this
desert, plants also have a waxy coating on their leaves which helps with dehydration as well.
Acclimatization in Humans
1. High Altitudes:
When we travel to higher altitude locations, we see acclimatization, and it is considered as one of the
best examples of the acclimatization process in humans.
For example: if a person rides to 3,000 meters above sea level and stays there for at least a week, then,
in that case, the individual becomes acclimatized to 3,000 meters. Further, if that person rides 1000
meter more than that they again have to acclimatize to 4000 meters in altitude.
● Some of the common changes which body undergoes, when they show acclimatization on high
altitude:
● Increased in the synthesis of Red blood cells
● Causes an increase in pressure in pulmonary arteries – thereby forcing blood into sections of
the lungs which are usually not used during normal breathing at lower altitudes.
● Increase in the synthesis of Red blood cells
● Causes an increase in pressure in pulmonary arteries – thereby forcing blood into sections of
the lungs which are usually not used during normal breathing at lower altitudes.
● It causes an increase in the depth of respiration
● It increases depth (volume) of breath during the inhalation process.
● Whereas in other humans, there may be acute mountain sickness when they are above 3000
meters from sea level.
● Whereas in normal conditions, there may be very common and mild conditions that can be
overcome if the body is given enough time to acclimatize.
● The main reason behind all this is reduced air pressure at high altitudes as well as the lower
oxygen levels. In extreme cases, it causes major disorder inside the body.
● It is High Altitude Cerebral Edema, where fluid builds up in the brain and it is a totally life-
threatening condition and requires immediate medical attention.

7. 2. Deep Diving:
● Deep-sea divers also undergo the acclimatization process when they ascend from a certain
depth.
● In this type of acclimatization process, there is another process which acts behind them and is
called decompression.
● In this case, the dissolved inert gases are eliminated from the diver’s body by pausing at several
stops during the ascent to the water surface.
● If drivers start descending in that case, it causes an increase in hydrostatic pressure as well as
ambient pressure.
● So, due to this, the breathing gas which is used with the dive is supplied at ambient pressure.
● In this process, the gases begin to dissolve in the diver’s body.
● On depressurization, the dissolved gases start forming bubbles inside the body, which lead to
debilitating pain.
● In adverse cases, it can also cause coma or even death.