Thermoregulation

Question 1

Describe the function of the hypothalamus?

Answer 1

• The hypothalamus is the highest level for integration and control of autonomic functions
• It provides rapid neural component for homeostasis
• It has a centre for heat conservation and a centre for heat dissipation

Question 2

Compare the regulation of body temperature between different groups of animals?

Answer 2

1. Birds & mammals maintain constant body temperature (are homeothermic)
2. Reptiles, amphibians,
   invertebrates & fish temperature varies with that of the surroundings (are poikilothermic)

Question 3

What is the cause of physiological variations in normal body temperature?

Answer 3

Variation due to site of measurement

Question 4

Describe what surface temperature depends on?

Answer 4

Because of direct exposure of the skin to environmental temperatures, its temperature varies with that of the environment & is usually lower than that of the core body
> Skin temperature is lowest at the extremities i.e. fingers & toes & highest at the axilla
Note: Scrotal temperature is maintained at 32 °C

Question 5

How do you measure core temperature?

Answer 5

1. Inner body temperature is measured orally with thermometer placed under the tongue & mouth closed
2. Or per rectum with the thermometer inserted through the anus into the rectum
   Note: Rectal temperature is higher than oral temperature by about 0.5 °C
3. Highest measurable temperature is from the tympanic membrane & it reflects closely the temperature of the blood reaching the hypothalamus

Question 6

Describe the individual variations in normal body temperature?

Answer 6

• Each individual maintain daily core temperature but average oral human temperature is from 36.3 to 37.7 °C
• Some individual have core temperatures regularly above the normal range & are said to be constitutionally hyperthermic

Question 7

Describe the variations in normal body temperature according to age?

Answer 7

• Newborn & young children have higher body temperatures than adults
• This is because of their higher metabolic rate

Question 8

Describe the variations in normal body temperature due to exercise?

Answer 8

Physical exercise raise the metabolic rate & body temperature
- Rectal temperature can reach 40 °C
during exercise

Question 9

Describe the variations in normal body temperature due to ovulation?

Answer 9

• There is a characteristic rise in basal body temperature at the time of ovulation
• This is due to the rise in progesterone levels (progesterone is thermogenic)

Question 10

Describe the variations in normal body temperature due to diurnal variations?

Answer 10

• Temperature fluctuates within 0.5 °C in relation to the time of day or night
• In general, it is lowest at 6 AM and highest in the evenings

Question 11

Why is the Thermoregulation important?

Answer 11

Most bodily chemical reactions depend on enzymes which are temperature dependent
- the body can tolerate deviations within +/- 2 degrees from 37 degrees

Question 12

What is hyperthermia?

Answer 12

Overheated body

Question 13

What is hypothermia?

Answer 13

A fall below 34 degrees leads to deleterious effects in the nervous function and heart leading to death at 28 degrees

Question 14

How is body temperature maintained?

Answer 14

1. Thermoreceptors
2. Modulators - in the hypothalamus
3. Effectors - skin, metabolic apparatus of cells

Question 15

Describe the location of thermoreceptors and their function?

Answer 15

• Receptors sensitive to temperature changes are located peripherally under the skin, & centrally in the hypothalamus
• Cutaneous thermoreceptors are mainly sensitive to cold, but some are sensitive to warmth
  > Cold receptors outnumber warm receptors by a ratio of 4:1
• Thermoreceptors in the hypothalamus are extremely sensitive to changes as little as 0.5 to 1 degree in warm or cold direction
• Central thermoreceptors are the main sensors for responses to rise in temperature
• For responses to cold, information is required from both peripheraland central thermoreceptors

Question 16

What are thermoeffectors?

Answer 16

The main tissues which generate heat or cause heat to be conserved or lost are the metabolic machinery of all cells
E.g. blood vessels, sweat glands & subcutaneous fat of the skin

Question 17

Describe cellular metabolism as a source of heat gain?

Answer 17

1. Rate of metabolism is under control of thyroxine & adrenaline & muscular exercise
   > Coldness leads to increase in thyroxine & adrenaline which increases metabolism
   > It also cause shivering which generates heat
2. Rate of metabolism increases after meals a phenomenon known as specific dynamic action of food substances
   > This arises from energy expenditure during the assimilation of food into the body
   > The increase lasts for up to 6 hours & is prominent with proteins & much less with carbohydrates and fats

Question 18

Describe the hot environment as a source of heat gain?

Answer 18

If the environment is hotter than body temperature, heat is gained from the environment through radiation, conduction convection

Question 19

Describe hot food as a source of heat gain?

Answer 19

Ingestion if large quantities of hot beverages and warm foods adds heat to the body

Question 20

Describe sources of heat loss?

Answer 20

Radiation, conduction, convection, faces & urine
> Between 20-31 °C, about 70% of heat loss is through radiation & conduction
> 27% is through sweat
> 3% is through urine & faeces
> In hot environment (>31 °C)
heat loss is through the secretion & evaporation of sweat from the sweat glands

Question 21

Describe the role of insulation of the skin in Thermoregulation?

Answer 21

• Subcutaneous fat is a poor conductor of heat, & it reduces heat loss or gain from the environment Infants have brown adipose tissue (BAT) which generates a lot of heat due to plenty of mitochondria and high metabolic rate (do not shiver to generate heat till about 6 months of age)
- BAT is primarily located in the interscapular, neck, axillary and perirenal regions in the human infant, with smaller depots behind the sternum and along the spine (human newborn)
Adult human BAT is mainly located in the cervical, axillary and supraclavicular regions, with smaller depots observed in the periaortic, perirènal and paravertebral regions.

Question 22

Describe the effects of cutaneous vasoconstriction or dilation of the skin in Thermoregulation?

Answer 22

Minor changes in body temperature are offset by constriction or dilation of peripheral blood vessels
1. When its cold, sympathetic stimulation cause vessels to constrict so that warm blood from the core is not exposed to the cooler environment
2. When it is hot sympathetic stimulation causes vessels to dilate so that warm blood from the core is exposed to outside environment
- The warm arteries from the limbs are in close contact with the veins so that there is counter current heat exchange between the arteries & veins

Question 23

Describe the role of sweating in Thermoregulation?

Answer 23

When temperature is high, the hypothalamus excites the sympathetic system to stimulate sweat secretion
- There are two types of sweat glands:
1. Eccrine glands which secrete in response to heat (thermal sweating)
2. Apocrine glands which secrete in response to emotions (emotional sweat)

Question 24

Describe location of eccrine glands and the sweat they produce?

Answer 24

• Eccrine glands are found all over the body but abundant on the palms, soles & head
• The sweat they secrete is hypotonic saline solution containing urea & uric acid
• Eccrine sweat is triggered by heat & is under cholinergic sympathetic control
- When the body is very hot, the rate of sweating can be 1.7L in one hour

Question 25

Describe apocrine glands and the sweat they produce?

Answer 25

• Apocrine glands develop from hair follicles in axilla, nipples, labia & mons pubis
• Are not under direct nervous control, rather they are stimulated by circulating adrenaline
• The secretion is milky & odourless but becomes foul due to bacteria
- Deodorants work by masking body odour or by targeting the bacteria that cause the smell in the first place.
- Antiperspirants work by preventing sweat glands from releasing sweat, in addition to counteracting body odour with their own fragrance

Question 26

Describe the hypothalamic thermoregulatory centers,

Answer 26

• There is evidence that heat-dissipating & heat-generating centres are found in the hypothalamus
1. Transection of CNS below the hypothalamus causes an animal to be poikilothermic
2. If transection is above the hypothalamus, the animal remains homeothermic
3. When a discrete lesion is placed in the preoptic area of anterior hypothalamus, the animal becomes unable to regulate temperature if placed in a warm environment & dies of hyperthermia. The animal can however regulate temperature in a cold environment
4. Stimulating the anterior hypothalamus with heat excites the body responses which increase heat loss
Cooling the anterior hypothalamus causes shivering and cutaneous vasoconstriction
6. Stimulation of posterior hypothalamus causes shivering & cutaneous vasoconstriction & does not provoke responses associated with heat loss
7. Heating the posterior hypothalamus does not abolish the shivering developed in a cold room
8. A discrete lesion in the posterior hypothalamus renders the animal poikilothermic for both warm and cold environments

Question 27

Describe disorders of Thermoregulation?

Answer 27

• Under conditions of extreme cold or extreme heat, the thermoregulatory mechanisms may fail
• In certain illnesses, body temperature may rise (fever), this is not necessarily due to a failure of the thermoregulatory mechanisms
• Thermoregulatory disorders include:
hypothermia, frost bite, heat stroke, heat exhaustion