Thermoregulation, Pyrexia & Hypothermia

What is a poikilotherm?

• Also provide some examples

'Cold blooded'

Temperature fluctuates with the environment (So blood not necessarily cold)

• Examples: reptiles

An animal that cannot regulate its body temperature except by behavioural means such as basking or burrowing. Temperature determines their activity.

What is a homeotherm?

• Also provide some examples

An animal that maintains a steady body temperature at a constant level, usually above that of the environment, by its metabolic activity.

• Examples: penguins, sheep, bears, etc.

Can increase or decrease their body temperature

• Remain active at all environmental temperatures.

Define the following alternative definitions:

• Endotherm

• Ectotherm

Endotherms - Internal heat production, are able to change their metabolic rate

'Broadly homeotherms'

Ectotherms - rely upon external heat sources (but muscle movement generates some heat), Low metabolic rates

'Broadly poikilotherms'

For body temp to increase, heat loss must _____.

If heat loss increases, heat production must ______.

increase, increase

Homeotherms/Endotherms want to maintain a _____ body temperature but there are regional variations, what does this mean?

stable

Core body temperature - temperature inside abdomen, cranium and thoracic cavity, important to keep at a stable temperature

• Very narrow range: rectal temp -1C higher than oral temp

Shell Temperature: outer surface: oral temp and skin temp has a wide range, changes vastly because they are used to gain or lose heat

• Physiologically regulated

• Varying shell temperature helps maintain the core body temperature

Describe how the circadian rhythm in homeotherms/endotherms influences the core temperature.

• In Diurnal Species (e.g. sheep)

• Body temp lowest in morning → highest in the day, drops off in the evening again

• In Nocturnal Species (e.g. the rat)

• Body temp highest in the evening → lowest in the morning

What other factors may impact an animal’s body temperature?

Seasonal Variations

Environment

Exercise - as a result of muscular contraction

Digestion - 'Specific dynamic action' associated with the packaging of nutrient derived energy into stores

Gender

• In humans, average core body temperature affected by gender

• Males 36.3 - 37.1 °C; Females 36.5 - 37.3 °C (but colder extremities!)

• Body temperature in females is influenced by steroids and thus varies with ovarian/menstrual cycle

What are the mechanisms in which heat can be transferred?

RADIATION - energy, in the infrared portion of the spectrum given off or absorbed by an object.

CONDUCTION - energy transferred between an object and the material next to the object by direct passage. Eg. Skin to air, or skin to water, skin to metal.

• Conductive losses are increased by convection.

EVAPORATION - loss of water from an organism in the form of water vapour requires significant heat input

What is the zone of thermoneutrality?

The range of environmental temperatures in which the body can maintain normal core temperature without increasing metabolic heat production or activating active cooling mechanisms.

Where is the central central control of thermogenesis?

Regulated at the level of the preoptic area of the hypothalamus

• Peripheral & central temperature sensors - send information to the POA.

• POA acts as an Integrating Centre - 'set point'

• POA sends information about the 'error' to either:-

  • heat promoting centre - more posterior region of the hypothalamus, to promote shivering, peripheral vasoconstriction etc.

  • heat losing centre - more anterior region of the hypothalamus, to promote sweating, panting, behavioral changes

A decrease in ambient temperature necessitates increased…

heat production / promotion

• The summit rate of heat production (often referred to as summit metabolic rate, ) is the maximum amount of heat an endothermic animal can produce, typically measured during an experimentally induced cold challenge. It represents the highest possible rate of aerobic metabolism, which serves as a critical indicator of an individual's cold tolerance and thermogenic limits.

What are the behavioral changes associated with heat production/promotion?

• Closed/curled up posture

• Huddling together

What are the main physiological changes associated with heat production/promotion?

1) Cutaneous vasoconstriction

• Sympathetic nervous system

• Peripheral vasoconstriction - a1 adrenoreceptors

  • Divert blood to the “core”

  • Periodic vasodilation - need to ensure tissues still periodically receive some blood flow

• Complications of vasoconstriction:

  • hypovolemia

    • Reduced effective circulating volume

      • Vasoconstriction can lead to hypovolaemia because it reduces blood flow to tissues and organs, especially the kidneys, which can reduce effective circulating volume and promote fluid loss or poor fluid regulation.

  • frost bite

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2) Countercurrent exchange systems

It’s a system where:

• Warm arterial blood flows toward the foot

• Cold venous blood flows back toward the body

• The vessels run very close together

Because they flow in opposite directions, heat transfers efficiently between them.

🔥 How it conserves heat

1⃣ Warm blood leaves the body core through arteries.
2⃣ Before it reaches the cold foot, it passes next to cold veins returning from the foot.
3⃣ Heat moves from the warm artery to the cold vein.
4⃣ The venous blood returning to the body is warmed.
5⃣ The arterial blood reaching the foot is already cooled.

❄ Why this saves heat

• The foot stays cool, so it loses less heat to the environment.

• The body core keeps more of its heat because the returning blood is already warmed.

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3) Piloerection / Horripilation

• Piloerection, commonly known as goosebumps or goose pimples, is the involuntary raising of hairs on the skin in response to cold, fear, or strong emotions

• Purpose: In animals with thick fur, this reaction traps air for insulation against the cold or makes them appear larger to threats. In humans, it is a vestigial reflex.

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4) Shivering Thermogenesis

• Spinal reflex - muscle movement producing heat

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5) Non-Shivering Thermogenesis

• SNS (via B adrenoreceptors) and T4 stimulate Na/K ATPase, increases Metabolic rate, increases heat production. (Increase 30%)

• In foetal animals utilise brown fat in which lipolysis is not linked to ATP synthesis - uncoupling protein THERMOGENIN

  • In adults, increased food intake

Increasing ambient temperature necessitates…

heat loss

What are the behavioral changes associated with increasing heat loss?

Open posture

Expose areas with low insulation

Minimal touching

Cool location

What are the physiological changes associated with increasing heat loss?

1) Altering conductance

• Cutaneous vasodilation increases skin temperature

PROBLEM - Effectiveness of conduction and convection decrease as ambient Temperature increases.....

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2) Evaporative heat loss

Works even at high temperatures

• 580kcal needed to evaporate 1kg water

• 25% heat loss in mammals is via evaporation

  • Sweating

  • Panting

  • Spreading saliva on fur

Describe the main features of heat loss by sweating.

• Not all animals have sweat glands

• Less effective if you have fur

Two types of sweat glands

• Apocrine glands - developed from hair follicles, produce membrane bound secretions and watery sweat. Thermoregulatory in horses and cattle. Respond to adrenaline.

• Eccrine glands - Thermoregulatory in man, cholinergic sympathetic innervation

Stimulated by increased central and skin temperature.

• Cattle, antelopes, camels, horses, but especially humans depend on sweating for heat loss.

Describe the main features of heat loss by panting.

Movement of air in 'non respiratory zone' of airways 'forced convection'

Side effects

• Loss of CO2

• Work - which can generate some heat

• Dogs - resp freq alternates between 30-40bpm and 300-400bpm

• Loss of salt

• Cools blood going to brain (nasal passages)

Dogs, Sheep, goats, gazelles - depend on panting

• Birds pant/gular flutter

Describe the main features of heat loss by spreading saliva on the fur.

Very ineffective - limited use for heat loss

Seen in few species, Marsupials and rodents

How are animals able to adapt to adverse environmental temperatures?

In response to adverse environmental conditions a number of responses depending on temporal characteristics and magnitude of change

• Phenotypic Adaptation during lifetime

•Genotypic Adaptation

•Acclimatization in response to natural forces

•Acclimation in laboratory

What are the mechanisms in which animals use to adapt to chronic cold temperatures?

Adjustments to Chronic Cold Temperatures

Increase thermal gradient

• Increased level of surface insulation

  • Summer vs winter fur, 52% difference in insulative value

• Behavioural

• Small mammals cant have enough fur!

  • Create microclimate e.g burrows, instead

Increase metabolic rate

Decrease core temperature

Hibernation

• Decrease Body temperature, heart rate, respiration rate metabolic rate.

• Fat used as energy source.

• If temperatures get too low, hibernating animals MUST wake and move (energy dependent).

What are the mechanisms in which animals use to adapt to chronic hot temperatures?

Decrease thermal gradient

• Decrease level of surface insulation

• Summer vs winter fur, 52% difference in insulative value

Increase in core temperature

• Camels, goats (without water)

Decrease the metabolic rate

• Estivation — decreased metabolic rate and water loss - snails, crabs, tortoises, crocodiles, lungfish, Fat-tailed Dwarf Lemur

What are some of the main thermoregulatory disorders?

1. Pyrexia

2. Heat stress - Hyperthermia, Heat Stroke, Heat Exhaustion, Heat Fatigue

3. Cold stress - hypothermia

What are the key features of pyrexia / fever?

Pyrexia/Fever

• alteration of the set point

Normal protective mechanism

• promotes interferon activity

  • Interferons (IFNs) are natural signaling proteins (cytokines) released by white blood cells and other tissues to help the immune system fight infections, viruses, and cancer

• Increases metabolic rate and accelerates tissue repair

• Decreases bacterial and viral replication

Describe how Pyrexia/Fever occurs.

1) Neutrophils and macrophages secrete pyrogen, such as IL1

• IL1 stimulates the anterior hypothalamus to secrete PGE

2) PGE raises the set point for Body temperature

• Triggers heat generating mechanisms to raise body temp

3) Individual shivers and peripheral vasoconstriction increases Body temp

4) Body temperature osscilates around the new set point

• Liver and spleen hoard Zinc and Iron depriving bacteria of the minerals needed to reproduce

5) When infection gone, pyrogen secretion land the set point is reset.

• When reset body must activate:

6) Heat loss mechanisms → return to set point

Describe how the following heat-stress based thermoregulatory disorders may occur:

• Heat Cramps

• Heat Exhaustion

• Heat Stroke

Heat cramps -excessive electrolyte loss (often after exercise and heavy sweating

• Muscle spasms

Heat Exhaustion - severe water loss/dehydration, electrolyte loss

• Hypotension dizziness, fainting, vomiting.

Heat Stroke (hot and Humid)

• Gain heat by radiation and conduction but can't loose by evaporation

• Delirium, convulsions coma tachycardia, hyperventilation, inflammation, multiorgan dysfunction

Describe how the following cold-stress based thermoregulatory disorders such as hypothermia may occur.

Stage 1 BT - decrease to 35-37°C, shivering, vasoconstriction. Breathing quick and shallow. Nausea, muscles become less responsive.

Stage 2 BT - decrease to 33-35°C. Shivering more violent. Muscle mis- coordination. Mild confusion. Surface blood vessels contract further. Extremities become blue.

Stage 3 BT - <32°C. Shivering stops. Cognitive impairment and physical inability present. Metabolic processes shut down. < 30°C, exposed skin, blue and puffy, muscle coordination very poor, incoherent/irrational behavior including terminal burrowing or stupor.

• terminal burrowing - irrational behavior, removing clothes + burrowing

Pulse and respiration rates J, heart rates (ventricular tachycardia, atrial fibrillation). Major organs fail. Clinical death occurs.