Thermoregulation, Pyrexia, and Hypothermia

Define Poikilotherms

“cold blooded” Temperature fluctuates with the environment, Activity levels related to body/environmental temeprature

Define homeotherms

Maintain a steady body temperature, Can increase or decrease their body temperature, Remain active at all environmental temepratures

Define Endotherm

Internal heat production, able to change their metabolic rate

Define Ectotherms

Rely upon external heat sources (muscle movement can generate some heat), Low metabolic rates

In order to maintain heat balance, if body temperature increases then

Heat loss must increase

In order to maintain heat balance, if heat loss increases then

Heat production must increase

In regards to endotherms, where is a very narrow range of temperatures maintained?

Cranium, thoracic cavity, abdominal cavity

In regards to endotherms, where is a wider range of temperatures maintained physiologically?

Shell temperature, oral temperature, skin

Homeotherm/Endotherm temperatures are considered stable but

NOT fixed

When would you expect a/an homeotherms/endotherms core temperature to fluctuate cooler?

During periods of inactivity/rest

Circadian rhythm temperature

Changes regardless of activity

What four factors can influence the thermoregulation of homeotherms/endotherms?

Seasonal variations, environment, exercise, digestion, gender

Exercise alters temperature due to

Muscular contraction

Body temperature in females is influenced by

Steroids and thus varies with ovarian/menstrual cycle

Heat can be transferred via what ways?

Radiation, Conduction, Evaporation

Define heat transfer via radiation

Energy, in the infrared portion of the spectrum given off or absorbed by an object

Define heat transfer via conduction

Energy transferred between an object and the material next to the object by direct passage (ex skin to air, skin to water, skin to metal).

Conductive losses are increased by

Convection

Define heat transfer via evaporation

Loss of water from an organism in the form of water vapor (requires significant heat input)

Control of Thermogenesis: Regulated at the level of the

Preoptic area of the hypothalamus

Control of Thermogenesis: What sends information to the preoptic area?

Peripheral and central temperature sensors

Control of Thermogenesis: preoptic area acts as a/an

Integrating center, or “set point”

Control of Thermogenesis: preoptic area sends information about ‘errors’ to

Heat promoting center or heat losing center

Where is the heat promoting center?

More posterior region of the hypothalamus

What does the heat promoting center promote physiologically?

Shivering, peripheral vasoconstriction, etc.

Where is the heat losing center?

More anterior region of the hypothalamus

What does the heat losing center promote physiologically?

Sweating, panting, behavioral changes, etc.

Decreased ambient temperature →

Increased head production/promotion

How can endotherms drive heat production/promotion and heat loss?

Through behavioral changes and physiological changes

What behavioral change occurs to decrease heat loss?

Closed/curled up posture, animals huddle together

What is the summit rate of heat production?

The point which rate of heat production is not enough to maintain body temperature

Heat production/promotion: Cutaneous vasoconstriction: What drives this change?

Sympathetic nervous system → Peripheral vasoconstriction (alpha1 adrenoreceptors)

Heat production/promotion: Cutaneous vasoconstriction: What is the outcome?

Divert blood to ‘core’ and periodic vasodilation

Heat production/promotion: Cutaneous vasoconstriction: What are some complications that may arise?

Hypovolemia (have to pee)

Frost bite

Heat production/promotion: Counter current exchange systems: How do they work?

As arteries travel down extremities, the blood cools as it moves distally. Venous return is warmed at the same level as arteries are cooled leading to warm blood returning to the core of the animal

Heat production/promotion: Piloerection/horripilation: How does it work?

Making hairs stand on end helps to trap a layer of air near your skin. This air is warmed by your body and helps to add an extra layer of warmth to aid in slowing heat loss

Heat production/promotion: Shivering thermogenesis: How does it work?

Spinal reflexes result in muscle movement → increased energy use → increased heat production

Heat production/promotion: Non-shivering Thermogenesis: What drives this change?

Sympathetic nervous system via beta adrenoreceptors and T4 stimulate Na/K ATPase

Heat production/promotion: Non-shivering Thermogenesis: How does this increase heat production?

Na/K ATPase stimulation → Increased metabolic rate → Increased heat production (~30%)

Heat production/promotion: Non-shivering Thermogenesis: What animals is this vital for?

Fetal animals utilize brown fat

Heat production/promotion: Non-shivering Thermogenesis: What makes brown fat excel at thermogenesis?

Brown fat can be uncoupled via thermogenin, not linked to ATP synthesis, leading to more heat production

Heat loss: Behavioral changes include

Open posture, expose areas with low insulation, minimal touching, cool location

Heat loss: Altering conductance means

Cutaneous vasodilation → increase skin temperature

Heat loss: What potential problem exists if cooling via altering conductance?

Effectiveness of conduction and convection decrease as ambient temperature increases

Heat loss: Evaporative heat loss occurs via

Sweating, panting, spreading saliva on fur

Heat loss: Evaporative heat loss benefits include

Works even at high temperatures

Heat loss: Evaporative heat loss: Disadvantages may include

Not all animals have sweat glands, less effective if fur is present

What species have apocrine sweat glands?

Horses and cattle

What do apocrine sweat glands look like physiologically?

Develop from hair follicles, produce membrane bound secretions and watery sweat

What helps to regulate apocrine sweat glands?

Adrenaline

What species have eccrine sweat glands?

Humans

What helps to regulate eccrine sweat glands?

Cholinergic sympathetic innervation

What stimulates sweat glands?

Increased central and skin temperature

What species rely on sweating for heat loss?

Cattle, antelopes, camels, horses, humans

Heat loss: Panting is defined as

Movement of air in ‘non respiratory zone’ of airways, or ‘forced convection’

Heat loss: Panting side effects include

Loss of CO2, increased work (by up to 10x), loss of salt, cools blood going to the brain (via nasal passages)

What species rely on panting?

Dogs, sheep, goats, gazelles

What species rely on pant/gular flutter?

Birds

Heat loss: Spreading saliva on fur is a very

ineffective way to increase heat loss

What species utilize spreading saliva on fur as a method to increase heat loss?

Marsupials and rodents

What adjustments can animals make in response to chronic cold temperatures?

Increase thermal gradient, Increase metabolic rate, Decrease core temperature

What is meant by increased thermal gradient in response to chronic cold temperatures?

Increased level of surface insulation

Examples of increased level of surface insulation

winter fur/coats

Behavioral

Small mammals cannot have enough fur → microclimate eg. burrows

What is meant by decreased core temperature in response to chronic cold temperatures?

Hibernation

What physiologically occurs during hibernation?

Decreased body temperature, heart rate, respiration rate, and metabolic rate. Fat used as energy source. If temp get too low, animals MUST wake up and move (energy dependant)

What adjustments can animals make in response to chronic hot temperatures?

Decrease thermal gradient, Increase in core temperature, Decrease metabolic rate

How can an animal decrease its metabolic rate?

Estivation

Define estivation

Decreased metabolic rate and water loss, especially during hot or dry periods

What animals estivate?

Snails, crabs, tortoises, crocodiles, lungfish, Fat-tailed Dwarf Lemur

What are exmaples of thermoregulatory disorders?

Pyrexia, Heat stress (including hyperthermia, heat stroke, heat exhaustion, heat fatigue), Cold stress

How could you describe pyrexia/fever using the metaphor of the body being a thermostat

Alteration of the set point

What is the normal function of pyrexia/fever physiologically?

Promote interferon activity, Increase metabolic rate and accelerate tissue repair, Decrease bacterial and viral replication

Pyrexia/Fever initiation and resolution mechanism: Step 1

Neutrophils and macrophages secrete pyrogen IL1→Stimulates the anterior hypothalamus to secrete PGE

Pyrexia/Fever initiation and resolution mechanism: Step 2

PGE raises the set point for body temperature

Pyrexia/Fever initiation and resolution mechanism: Step 3

Individual shivers and peripheral vasoconstriction

Pyrexia/Fever initiation and resolution mechanism: Step 4

Body temp oscillates around the new set point. Liver and spleen hoard Zn and Fe depriving bacteria of the minerals needed to reproduce

Pyrexia/Fever initiation and resolution mechanism: Step 5

When infection is gone, pyrogen secretion decreases→ set point is reset

Pyrexia/Fever initiation and resolution mechanism: Step 6

Heat loss mechanisms activated

What causes heat cramps?

Excessive electrolyte loss (often after exercise and heavy sweating)

What is the symptomatic result of heat cramps?

Muscle spasms

What causes heat exhaustion?

Severe water loss/dehydration, electrolyte loss

What is the symptomatic result of heat exhaustion?

Hypotension, dizziness, fainting, vomiting

What causes heat stroke?

Gain heat by radiation and conduction but can’t lose by evaporation

What is the symptomatic result of heat stroke?

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

Stages of Cold stress/Hypothermia: Stage 1

Body temp drops to 35-37C, shivering, vasoconstriction. Breathing is quick and shallow. Nausea, muscles become less responsive.

Stages of Cold stress/Hypothermia: Stage 2

Body temperature drops to 33-35C. Shivering more violent. Muscles mis coordinate. Mild confusion. Surface blood vessels contract further, extremities become blue.

Stages of Cold stress/Hypothermia: Stage 3

Body temperature drops to <32C. Shivering stops, Cognitive impairment and physical inability present. Metabolic processes shut down. <30C, exposed skin is blue and puffy, muscle coordination is very poor, incoherent/irrational behavior including terminal burrowing.

Pulse and respiration rates drop, heart rate increases. Major organs fail, clinical death occurs

What is terminal burrowing?

Stage of hypothermia where someone will take off all their clothes and try to dig