Temperature Regulation Flashcards
What is the average core body temperature for human adults?
36.5 - 37.5 degrees Celsius
What happens if body temperature falls outside the normal range?
Many physiological processes are impaired, predominately because of reduced enzyme activity
What temperature does hypothermia occur at?
= or < 35 degrees Celsius
What temperature is classified as hyperthermia aka pyrexia?
> 38 degrees Celsius
What are the two types of thermoreceptors?
Cold and Warm Receptors
Where are cold receptors mainly found?
Peripherally in skin
Where are warm receptors mainly found?
Centrally in hypothalamus
A few peripherally in skin
What are cold receptors conducted by?
myelinated A fibres
What receptors identify extreme temperatures?
Nociceptors - extreme temperatures are perceived as pain
What controls thermoregulation?
Anterior or preoptic area of hypothalamus
Where does the anterior area of the hypothalamus have extensive connections with to effect responses?
Higher (cortical) centres
Vasomotor centre - blood vessels
Motor output - skeletal muscle
What are the behavioural response mechanisms to temperature change?
Voluntary actions to:
- Increase muscle activity
- General movement
- Change the body’s immediate environment
- Seeking shelter/shade
- Adjusting clothing/ vocer
- Find heat (fire etc.)
What are the physiological response mechanisms to changes in temperature?
Involuntary efforts that influence the:
- rate of heat production
- rate of heat loss
What are the two physiological effector systems to change temperature?
Neurological
Hormonal
Why do neonates have a high risk for hypothermia?
High body surface area to weight ratio
Unable to make behavioural changes
What is brown fat?
Brown adipose tissue is a specialised heat source used by neonates
Where is brown fat located?
Between scapulae
How does brown fat produce heat?
Oxidative metabolism in mitochondria is uncoupled to phosphorylation and thus produces heat rather than ATP
How is brown fat activated?
Activated by thyroid hormones sensitising adipocytes to action of adrenaline
Why are those who have suffered a near drowning experience at a risk for hypothermia?
Huge increase in heat loss by conduction
Why are those who have suffered a drug overdose at a risk for hypothermia?
Reduced metabolism and heat production
Unable to make behavioural changes
Why are those who have suffered a major trauma at risk for hypothermia?
Exposure increasing heat loss (including evaporative)
Replacement of blood loss with cold fluid
Note: Hypothermia impairs blood clotting
Why might doctors want to induce hypothermia?
To reduce metabolic requirements:
- during surgery - brain, heart
- following injury - brain
How could doctors induce hypothermia?
Surface cooling (ice bath/packs)
Cardio-pulmonary bypass
Summarise the response cold receptors in the skin becoming activated
Prevent heat loss:
- Vasoconstriction
- Person puts on warm clothing
Increase heat production:
- Thyroxine (T3) released
- Catecholamine release
- Activity - shivering
Summarise the response to warm receptors in the hypothalamus becoming activated
Increase heat loss:
Sweating
Vasodilation
Exposure
Decrease heat production:
- Decreased activity
What are the two physiological process that can cause variation in core body temperature?
Circadian Rhythm
Menstrual Cycle
What occurs to the body when its temperature reaches 35 degrees Celsius?
Severe shivering, peripheral circulation shut down
What occurs to the body when its temperature reaches 33 degrees celsius?
Confusion, drowsiness, coagulation impairment
What occurs to the body when its temperature reaches 31 degrees celsius?
Unconciousness, major risk of arrythmia (VF)
What occurs to the body when its temperature reaches 28 degrees Celsius?
Respiratory Muscle Failure
What happens when a person reaches 26 degrees celsius or below?
Death (normally)
What occurs to the body when its temperature reaches 39 degrees celsius?
Severe sweating, vasodilation, breathlessness
What occurs to the body when its temperature reaches 40 degrees celsius?
Vomiting, dehydration, weakness, dizziness
What occurs to the body when its temperature reaches 41 degrees celsius?
Fainting, confusion, drowsiness
What occurs to the body when its temperature reaches 42 degrees celsius?
Brain Protein Denaturing
What happens when a person reaches 43 degrees celsius or above?
Death
What is heat stroke?
Rapid, extreme rise in body temperature (40+ degrees celsius)
What is heat stroke usually related to?
Exercise
What exacerbates heat stroke?
Drugs
What happens in the body when heat stroke occurs?
Heat stroke outstrips ability to compensate
Hyperthermia impairs hypothalamus leading to failure of any response
Mortality rate is high even with rapid intervention
What is a fever?
Elevated temperature from infection or inflammation
What are pyrogens?
A pyrogen is a substance that induces fever.
How do pyrogens induce a fever?
Endogenous pyrogens, such as cytokines (IL-1, IL-6 and TNF-alpha) release from macrophages, inhibit heat sensing neurons in the hypothalamus, and excite cold sensing neurons, which causes the hypothalamus to raise body temperature above normal.
- Pyrogens act via PGE2
By how many degrees to pyrogens increase temperature?
1 - 2 degrees celsius
By what mechanisms to pyrogens increase temperature?
With normal mechanisms (e.g. shivering)
How is heat produced in the body?
Virtually all body heat comes from cellular metabolism
How much oxygen is needed to burn 5 calories?
1 mL O2
What does body temperature represent the balance of?
Body temperature represents the balance between heat production (and uptake) and heat loss
What are the two mechanism the body uses when body temperature is rising?
Decreases heat production (and uptake)
Increases heat loss
What are the two mechanisms the body uses when body temperature is falling?
Increases heat production
Decreases heat loss
What is Basal Metabolism equivalent to?
1 MET (Metabolic Equivalent of Task)
What are the two main route of heat loss?
Skin (90%)
Lungs (10%)
What system regulates skin blood flow?
The sympathetic nervous system
What are the four modes of heat loss?
Conduction
Convection
Radiation
Evaporation
What is conduction?
Heat lost via direct contact with adjacent material
What is convection?
Warming of the adjacent air which rises creating a heat-losing current
What is radiation?
Transfer through infra-red rays to a distant object at a lower temperature (radiated heat is gained by the body by objects at higher temperature e.g. the sun)
What is evaporation?
Latent heat of vaporization is lost as sweat/respiratory humidity evaporates
What are Eccrine glands (merocrine glands)?
Eccrine glands (sometimes called merocrine glands) are the major sweat glands of the human body, found in virtually all skin, with the highest density in palm and soles, then on the head, but much less on the trunk and the extremities.
Which system controls sweat glands?
The sympathetic nervous system
What neurotransmitters are used for sweat glands?
ACh
Eccrine sweat glands receive sympathetic innervation via which fibres?
Cholinergic fibers
When is sweating ineffective?
In very high humidity
At what environmental temperature is sweating the only way of losing heat?
> 37 degrees celsius
What is the composition of sweat?
The composition varies:
- [Na+] 20 - 100 mmol/L
- Falls as rate of sweat production increases
- Adapts with chronic high temp exposure to reduce Na+ loss
Within the thermal neutral zone (TNZ) the basal rate of heat production is equal to what?
The rate of heat loss to the environment.
