Lecture 3 thermoregulation Flashcards
Define homeostasis.
The dynamic maintenance of physiological variables within a predictable range
What does dynamic mean, within the context of homeostasis?
The variable fluctuates within a normal range, and the average value is predictable over longer periods of time
What is the short term purpose of homeostasis?
Immediate survival
What is the medium to long term purpose of homeostasis?
Health and well-being
Reproductive capability
What does homeostatic control mechanisms ensure?
Changes in physiological variables are normalised
Negative feedback is also known as?
Reflex arcs
Describe negative feedback.
Change in regulated variable compared against set point, response moves variable back to set point (normalisation)
When may set points need to be changed or over-ridden?
When physiological circumstances change (fever, exercise)
What are the three types of negative feedback?
Neuronal
Endocrine
Local (chemical/physical)
What is feed-forward?
Anticipation of a change brings about response to change before detected by negative feedback sensors
What is positive feedback?
Change in variable triggers response that causes further change to variable (amplification)
How much ATP does the heart consume per day?
6KG
20-30x it’s own weight
How many times does the heart beat per day?
100,000
How much blood does the heart beat per day?
10 tons
What physiological variables are required for immediate survival?
Oxygen and glucose supply to all cells (more immediate importance to brain)
What physiological variables are responsible for optimal functioning of organs and tissues?
Temperature, metabolic rate
What physiological variables are responsible for fulfilling dietary requirements?
Appetite
Gastro-intestinal secretions, motility and absorption
What variables are responsible for gene expression?
Environmental factors and hormone levels
What physiological variables are responsible for reproductive capability?
Hormone level
Control of body temperature is an example of what type of negative feedback reflex arc?
Neuronal negative feedback reflex arc
What name is given to organisms whose body temperature varies with environmental temperature?
Poikilotherms
What names is given to organisms whose core body temperature is maintained within narrow constant range despite variations in environmental temperature?
Homeotherms
What is the heat source for homeotherms?
Internal: by-product of metabolism
What is basal metabolic rate?
The minimum level of energy required to live, generates heat
Why control temperature?
Metabolic rate is very temperature sensitive
At what high temperature does death occur?
42.5 degrees C
What happens to the body at lower temperatures?
Loss of consciousness, decrease in HR/arrhythmia’s, death (27.5 degrees)
What is the name given to maintenance of the core temperature?
Thermoregulation
What is core body temperature?
Temperature of the body around the main organs
Where is the core body temp 37.8 degrees C?
CNS, abdominal and thoracic cavities
Where is the shell temp found?
Skin and subcutaneous fat
What temperatures does the shell temp fluctuate between?
20 and 40 degrees C
How is heat exchanged between the core and the shell?
Via blood
Why is thermoregulation important for cellular function?
Ensures optimal temp for cellular metabolism
When does body temperature change?
Diurnal variation Menstrual cycle Exercise Age Ambient temperature
What happnes to body temperature after ovulation?
Increases
Advantages and disadvantages of oral measurement of core temperature.
- Simple, non-invasive
- Can underestimate Tc
Advantages and disadvantages of Aural measurment of core temp?
- Fast
- Can be uncomfortable, can underestimate Tc
Advantages and disadvantages of rectal measurement of core temperature.
- Continuous, close to Tc
- Slow
Advantages and disadvantages of oesophageal measurement of Tc
- Continuous, close to Tc
- Slow, affected by food and drink
Advantages and disadvantages of GI tract measurement of core temp.
- Temperature pill radio transmitter continuous
- Varies along track, has to be retrieved
What are the 4 mechanisms of heat transfer?
Radiation, convection, conduction, evaporation
What % of heat transfer does evaporation account for?
22%
What % of heat transfer does radiation account for?
60%
What % of heat transfer does conduction to the air account for?
15%
Give an example of convection.
Air currents
What mechanism of heat transfer accounts for 3% of heat transfer?
Conduction to objects
What is radiation as a mechanism of heat transfer?
Emission of heat energy from surface of warm body via electromagnetic waves
What is convection?
A gravitationally-induced heat transport, driven by the expansion of air or fluid containing heat
What are convection currents?
Warm air rises and is replaced by cool air
Give two examples of convection as a mechanism of heat transfer.
Air currents
Water currents
Why is little heat lost by conduction under normal conditions?
Still air (in clothing) is a poor conductor
What is the thermal conductivity of water compared to air?
25 x
Define conduction
Transfer of heat between objects in direct contact
In what way does water evaporate from the skin and lungs?
Insensibly
What does it mean that water evaporates insensibly from the skin and lungs?
The heat loss cannot be controlled for purposes of temperature regulation
(passive)
How much energy is there in the evaporation of 1L of water?
2,400KJ
What is the daily insensible water loss?
800ml, around 20 Watt
How does sweat cool things down
Requires thermal energy, absorbed from the skin which cools the body
What is sensible loss of heat/
Regulated by sympathetic nervous control (sweating from eccrine sweat glands)
How much water is loss through sensible loss on a normal day?
100ml
How much water is lost through sensible loss in a hot environment in a day?
1.5L
How much water is lost through sensible loss during vigorous exercise per day?
4L
What are the possible heat inputs?
Oxidation of fuel
Environment
What is the efficiency of heat input from oxidation of fuel?
50%
What centers are involved in the negative feedback reflex arcs in the autonomic nervous system?
Neuronal integrating centers
Where are neuronal integrating centers found?
Midbrain and brain-stem
What parts of the brain make up the neuronal intergrating centers within the midbrain and brain-stem?
Hypothalamus, pons, medulla
Name 4 things under the control of the autonomic nervous system.
Temperature control
Osmolarity control
Blood pressure/flow control
Blood gas/ventilation control
What is the hypothalamus?
Thermoregulatory integrating centre of central and peripheral sensory information
What region of the hypothalamus is activated by the cold?
Posterior region
What region of the hypothalamus is activated by the heat/
Anterior region
What size changes does the hypothalamus respond to?
0.01 degrees C
What is the centre of thermoregulation?
No one area, a heirachy extends through hypothalamus, brainstem and spinal cord
What parts of the nervous system provide higher control of thermoregulation?
Medial and lateral parts of preoptic nucleus, anterior hypothalamus and nearby regions of septum (preoptic region)
What parts of the nervous system crudely sense changes in Tc and initiate certain responses?
Lower brain stem and spinal structures
What are the three types of effectors involved in thermoregulation?
Peripheral blood vessels, sweat glands (eccrine), skeletal muscles
What are the two types of receptors involved in thermoregulation?
Peripheral thermoreceptors in skin
Central thermoreceptors in hypothalamus, abdominal organs and elsewhere
How do peripheral blood vessels act as effectors during thermoregulation?
Vasoconstriction/dilation alters blood flow to the skin
How does vasoconstriction/dilations change body temperature?
Controls conductive and radiative heat loss
What is another mechanism by which blood vessles can control blood temperature?
Countercurrent exchange mechanism
How are the blood vessles involved in the countercurrent exchange mechanism arranged?
Arteries transport warm blood, deep veins positioned alongside arteries
What happens in the cold with respect to the countercurrent exchange mechanism
Blood returns through deep veins, heat transfers from arteries to veins which conserves heat
What happens when heat wants to be lost with respect to the countercurrent exchange system?
Deep veins vasoconstrict, more blood flows through superficial veins
How do sweat glands act as effectors during thermoregulation?
Control evaporative heat loss
How do skeletal muscles bring about thermoregulation?
Shivering- rapid involuntary rhythmic contractions (10-20/sec) liberated energy converted to heat
What increase in heat can shivering cause?
2-5 fold increase in heat within seconds/minutes
What can inhibit shivering?
Heat signals from preoptic region
How do babies produce heat?
Non-shivering thermogenesis via brown adipose tissue
How does brown fat generate heat
Oxidative phosphorylation in the mitochondria is uncoupled
How is temperature gradient between skin and environment altered?
Radius of blood vessles determine blood flow
By what amount does skin blood flow vary?
400-2500ml/min
An increase in sympathetic activity leads to what vasomotor response?
Vasoconstriction
What part of the brain is responsible for behavioural heat production/conservation?
Hypothalamus and limbic system
What part of the nervous sytem is responsible for piloerection?
Sympathetic NS
How does piloerection decrease thermal gradient?
Muscles at base of hair contract, creating dead space
Define fever.
Elevation of body temperature as a result of infection/inflammation (trigger)
What is the response to a fever trigger?
White blood cells proliferate and secrete chemical substances
What ‘fever-inducers’ do white blood cells secrete in response to triggers?
Endogenous pyrogens
What do endogenous pyrogens do?
Stimulate release of prostoglandins
What do prostoglandins do?
Increase the hypothalamic set point
