Thermoregulation Flashcards
Major Functions in the Skin
- Nutrition of the skin tissues
- Heat loss
Dermal circulation
Cutaneous plexus
* A network of capillaries
* Located between subcutaneous layer and dermis
* Supplies adipocytes, hair follicles and sweat glands
Papillary plexus
* Formed from branches of the cutaneous plexus
* Gives rise to papillary loops
Skin Vascularity
Arteriovenous Shunts
* Shunts blood between arteries and veins to conserve heat
* Have muscular walls innervated by the sympathetic division of the ANS
* Activation of sympathetic division results in blood flow to plexus and heat loss.
* Blood flow to skin & metabolism is influenced by to of surroundings.
Nervous Control
* Vasodilator control
* Vasoconstrictor control
Sensory Nerves (Innervation and Skin)
- From pain, temp, and touch receptors
- Free nerve endings (dendrites of sensory nerve)
- Or sensory receptors:
- Merkel Discs
- Messiners Corpuscles–
sensitive to light touch (in
dermal papillae) - Pacinian corpuscles - sensitive
to deep pressure and vibration
(in reticular layer)
Motor Nerves (Innervation and Skin)
- To sweat glands
- To erector pili muscle
- To blood vessels
Sensory Nerves in Thermoregulation
- Detect increase or decrease in skin temperature
- Carry the information to the hypothalamus of the brain for
thermoregulation
Motor Nerves in Thermoregulation
- Activation of Sweat glands and pili muscle
- Contraction of skeletal muscle- shivering
- Shunting of blood in skin and limbs
Metabolic rate
- the sum of all the catabolic and anabolic processes
- Rate at which heat is produced in the body
- The heat is produced as food is oxidised
- Energy from food = ATP + Heat
- Significant amount of energy used to synthesise ATP, but much of it is lost to environment as heat
- The heat produced is carried in H2O (plasma) throughout the body
- For body temperature to remain constant, heat must be lost to the
environment at same rate it is generated
Factors that Affect Metabolic Rate
- Exercise may increase BMR x15% (20% in well trained athlete)
- Nervous system, in stress ANS releases noradrenaline which increases BMR
- Hormones; adrenaline, noradrenaline, hGH, testosterone and thyroid hormones increase BMR
- Body T°; a 1°C rise in body To increases BMR X 10% (fever)
- Digestion of food increases MR X 10-20% (diet induced thermogenesis)
- Age; MR is higher in children
- Other factors:
- Sex (female MR is lower except in pregnancy)
- Climate (↓ if hot)
- malnutrition (↓ if malnourished)
Homeostasis of Body T° Regulation
- Core T° is the body T° below the skin
- Shell T° is the surface T°
- Usually the core T° is higher
- If the heat loosing mechanisms give off more heat than is produced the core T° ↓
- If heat producing mechanisms generate more heat than is lost the core T° ↑
Importance of temperature
- Too high core T° kills by denaturing protein (it also causes convulsions)
- Too low core T° causes arrhythmias (it also causes disorientation)
- Core Temperature kept between 36.7°C and 37.2°C by homeostatic control
Evaporation
- Absorbs energy and cools surface where evaporation occurs
- Insensible water loss– each hour 20-25ml of water crosses epithelia and evaporates from alveolar surfaces and surface of skin (accounts for 20% of boy’s average indoor heat loss).
- Sensible heat loss– evaporation of sweat
Radiation
- Objects warmer than environment lose heat as radiation
- Heat from sun is radiant heat
- More than 50% of heat you lose indoors is lost through radiation
Conduction
- Direct transfer of heat through physical contact e.g. sitting on a cold chair
- Not effective way of gaining or
losing heat – impact depends on the temperature of the object and the amount of skin surface area
involved
Convection
- Heat loss to cooler air that moves
across the surface of your body. As
body loses heat to air next to skin,
the air warms and rises away from
skin surface, and is replaced by
cooler air which is then warmed - Accounts for roughly 15% of
body’s heat loss indoors
Hypothalamic Thermostat
Tries to keep heat production and
heat loss in balance – i.e. maintain
Temp range
Sensory thermoreceptors
(in skin, mucous membrane and in
hypothalamus) bring information to
hypothalamus of the brain
Hypothalamus
has a special region called the Preoptic area output that has:
* heat losing centre
* heat production centre.
Preoptic area - regulates body temperature
Heat Loss Centre (Heat loss)
stimulated when temperature at pre-optic area rises above its set
point
Mechanisms for heat loss
- Vasodilation
- Perspiration
- Decreased Metabolic Rate and Decreased Shivering
- Increased Respiration
- Behavioural change
Vasodilation
inhibition of the ANS vasomotor output. This increases blood flow to the skin, warms the skin and increases heat transfer
Perspiration
activation of the sweat glands and evaporation of H2O cools the skin
Decreased Metabolic Rate and Decreased Shivering
mediated through hypothalamic
reflexes
Increased Respiration
depth of breathing increased and breathing through the mouth will increase evaporation
Behavioural change
seek shade or remove clothing
Function of the Heat Promoting Centre (Heat gain)
to prevent hypothermia, or below-normal body temperature
Mechanisms of heat gain
- Vasoconstriction
- Skeletal muscle
- Sympathetic release
- Thyroxine
- Behavioural responses
Vasoconstriction
increase vasomotor output (decrease blood flow to skin surface)
Skeletal muscle
by shivering increases muscle tone and increases heat production.
Sympathetic release
from adrenal gland (adrenaline and noradrenaline stimulate cellular metabolism, i.e. chemical thermogenesis)
Thyroxine
increases cellular metabolism by increasing catabolic reaction
Behavioural responses
put on more clothing or move to a heated area
Shivering Thermogenesis
- muscle metabolism in contraction/relaxation
- Gradual increase in muscle tone increases energy consumption of skeletal muscle tissue -> more energy used the more heat produced
- Heat produced warms deep vessels where blood has been shunted
- Shivering can increase rate of heat generation as much as 400%
Non Shivering Thermogenesis
- increased metabolism (by liver) and BAT adipose breakdown
- Involves release of hormones that increase tissue metabolic activity
- Adrenal medulla – releases
epinephrine, increasing
glycogenolysis - Thyroid gland – releases
thyroxine, increasing nutrient
catabolism
- Adrenal medulla – releases
Pyrexia
-Fever
* Usually bacterial or virus stimulated
* Other causes are heart attack, tumour, surgery or trauma
Mechanisms of Pyrexia
Phagocyte ingests bacteria
↓
Interleukin-1 secreted and circulated in the blood
↓
Prostaglandin production in the hypothalamus
↓
Resetting of T° thermostat
Stages of fever
- Core T° thermostat ↑ 39.4°C,→ activation of the heat production mechanism resulting in vasoconstriction, increased metabolic rate and shivering→ Chill
- When skin T° is increased to 39°C the chill disappears
- When stress is removed the thermostat is reset→ the heat losing mechanism goes into operation. Resulting in Vasodilation and sweating. Crisis → T° is falling
Why is T* increased during a fever?
- The high T° is beneficial:
- Inhibits growth of bacteria
- WBC delivered to site of infection faster
- Increases Ab production
- Increases rate of repair
Complications of fever
- Dehydration
- Acidosis – Blood pH
- Permanent brain damage
- Death of To rises to 44.5 – 45.5°C
Heat Cramp
- Result of profuse sweating that removes NaCl from the body
- Salt loss causes painful muscle contraction called heat cramp
- Cramp in muscles used in working only felt when relaxed
- Salted liquid relieves cramp.
Heat Exhaustion
- Difficulty in maintaining blood volume
- Normal or slightly low T°.
- Cool clammy skin due to loss of fluid and electrolytes as sweat.
- Muscle cramps, dizziness, vomiting, fainting, low blood pressure and low blood volume.
- Body cannot compensate for low blood pressure.
- Relieved by rest and salt tablets.
Heat Stroke (sunstroke)
- Occurs when T° and relative humidity are high.
- Body heat cannot be lost by evaporation and radiation
- Decreased blood flow to skin and decreased sweating→ Core T° rises.
- May destroy brain cells.
- Relieved by immersion of the body in cool water (increased conduction) and rehydration.
