Thermoregulation (wk8) Flashcards
Body temperature regulation
-Endotherms, heat + metabolism and maintenance of body temperature
-Humans are endotherms. They generate their own internal body heat and do not rely on the energy of sunlight to warm the body. Heat is generated during metabolism.
-Heat is generated during metabolism. At least 75-80% of energy released in physical activity is ‘wasted’ as heat, while only 20-25%yields external work
-Humans maintain their body temperature within very narrow limits despite wide fluctuations in ambient temperature and are, therefore, known as homeotherms
-Thermoregulation = Maintaining a constant core body temperature.
Draw the system for thermoregulation
Body temperature regulation
-body core v body surface temp, circadian fluctuation and women
-Body core v Body surface temperature -> Humans maintain their body temperature within very narrow limits. In contrast, body surface temperature can vary considerably depending on the environmental conditions and eating and exercise.
-There is a characteristics circadian fluctuation in body core temperature of about 1 degrees. Body temperature is lowest during the night and highest during the day.
-In women, body core temperature is higher during the second half of the menstrual cycle due to the effects of the hormone progesterone.
Mechanisms of heat exchange
-Evaporation of sweat -> Sweating. Transformation of a liquid to gas requires energy that is derived from the skin, thereby cooling it.
-Convection from air movement -> Movement of the layer surrounding the skin (either air or water). Convection is a form of conduction.
-Radiation from the sun -> Emission of heat in form of electromagnetic waves. The body can absorb or emit heat depending on the ambient conditions.
-Conduction to clothing -> Direct conduct. The body can gain or lose heat depending on the temperature of the adjacent surface.
Body temperature control
-Thermoregulation -> Body temperature is maintained by balancing heat gain and heat loss.
* Heat conservation/ production -> Shivering thermogenesis, voluntary muscular activity, non-shivering thermogenesis
* Heat loss -> Blood reaching the skin and sweating
-Heat conserving mechanisms -> increase in shivering and a decrease in skin blood flow to the periphery (of the body) (behavioural)
-Heat loss mechanisms -> Increase in sweating and increase in skin blood flow to the periphery (behavioural).
Physiological responses to heat
-Vasodilation and sweating
-Vasodilation -> Vasodilation of blood vessels in the skin near the extremities happens due to a reduction of sympathetic stimulation. In skin away from the extremities vasodilation also happens due to the release of acetylcholine from specialised sympathetic neurons.
-Sweating -> Sweat production rates affect the composition of sweat with a larger amount of Na+ and Cl- being lost at high sweat rates. Sweat glands are innervated by cholinergic nerve fibres via stimulation of muscarinic receptors. Circulating catecholamines also stimulate sweat production. Sweat glands can either by eccrine or apocrine.
Physiological responses to cold
-Vasoconstriction and shivering
-Vasoconstriction -> The blood vessels in the skin are innervated by sympathetic adrenergic fibres. Vasoconstriction happens as a result of norepinephrine acting on adrenergic receptors. Blood flow can be reduced to 0 at extreme temperatures. However, during long exposure to cold, the skin circulation will often show intermittent periods of vasodilation. This is called the ‘Hunting reaction’ and it is considered to be a protective mechanism against ischaemic injury (frostbite). The mechanism is unclear but it may be due to a period insensitivity to norepinephrine.
-Shivering -> Muscular contraction releases heat because ATP is broken down. In response to cold, voluntary movements increase and this prevents core temperature to drop. Shivering is a specialised form of muscular contraction in which the muscles perform no external work and virtually all the energy of contraction is converted into heat. Shivering is a result of stimulation by somatic motor neurons. Non-shivering thermogenesis is from brown fat metabolism but this is not important in adult humans.
Problems with being too cold - hypothermia
-Problem with being too cold -> Leads to hypothermia.
* Roughly 37 degrees – resting zone
* 36 degrees – Moderate shivering, reduced strength, power and dexterity
* 35 degrees – Maximal shivering, decreased physical performance
* Under 33 degrees – Cardiac arrythmias (abnormal heart rhythm), highly variable and inter-individual responses
* Under 29 degrees – Severe cardiac arrhythmias and unconsciousness common
+Draw diagram of effects
Problems with being too hot - Hyperthermia
-Problem with being too hot -> Leads to hyperthermia – when core body temperature becomes dangerously high (38-40.5 degrees).
* Mild hyperthermia – heat exhaustion
* Severe hyperthermia – heatstroke (heatstroke is defined as a failure of thermoregulation with a body temperature of more than 40.6 degrees)
-Signs of hyperthermia -> dizziness, disorientation, skin can appear dry, heart rate is elevated, collapse
+ draw diagram of the problems of hyperthermia
Exercise and thermoregulation:
-During exercise, body temperature is maintained higher than at rest. This is an example of set-point resetting and it has functional implications for muscle contraction.
-Exercise and heat stress -> Thermoregulation reduces exercise capacity. Thermoregulation capacity is finite and further limits by other homeostatic demands
Describe acclimatization:
-Acclimating to heat means that an individual can regulate body temperature more efficiently. This is crucial for exercise performance in a hot environment e.g. Tokyo 2020.
-Acclimatization. Acclimation involves the repeated exposure to heat either by habitation in a hot climate or the use of environmental chambers. Acclimatization is considered to begin after 5-10 days of prolonged exercise in the heat. This happens via adjustments in blood flow and sweating.
-Am acclimated individual will:
* Sweat more and sooner
* Increased blood flow to skin
* Have more dilute sweat (conserve electrolytes)
* Increased blood volume and stroke volume
* Have a lower body core temperature, heart rate, glycogen use, and therefore fatigue
Draw/ describe the physiological control of thermoregulation:
