Thermoregulation Flashcards
Heat is produced in large part by ____.
oxidative metabolism
Calorie
energy capable of raising 1 gram of water from 0 to 1 degree celsius
Glucose provides ____ of energy.
686 Kcal/mole (as heat)
or 420 Kcal/mole as heat and 266 Kcal/mole as ATP high energy bonds
At rest, ____ are responsible for the highest percentage heat production.
splanchnic organs (then the brain, followed by muscle)
Physical factors that govern temperature control
Evaporation (Insensible and sensible), Conduction and Convection, and Radiation
Transition of water from liquid to gas phase requires ___ at room temperature.
584 cal/g
Insensible evaporation
Loss due to saturation of expired air and diffusive loss from epidermis via the stratum conium. (1 Liter/day, requires 584 Kcal)
25% of caloric intake
Sensible evaporation
Sweating, occurs due to cholinergic sympathetic stimulation of sweat glands, and it can exceed 2 liters/hour. Sweat is a dilute salt solution containing mainly sodium chloride, with some potassium chloride, urea, organic acid, and other trace electrolytes. Excessive sweating can lead to both serious salt and water loss. Effectiveness in cooling will depend on environmental conditions. For instance, a dry breeze will facilitate evaporation and heat transfer to the environment. If aire temperature > skin temperature and air is saturated, then sweat cannot evaporate, so sweat simply drips off without removing heat
Conduction
Heat conductive = AcKc(Tskin-Tambient)
Where Ac = surface area available for conductive exchange
Kc = Conductive characteristics of the medium (still air is 6 Kcal/hr/m2/1degreeC, water is 150)
Tskin = skin surface temperature
Tambient = Temperature of conducting medium
Convection
Bulk movement of conductive fluid as a function of temperature differences within the fluid (fans)
Radiation
Same equation as conduction, but with radiant exchange instead of conductive exchange
Heat exchange is dominated by ___.
Conduction between capillary blood and the normally cooler skin surface
Compensatory changes in hypothermia include:
vasoconstriction to reduce blood flow near skin
Core thermoreceptor locations
Pre-optic area of hypothalamus and in the spinal cord (which respond to direct heating or cooling of these areas)
Vasomotor regulation
Changes in skin blood flow regulate temperature in normal resting state or mild levels of thermal stress or exercise
Metabolic regulation
Occurs when maximal vasoconstriction is ineffective in preventing some heat loss. Responses include voluntary exercise and shivering. Shivering occurs when Tc drops below a certain threshold. It is of limited effectiveness in elevating temperature because it increases conductive and convective loss by increasing blood flow from core to muscles and by increasing movement of the body (but produces 3-fold increase in heat production)
Sudomotor and vasomotor regulation combined
progressive increases in sweating and active vasodilation in response to heat accumulation
Fever
Hypothalamic temperature set-point is increased to a new higher temperature, and body temperature is elevated above normal.
Pyrogens
chemicals that cause a febrile response
exogenous pyrogens
heat-stable, high molecular weight polysaccharides produced from gram-negative bacteria
Endogenous pyrogens
Heat-labile proteins released by monocytes, macrophages and Kupffer cells in response to bacterial pyrogens.
PGE2 (prostaglandin E2) is activated by IL-1beta
Tumor necrosis factor
Causes of hyperthermia
Delirium tremens, drug abuse, exertional tetanus, heatstroke (exertional), lethal catatonia, malignant hyperthermia of anesthesia, neuroleptic malignant syndrome, pheochromocytoma, salicylate intoxication, status epilepticus, thyrotoxicosis, anticholinergic drugs, autonomic dysfunction, dehydration, heatstroke (classic), neuroleptic malignant syndrome, occlusive dressings, cerebrovascular accidents, encephalitis, idiopathic hypothalamic dysfunction, neuroleptic malignant syndrome, sarcoidosis and granulomatous infections, trauma, and tumors
Blood flow to the skin can increase to ____, or 60% of total cardiac output.
8 liters
Classical Heatstroke
Due to excessive heat and humidity, causing increased body temperature with poor heat dissipation due to ineffectiveness of evaporative cooling. Dehydration accelerates hyperthermia. (May need electrolytes if replacing water, lowered blood pressure)
Exertional heatstroke
voluntary muscle activity coupled with environmental heat stress. Drop in blood pressure rapid.
How to manage heatstroke
physical cooling (sponging with cool water or fanning)
Hypothermia
Core cooling to below approximately 92 degrees F
Reduces oxygen requirements and blood pressure/bleeding
Below 90 degrees F, there is a failure of ___ and there is general ____.
hypothalamic-based compensatory mechanisms,
CNS depression
With ventilatory and cardiac assists, core temperature can be maintained at ____ for many hours.
70-75 degrees F.
