Homeostatic Mechanisms: Thermoregulation Flashcards
What does skin temperature rise and fall with?
The environmental temperature.
Note: rectal thermometers are more accurate and read typically .6C warmer (1.0F) warmer than oral
Core temperatures vary with? What effect does the variable have on the temperature (i.e. raises it or lowers it?)
Time of day - lowest between 3am and 6am; highest between 3pm and 6pm
Stage of menstrual cycle - increasing 1C during the post-ovulatory phase (progesterone)
Level of activity - increasing with exercise and emotional states
Age - higher temp in active children and lower in aged adults.
Maintenance of a stable body temperature involves ______ feedback control with a ______ gain
Negative. Very high gain (~25-30).
Ex. The core body temperature of humans changes 1C for every 25-30C change in environmental temperature.
A body immersed in water exchanges most heat by?
Convection
Which heat removal mechanism (i.e. radiation, conduction, convection, evaporation) dissipates the most heat?
Evaporation by sweating normally dissipates nearly all of the heat produced during exercise.
Most body heat is generated how and where?
By cellular metabolism in the deep organs (liver, brain, heart, active skeletal muscle)
The rate of heat loss is determined largely by?
(1.) How rapidly heat is carried from the core to the skin and (2.) Transferred from the skin to the surroundings. (mostly by convection through the blood.
What is passive or unregulated heat transfer?
In the steady state, the rate of heat production by the body core must be matched by the flow of heat from the core to the skin, and from skin to environment.
*Note various homeostatic controls. Systems not directly involved in temperature regulation can also affect heat flow.
The skin is a highly effective controlled heat radiator system, how does it work?
Low env. temps: arterioles and arteriovenous anastomosis (connected artery and vein) that supply blood to the venous plexus of the skin are CONSTRICTED.
Higher env. temps: vasodilation subserves heat conductance through skin.
Controlled by sympathetic nervous system.
Radiation
Transfers heat as EM waves for objects not in contact, rte proportional to temp difference
60% of body heat lost indoors/at rest
Conduction
Transfer of heat between objects in direct contact. Ex. lying on hot sand causes body to gain heat).
Usually minimal in a person wearing clothes/shoes.
Convection
Loss or gain of heat by the movement of air/water over body. Because heat rises, air carries away heat from body. Most heat lost by convection when body is submerged in water.
Evaporation
Large amounts of heat lost this way because a lot of heat (energy) is required to transform water (sweat) from liquid to gas
Air circulation = more effective
High humidity = less effective
What happens when the body acclimates to hot weather?
Takes about 1-6 weeks. Sweat glands change, allow a change from 1L/hr in sweat to up to 2-3L/hr loss in sweat.
Aldosterone secretion from the adrenal cortex also leads to a decrease in loss of NaCl in sweat (conserves body salt).
How are sweat glands innnervated?
By an acetylcholine-secreting sympathetic nerve
Where are free nerve endings that function as thermal sensors located? How do they respond to changes in local temperature?
The skin and in the hypothalamus. They alter their frequency of firing of action potentials. They anticipate changes in core temp.
Does the skin have anatomically distinct receptors for warmth and cold?
Yes. There are 10-fold as many cold receptors in many parts of the skin. The skin receptors work with the distinct deep body receptors which are sensitive primarily to cold in the body core to prevent hypothermia. They project to a control center in the hypothalamus.
How does the body respond to increased temperature?
Skin vasodilation, sweating, decreased hat production to reduce body heat
How does the body respond to decreased temperature?
Skin vasoconstriction and piloerection (goosebumps, important for animals)
Thermogenesis/heat production: shivering, sympathetic/chemical excitation (epi/NE important in infants), thyroid hormone production
When do the cold and the warmth receptors fire with the same frequency?
When the skin temperature is at 37C
There are both heat and cold-sensitive neurons in the hypothalamus. Which are there more of?
Proportionately more heat-sensitive neurons.
“Anticipatory” Feedback
Changes in skin temperature reflect the environment and the resulting reflexes prevent corresponding changes in body core temperature.
What is the most common cause of lethal hypothermia?
Immersion in cold water for an extended period of time
Why is the rise in core temperature an essential feature of homeostatic thermoregulation?
It provides the error signal that sustains the sweating response during exercise
What effect do pyrogens (fever-producing stimuli) have on the set point of the hypothalamic temperature-regulating center?
They increase the set point (cross BBB to act on hypothalamus - sensed by neurons) thereby enhancing heat conservation and heat production.
Note: fever can be beneficial during infection, some immune cells work best at higher temps
How do aspirin and acetaminophen inhibit the raising of the set point?
They inhibit prostaglandin synthesis and reduce fever.
Other effects: decrease muscle and joint pain that often accompany fever
What effect does raising the set point (fever) have on the hypothalamus and the body?
The hypothalamus ‘thinks’ that the body is too cold. The person develops chills leading to heat conservation mechanisms such as; vasoconstriction, piloerection, Epi secretion, and shivering.
Fever “breaks” when pyrogen is removed and temp returns to normal set point (sweating, vasodilation)
How does heat stress and cold stress affect the body? (LO4)
Heat stress and cold stress both compromise the cardiovascular system. Heat stress compromises the fluid and electrolyte balance while cold stress can compromise neuronal function.
Describe heat exhaustion (heat collapse)
Common, due to a failure of cardiovascular homeostasis in a hot environment. Decrease in circulating blood volume caused by skin vasodilation and a sweating-induced decrease in central venous pressure.
Patient typically has dilated pupils and sweats profusely. Rest in a cool environment plus fluid/electrolyte replacement. Core temperature may be normal or mildly elevated.
Describe heatstroke
Elevated core body temperature in conduction with severe neurological disturbances (LOC and confusion). Cell/tissue damage occurs throughout the body. The weather conditions can affect heatstroke (dry air versus humid due to differences in convective currents and evaporation). Treatment requires rapid lowering of core body temperature, vigorous hydration, airway maintenance, avoidance of aspiration.
What is the difference between classical heatstroke and exertional heatstroke?
Classical is when environmental stress overwhelms and impairs thermoregulatory system (most patients have preexisting chronic disease).
Exertional is where the primary factor is high metabolic heat production. Patients are generally soldiers or athletes.
Describe malignant hyperthermia
A massive increase in metabolic rate, oxygen consumption, and heat production in skeletal muscle that can be lethal. Many pt have a mutation in the ryanodine receptor which disrupts calcium homeostasis in skeletal muscles.
Also triggered by inhaling anesthetics.
Treatment involves discontinuation of the triggering agent (if chemical), use of ryanodine receptor antagonists, and cooling of the body.
Describe hypothermia
Core temperature below 35C. Heat production cannot increase to compensate for heat loss. Hypothermia leads to drowsiness, slurred speech, bradycardia, and hypoventilation associated with cold-induced decreases in metabolic rate. Severe hypothermia can lead to coma, hypotension, and fatal cardiac arrhythmias (V-fib).
Describe frostbite
Exposure to extremely low temperatures casuses freezing of surface areas (vulnerable - earlobes, hands, feet.) Permanent necrotic damage occurs when extensive ice crystals form in the cells of the skin and subcutaneous areas. Gangrene often follows and surgical removal is necessary. Sudden cold induced vasodilation can occur but is more developed in lower animals
