Body Temperature Regulation and fever

Created by cmagliulo 

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range of normal core temperature

97 F (36 C) - 99.5 F (37.5 C)

generally b/w 98-98.6 F when measured orally and 1 F higher rectally

The rate at which heat is lost is determined by (2)

1) how RAPIDLY heat can be conducted from where it is produced in the body CORE (deep organs such as heart, brain, liver and skeletal muscle during exercise) to the skin

2) how rapidly heat can then be transferred from the skin to the surroundings

Insulator system of the body

skin, subcutaneous tissues, and especially fat of the subcutaneous tissue

*fat is important because it conducts heat only ONE THIRD as readily as other tissues
-women have better insulation

Blood flow to the skin from body core

-arteriovenous anastomoses provide connection to a continuous venous plexus that is supplied by inflow of blood from the skin capillaries

-vasodilation of this system --> more heat conductance

radiation

-loss of heat in the form of Infrared heat rays
-60% of total heat loss
- wavelengths from human bodies = 5-20 micrometers
-all objects not at absolute zero radiate these rays
-heat is also being radiated from objects around the room
*If the temp of the body is greater than the temp. of the surroundings, a greater quantity of heat is radiated from the body than is radiated to the body

conduction

-3% -from object
-lost to solid objects

-loss of heat by conduction to air = 15%
-energy of motion on skin is transferred to the air if it is colder than the skin, thus increasing the velocity of the air molecules' motion
*once the heated air equals the temp of the body, no more heat is lost; unless heated air moves away from the skin (air convection)

convection

-heat loss by convection
-the heat must be FIRST conducted to the air, and then be carried away by the convection air currents
-small amount of this always occurs because heated air around skin tends to rise and move away from the skin (15% in this described situation with no gross air movement)

cooling effect of wind

-rapid convection removal of heated air
-at low velocities the cooling effect is about proportional to the SQUARE ROOT OF THE WIND VELOCITY

Conduction and convection of heat from a person suspended in water

-water has a specific heat several thousand times as great as air
-more heat will be lost from skin
-the conductivity is also great in water
-can't form "insulator zone" around body
-great heat loss

evaporation

-when water evaporates from the body surface, .58 Calorie (kilocalories) of heat is lost for each gram of water that evaporates
-insensibly - even when the body is not sweating (can't be controlled)

Stimulation of the ______ in the brain either electrically or by excess heat causes sweating

***anterior hypothalamus-preoptic area in the brain** - more heat receptors than cold receptors (it is the thermoregulator center)
-nerve impulses are transmitted in the autonomic pathways to the spinal cord and then through sympathetic outflow to the skin everywhere in the body
-sweat glands are innervated by CHOLINERGIC nerve fibers (fibers that secrete acetylcholine but that run in the sympathetic nerves along with adrenergic fibers)
-these glands can also be stimulated, to some extent by epinephrine and norepinephrine circulating in the blood (even those these glands do not have adrenergic innervation). Important during exercise

Sweat Gland (2)

1) a deep subdermal coiled portion that secretes the sweat

2) duct portion that passes outward through the dermis and epidermis of the skin

**as is true of many other glands, the secretory portion of the sweat gland secretes a fluid called the PRIMARY SECRETION or PRECURSOR SECRETION; the concentration of the constituents in the fluid is then modified as the fluid flows through the duct (this secretion is initiated by CHOLINERGIC sympathetic nerve fibers ending on or near the glandular cells)

Composition of the precursor secretion

-similar to plasma but does not contain plasma proteins
*as it flow through the gland it is modified by reabsorption of most of the SODIUM and CHLORIDE ions
*the degree of reabsorption depends on the rate of secretion:
-when the sweat glands are stimulated only SLIGHTLY, the precursor fluid passes through the duct slowly. In this instance, essentially all the sodium and chloride ions are reabsorbed , and the concentration of each falls to as low as 5 mEq/L. $$This reduces the OSMOTIC PRESSURE of the sweat fluid (water follows salts) to such a low level that most of the water is reabsorbed

-acclimation plays a role in how much electrolyes you lose

Acclimatization of the sweating mechanism

-increased secretion of aldosterone

Skin temp. receptors

-both cold and warmth
**far more cold receptors (10X)

Cold receptor skin reflex

1) provide a strong stimulus to cause shivering
2) inhibiting the process of sweating, if this is already occurring
3) promoting skin VASOCONSTRICTION

Deep Body temperature receptors

-found mainly in the SPINAL CORD in the abdominal viscera, and in or around the GREAT VEINS in the upper abdomen and thorax
-detect mainly COLD in the core rather than the skin

Peripheral receptos stimulate ____

-an area of hypothalamus located bilaterally in the posterior portion at the level of the mamillary bodies
**The temp. sensory signals from the anterior hypothalmic-preoptic area also transmit here
*all signals combine here to regulate

Temp-decreasing mechanisms when the body is too hot

1) Vasodilation of skin blood vessels - caused by inhibition of the sympathetic centers in the post hypothalamus that cause vasoconstriction

2) Sweating

3) decrease in heat production- shivering and chemical thermogenesis are strongly inhibited

" " too cold

1) skin vasoconstriction - stim. of post hypothalamic symp centers

2) Piloerection - hair "standing on end" caused by symp (not important in humans because hair is too short to entrap "insulator air"

3) Increase in thermogenesis

Primary motor center for shivering

-located in the dorsomedial portion of the posterior hypothalamus near the wall of the third ventricle
-normally inhibited by signals from the heat center in the anterior hypothalamic-preoptic area
-is excited by cold signals from skin and spinal cord
-stimulated at a threshold temp
-signal transmitted through a bilateral tract down the brain stem, into the lateral columns of the spinal cord, and finally to the anterior motor neurons

*increase the tone of muscle. shivering results from feedback oscillation of the muscle spindle stretch reflex mechanism
*during maximum shivering, body heat production can rise to 4-5 times normal

Chemical excitation for heat production

-increase in sympathetic stimulation or circulating epinephrine or norepinephrine increases rate of CELLULAR METABOLISM = chemical thermogenesis or nonshivering thermogenesis
**results at least partially from the ability of epinephrine and norepinephrine to UNCOUPLE oxidative phosphorylation - which means that excess foodtsuffs are oxidized and thereby release energy in the form of heat but DO NOT CAUSE ATP TO BE FORMED
-10-15% increase in heat production

Brown Fat

-type of fat that contains large numbers of special mitochondria where uncoupled oxidation occurs
-richly supplied with sympathetic nerves that release norepinephrine, which stimulates tissue expression of MITOCHONDRIAL UNCOUPLING PROTEIN (thermogenin) and increases thermogenesis
-human adults have almost no brown fat
-infants do have a small amount of brown fat in the interscapular space --> can increase heat production 100%

Increased thyroxine output

-cooling the anterior hypothalamic-preoptic area also increased production of THYROTROPIN RELEASING HORMONE (TRH) --> AP --> TSH-->thyroxine

*thyroxine activated uncoupling protein and increases the rate of cellular metabolism (requires several weeks exposure to cold) --> increase in thyroid size with prolonged cold exposure

critical core body temperature

-37.1 C (98.8 F)
-the "setpoint"

feedback gain of the temperature control system (measure of effectiveness of a control system)

= the ratio of the change in environmental temperature to the change in body core temperature minus 1.0
-the human body changes about 1 C for each 25 C to 30 C change in the environmental temperature

28/1 - 1 = 27 *an extremely high gain

Skin temp. alters set point (in hypothalamus)

-when skin temp is high sweating began at a lower hypothalamic temp (lowering of set point so that more temps. fall under too high category
-opposite is true
*anticipator mechanism

Behavioral control of body temperature

-whenever the internal body temp. becomes too high, signals from the temperature-controlling areas of the brain give the person a psychic sensation of being overheated
-opposite is true
*person will then make appropriate environmental adjustments to re-establish comfort, such as moving into a heated room or wearing well-insulated clothing in freezing weather

Local skin temp reflexes

-caused by local effects of temp. directly on blood vessels and also by local cord reflexes conducted from skin receptors to the spinal cord and back to the same skin area and sweat glands
-also have the central working as well

Fever
-what causes set-point to rise

-many proteins, breakdown products of proteins (from degenerating body tissues), LIPOPOLYSACCHARIDE TOXINS released from bacterial cell membranes = PYROGENS

Mechanism of pyrogens

-some directly alter set point in hypothalamus
-others work indirectly and require several hours of latency --> true of many of the bacterial pyrogens, especially endotoxins from gram-nefative bacteria

1) when bacteria or breakdown products of bacteria are present in the tissues or in the blood, they are phagocytized by the blood leukocytes, by tissue macrophages, and by large granular killer lymphocytes
2) they all digest and then release cytokines- one of the most important of which is IL-1 (LEUKOCYTE PYROGEN/ or endogenous pyrogen) released from MACROPHAGES into the body fluids, and on reaching the hypothalamus, almost immediately activated the processes to produce fever (8-10 mins)

3) as little as one ten millionth of a gram of endotoxin lipopolysaccharide from bacteria can cause fever

**IL-1 may induce fever by first inducing the formation of PGE2 which then acts of the hypothalamus to elicit fever

Fever caused by brain lesions

-when a brain surgeon operates in the area of the hypothalamus a fever almost always occurs (rarely hypothermia)
-also compression of the hypothalamus by a brain tumor can cause PROLONGED FEVER

Chills

-because of the new high set body temp he body feels cold
-skin becomes cold because of vasoconstriction and the person shivers

Crisis or "Flush"

-when the the stimulus for high temp is removed and the set point begins to fall
-vasodilation and mechanisms to get rid of heat begin to happen

Heatstroke

-body temp. into the range of 105-108 F
-dizziness, abdominal distress, sometimes vomiting, delirium, loss of consciousness, a degree of circulatory shock brought on by excessive fluid loss

-immediate tx by placing in a cold bath (but it induces uncontrollable shivering)
-other suggest sponge or spray cooling of skin

pathological findings of heat stroke

-local hemorrhages
-parenchymatous degeneration of cells throughout entire body but especially the brain
-a person can die several days later because of the damage done to major organs

-takes about 1-3 weeks to slowly acclimate to extreme temps (a ~ two fold increase in the maximum rate of sweating, an increase in plasma volume, and a diminished loss of salt and sweat in the urine to almost none; last two effects are from effects of aldosterone

extreme cold

- a person in ice cold water for 20-30 mins ordinarily dies b/c of heart standstill or heart fibrillation (body temp will have fallen to 77 F)

Frostbite - surface areas freeze; occurs especially in the areas of the ears and in the digits of the hands and feet. If crystal form in the cells = permanent damage. gangrene follows thawing and frostbitten areas must be removed

Artificial hypothermia

-first administer a strong sedative to depress reactivity of the hypothalamic temp. controller and then cooling the person
*important in heart surgeries so that the heart can be stopped artificially for many minutes at a time
-cooling to this extent does not cause tissue damage but it does slow the heart and cause depressed cell metabolism so that the body's cells can survive 30 mins to more than an hour without blood flow during the surgical procedure

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