Homeostatic Mechanisms

Question 1

Core Body Temp

Answer 1

• 95-97F (34-36 Rectal)- early, morning cold weather
• 97-99F (36-37.5 Rectal)- normal range
• 99-101F (37.5-38.5 Rectal)- a few normal adults, many active children, hardwork emotion
• may vary between time of day (low in morning, high between 3-6); stage of menstrual cycle (increase 1C during the post-ovulatory phase); level of activity (increase with exercise and emotion); age (high in active children, lower with adults)

Question 2

Endotherms and Homeotherms

Answer 2

• humans and mammels are endotherms- generate their own body heat
• humans also homeotherms- maintain their core body temp within a narrow range despite large fluctuations in the environment

Question 3

Consequences of Deviations in body temp

Answer 3

106-114F- temp regulation seriously impaired
97-103- temp regulation efficient in febrile disease, health and work
85-94- temp regulation impaired
74-82- temp regulation lost
41-44C- heatstroke, brain lesions
-hypothalamic regulation of temp is compromised below 94F/34.4C and lost below 85/29.4C
-cardiac standstill at low temp
-heat stroke with multiple organ failure and brain lesions is a threat at high temp
-maintenance of stable body temp with neg feedback with a very high gain

Question 4

Body Temp maintenance in atmosphere

Answer 4

• nude person can maintain their core body temp within 97 and 100F after exposure for a few hours to dry air between 55F and 130F
• wind and moisture influence shape of curve (water better conductor of heat). Rate of heat loss to water is much greater than the rate of heat loss to air
• regulatory mechanism is largely from detectors in the skin and hypothalamus. Receptors in core (spinal cord, viscera, great veins) also contribute

Question 5

Radiation

Answer 5

• transfers heat as electromagnetic waves between objects that are not in contact
• rate of temperature transfer is proportional to the temp difference between the body surface and the environment
• at rest outdoors 60% of body heat is lost by radiation

Question 6

Conduction

Answer 6

-intermolecular thermal heat transfer between solid objects in direct contact (lying on hot sand causes body to gain heat by conduction, ice pack conductive heat loss)- normally minimal in person with shoes and clothes

Question 7

Convection

Answer 7

• loss or gain of heat by movement of air or water over the body
• heat rises and air carries heat away from body
• loss heat in water the most this way

Question 8

Evaporation

Answer 8

• of water from skin and respiratory tract can carry large amounts of heat generated by the body because of the amount of heat required to transform water from liquid to gas phase (insensible loss is ~650ml/day)
• air circulation improves the rate of evaporation of sweat from skin, and high humidity makes it less effective
• dissipates most heat during exercise

Question 9

Homeostatic mechanisms and mass balacne

Answer 9

• most body heat is generated in deep organs (liver, brain, heart, skeletal muscle) by cellular metabolism
• rate of heat loss is determined by how rapidly heat is carried from core to skin and transferred from skin to surrounding
• heat transferred to skin via convection in blood where it is then lost to the air and surroundings
• sympathetic nervous systems regulates blood flow to the skin and sweating
• sweating in response to hypoglycermial changes in blood flow patterns in response to a fall in blood pressure, and changes in metabolism in response to alterations in thyroid metabolism

Question 10

Blood flow and heat transfer

Answer 10

• skin and subcutaneous fat are heat insulators (fat 1/3 as effective as other tissues)
• blood vessels beneath skin are produce and include continuous venous plexus, in most exposed areas there are blood from plexus directly to small arteries via arteriovenous anastomosis
• dry clothing insulates the body from the environment by entrapping air next to the skin in the weave of the cloth- normal clothes decrease heat loss by 1/2, artic clothing decrease heat loss to as little as 1/6

Question 11

Skin highly controlled radiator system

Answer 11

• changes in environmental temp result in 8 fold increase in heat conductance compared to the fully vasoconstricted state
• at low temp, arterioles and arteriovenous anastomosis that supply blood to the venous plexus of the skin are constricted
• vasodilation subserves heat conductance through skin as temp increases
• SNS is inhibited =vasodilation when temp rises, and SNS increases when temp falls

Question 12

Eccrine sweat glands promote evaporative heat loss

Answer 12

• when hot thermal sensors in the skin increase blood flow to the skin and also sweat production
• if core temp increases, sweat production can increase profusely to greatly increase heat loss by evaporation (1L/hr)
• acclimization to hot weather involves a change in sweat glands to increase sweating capability (2-3L/hr), also decrease loss of NaCl in sweat due to aldosterone secretion from adrenal cortex
• congenital anhidross- inability to secrete sweat
• lower animals pant to dissipate heat
• sweat glands innervated by an acetylcholine-secreting sympthetic nerve- a primary protein-free secretion is formed by the glandular portion but most of the electrolytes are reabsorbed in the duct leaving a dilute watery secretion

Question 13

Cold and Warmth Receptors in the Skin

Answer 13

• free nerve endings are themal sensors in the skin and hypothalamus- changes in local temperature by altering their frequency of firing action potentials- anticipate changes in core temp
• 10 fold more cold receptors in many parts of skin+ distinct deep body receptors sensitive primarily to cold in the body core they prevent hypothermia- project to control center in hypothalamus
• firing rate of cold and warm receptors are equal at skin temp 37C
• up to 44-46 firing rate of warmth receptor fibers increases, as temp decreases below 37C the firing rate of cold receptor fibers increase- travel up afferent fibers via the spinal cord to the hypothalamic regulatory center

Question 14

Negative feedback control of body temperature

Answer 14

• hypothalamic center integrates thermal information from the skin and central temperature receptors and directs changes in efferent activity resulting in vasoconstriction to conserve heat or shivering to produce more heat, or vasodilation and sweating to increase loss of heat
• both heat and cold sensitive neurons in the hypothalamus with proportionately more heat-sensitive neurons
• feel temp in skin, result in reflexes prevent corresponding changes in body core temperature- anticipatory feedback

Question 15

Hypothalamic temperature

Answer 15

• increase body temp engages skin vasodilation, sweating, and decreased heat production
• decrease body temp- skin vasoconstriction, piloerection, thermogeneis/ heatproduction (shivering, sympathetic/ chemical excitation, thyroid hormone production
• hypothalmic temp on evaporative heat loss from body and on heat production caused primarily by muscle activity and shivering

Question 16

Homeostatic control- decrease heat loss and increase heat generation

Answer 16

• rapid decrease in temperature increases the loss of heat from the subjects warm skin upsetting the balance between heat gain and heat loss
• body temp decreases
• constriction of skin blood vessels reduces blood flow to skin reducing heat loss
• curling up reduces surface area for skin available for heat loss
• if heat loss continues then heat input must increase and shivering generates large quantities of heat to help restore a steady state
• chronic exposure to cold involves adaptive responses that are poorly understood- constrict blood vessels and inhibit sweat
• lethal hypothermia from immersion in cold water

Question 17

Components of negative feedback homeostatic reflex arc in thermoregulation

Answer 17

Regulated variable- body temperature
Stimulus- decreased body temperature
Sensors- temperature sensitive neurons in the periphery and central nervous systems
Integrator- hypothalamic neurons that compare input to set point
Effectors- sympathetic nerves regulating blood vessels in the skin and sweat glands; hypothalamic motor centers regulating shivering

-hypothalamic intregrating center, if preoptic area of the hypothalamus is heated in experimental subjects, heat sensitive neurons/receptors in the hypothalamus are activated, the skin sweats profusely and the skin vessels vasodilate

Question 18

Body Heat Balance During Exercise

Answer 18

• steady state core temp during exercise is not regulated at the elevated level
• exercise induced hyperthermia is due to an initial imbalance between rates of heat production and dissipation
• at onset the rate of heat production increases in proportion to the exercise intensity and exceeds the current rate of heat dissipation- heat storage and rise in core temp
• the hypothalamic thermoreceptors sense increaste, the integrator detects an error signal and activates heat dissipation then there is evaporative heat loss
• body core temp raises by 1C because of delayed onset of sweating

Question 19

Fever producing stimuli (pyrogens)

Answer 19

• triggers an increase in temp set point and core temp- usually due to pathological process such as infection
• when set point raised the mechanisms for raising body temp are engaged, thereby enhancing heat conservation and heat production
• hypothalamus has fenestrated capillary endothelium that allows endogenous pyrogens (TNF ) to cross blood brain barrier and act to increase temp set point
• endogenous pyrogens are sensed by hypothalamic control neurons, via local release of prostaglandins
• fever- degenerating body tissues, hypothalamic brain lesions, tumors that compress the hypothalamus and thyroid storm
• aspirin and acetaminophen inhibit prostaglandin synthesis and reduce fever, and also attenuate muscle and joint pain that often accompany fever. Note that fever may be beneficial during an infection bc immune cells may operate optimally at higher temp

Question 20

Effect of increasing the set point

Answer 20

• raising the set point to 103F triggers an error signal that the blood temp is now less than the set point. Hypothalamus thinks the body is too cold- they have chills, vasoconstriction, the skin feels cold, shivering generates heat so does curling up
• body can take several hours to reach new set point, when it does reach it the higher temp is maintained as the new normal as long as the pyrogen is there
• pyrogen removed the set point returns (fever breaks, crisis)
• lower set point activates mechanisms to dissipate heat leading to hot skin, intense sweating and vasodilation, takes about an hour

Question 21

Heat exhaustion

Answer 21

• failure in cardiovascular homeostasis in hot environment
• decrease in circulating blood volume caused by skin vasodilation and sweating induced decrease in central venous pressure
• decrease CVP can allow blood to pool in limbs causing syncope
• typically dilated pupils and sweating
• collapse preceded by weakness, confusion, ataxia, anxiety, vertigo, headache, nausea or vomiting
• core temp normal or mildly elevated
• rest in cool environment, fluids

Question 22

Heat stroke

Answer 22

-elevated core body temp in conjunction with severe neurological disturbances including loss of consciousness and convulsion
-cell/tissue damage occurs throughout the body
-classical- environmental stress overwhelms an impaired thermoregulatory system
exertional- high metabolic heat production, generally young fit people
-core temp over 41 neural death and organ system failure
-if air dry convective currents promote evaporative heat loss, in humid air can take less temp
-treatment by lowering core body temp in cold water, hydration, airway maintenance

Question 23

Malignant hyperthermia

Answer 23

• massive increase in metabolic rate, oxygen consumption and heat production in skeletal muscle that can be lethal
• most people have gene mutations in ryanodine receptor which disrupt calcium homeostais in skeletal muscles
• can be triggered by inhaling anesthetics and some depolarizing muscle relaxants (succinylcholine)
• treat with discontinuation of trigger, use ryanodine receptor anatagoists

Question 24

Hypothermia

Answer 24

• core temp below 35C or 95F is common after immersion in cold water
• heat production cannot increase enough to compensate for heat loss
• leads to drowsiness, slurred speech, bradycardia, and hypoventilation with cold induces decreases in metabolic rate
• can lead to coma, hypotension, and fetal cardiac arrhythmias (ventricular fibrillation)

Question 25

Frostbite

Answer 25

• exposure to extremely low temp causes freezing of surface areas
• vulnerable= earlobes, fingers and toes
• permanent necrotic damage occurs when extensive ice crystals form in the cells of the skin and subcutaneous areas
• gangrene often follows thawing and those areas must be surgically removed
• sudden cold-induced vasodilation is a final protective response- it occurs near freezing temperatures when smooth muscle in the vascular wall becomes paralyzes by the cold itself