11/14/2014 Medical Physiology: Thermoregulation Mary Lou Vallano

Question 1

Why are humans considered endotherms?

Answer 1

They generate their own body heat

Question 2

Why are humans considered homeotherms?

Answer 2

They maintain their core body temperature within a narrow range (~0.6oC/1.0oF) despite large fluctuations in the environment

Question 3

How do skin temperature and core temperature differ?

Answer 3

Skin temperature rises and falls with the environmental temperatures Core temperatures are more stable (can be maintained over a wide range of atmospheric temperatures) and normally vary relative to other physiological factors

Question 4

What are the different physiological factors regulating core temperature?

Answer 4

• Time of day *lowest between 3 and 6am *highest between 3 and 6pm - Stage of menstrual cycle *increasing ~1oC during the post-ovulatory phase (progesterone) - Level of activity *increasing with exercise and emotional states - Age *higher in active children *lower in aged adults

Question 5

How do wind and moisture influence the shape of the body temp vs. atmosph temp curve?

Answer 5

Specific heat of water >> specific heat of air, (also a much better conductor of heat) –> rate of heat loss to water >> rate of heat loss to air

Question 6

Where are the regulatory mechanisms for temperature located?

Answer 6

Detectors mostly in skin and hypothalamus Core receptors also –> spinal cord, viscera, great veins

Question 7

Why is maintenance of internal body temperature one of the most important regulated variables in humans?

Answer 7

Because enzymes, cells, and organs function optimally in a narrow range of temperatures, whereas environmental temperatures can vary widely

Question 8

What happens below ~94oF/34.4oC?

Answer 8

Hypothalamic regulation of temperature is compromised

Question 9

What happens below ~85oF/29.4oC?

Answer 9

Hypothalamic regulation of temperature is lost

Question 10

What happens to cellular heat production with every 10oF decrease in body temperature?

Answer 10

Decreases ~2-fold

Question 11

What is a threat at low temperatures?

Answer 11

Cardiac “standstill” or fibrillation

Question 12

What is a threat at high temperatures?

Answer 12

Heat stroke with multiple organ failure and brain lesions

Question 13

What does maintenance of a stable body temperature involve?

Answer 13

Negative feedback control with a very high gain (~25-30)

Question 14

Describe heat exhaustion (heat collapse)

Answer 14

1. Failure in cardiovascular homeostasis in a hot environment 2. Core temp normal or mildly elevated) 3. Decrease in circulating blood volume

Question 15

Describe heatstroke

Answer 15

1. Elevated core temp 2. Severe neuro disturbances (LOC + convulsions) 3. Cell/tissue damage 4. Classical (preexisting chronic illness) vs. exertional (high metabolic heat production; young people) 5. Humid environment and heavy exercise 6. Treat: immersion in cold water, vigorous hydration, airway maintenance

Question 16

Describe malignant hyperthermia

Answer 16

1. Massive increase in metabolic rate –> oxygen consumption –> skeletal muscle heat production 2. Associated with gene mutations in ryanodine receptor –> calcium homeostasis disruption 3. Triggered my anesthetics inhalation and depolarizing muscle relaxants 4. Treat: discontinuation of triggering agent; use ryanodine receptor antagonists; cool down body

Question 17

Describe hypothermia

Answer 17

1. Common after immersion in cold water 2. Drowsiness, slurred speech, bradycardia, hypoventilation 3. Severe cases: coma, hypotension, fatal cardiac arrhythmias

Question 18

Describe frostbite

Answer 18

1. Freezing of surface areas after exposure to extremely low temps 2. Most vulnerable: earlobes, fingers and toes 3. Ice crystals in skin cells –> permanent necrotic damage –> amputation of affected areas 4. Sudden cold-induced vasodilation (last resort)

Question 19

What are the major mechanisms of heat gain or loss?

Answer 19

1. Radiation 2. Conduction 3. Convection 4. Evaporation

Question 20

Describe radiation

Answer 20

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

Question 21

Describe conduction

Answer 21

Intermolecular thermal heat transfer between solid objects in direct contact *normally, heat exchange by conduction is minimal in a person wearing shoes and clothing

Question 22

Describe convection

Answer 22

Loss or gain of heat by movement of air or water over the body *because heat rises, air carries heat away from the body by convection *a body immersed in water exchanges most heat by convection

Question 23

Describe evaporation

Answer 23

Evaporation of water from the 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 *air circulation improves the rate of evaporation of sweat from skin, and high humidity makes it less effective *evaporative losses from the surface of the skin by sweating normally dissipate nearly all of the heat produced during exercise

Question 24

Where is most body heat generated?

Answer 24

In deep organs (liver, brain, heart, active skeletal muscle)

Question 25

Who generates most body heat?

Answer 25

Cellular metabolism, which is inherently inefficient *rate of heat production ~ metabolic rate

Question 26

What determines te rate of heat loss?

Answer 26

How rapidly the heat is: 1) carried from the core to the skin 2) transferred from the skin to the surroundings *most heat is transferred from the core to the skin by convection in the blood, where it is then lost to the air & surroundings

Question 27

What regulates blood flow to the skin and sweating?

Answer 27

The sympathetic nervous system

Question 28

What mechanisms are involved in passive or unregulated heat transfer?

Answer 28

Various homeostatic controls-systems not directly involved in temperature regulation can also affect heat flow *sweating in response to hypoglycemia *changes in blood flow patterns in response to a fall in blood pressure *changes in metabolism in response to alterations in thyroid metabolism

Question 29

What do skin and subcutaneous fat do?

Answer 29

They are heat insulators that help maintain core temperatures *fat is a poor heat conductor

Question 30

What is the role of blood vessels beneath the skin in heat transfer?

Answer 30

-they are profuse -includes a continuous venous plexus supplied by inflow of blood from the skin capillaries -in the most exposed areas (hands, feet, ears) blood is supplied to the plexus directly from small arteries through muscular arteriovenous anastomosis

Question 31

How does dry clothing insulate the body from the environment?

Answer 31

1) entrapping air next to the skin in the weave of the cloth, reducing conduction 2) reducing the flow of convective air currents –> reducing heat transfer from the skin

Question 32

What happen as the environmental temperature increases?

Answer 32

Vasodilation subserves heat conductance through the skin

Question 33

How does the sympathetic nervous system react to body temperature changes?

Answer 33

The SNS supplying the skin vasculature is inhibited (less vasoconstriction=vasodilation + active vasodilation) when the body temperature rises, and activated (more vasoconstriction) when the body temperature falls

Question 34

What do free nerve endings functioning as thermal sensors and located over the skin surface and in the hypothalamus do?

Answer 34

1) respond to changes in local temperature by altering their frequency of firing action potentials 2) anticipate changes in core temp.

Question 35

Describe the function of skin receptors involved in temperature regulation

Answer 35

1) skin has anatomically distinct receptors for warmth and cold *10-fold more cold receptors in many parts of the skin 2) skin and distinct deep body receptors (spinal cord, abdominal viscera, great veins) sensitive primarily to cold in the body core prevent hypothermia *project to a control center in the hypothalamus 3) the firing rate of the cold and warmth receptor fibers are equal at a skin temperature of ~37oC 4) up to 44 - 46oC, the firing rate of the warmth receptor fibers increases 5) when exposed to a hot environment, thermal sensors in the skin increase blood flow to the skin and also sweat production, increasing heat loss 6) as the temperature decreases below the set point of 37oC, the firing rate of the cold receptor fibers increases *these action potentials, firing at temperature-dependent frequencies, travel up afferent fibers via the spinal cord to the hypothalamic regulatory center (INTEGRATOR/COMPARATOR)

Question 36

Describe acclimatization

Answer 36

Acclimatization to hot weather (1-6weeks) involves a change in the sweat glands to increase sweating capability (up to 2-3L/hr). *decrease in the loss of NaCl in the sweat to conserve body salt

Question 37

What is congenital anhidrosis?

Answer 37

A genetic inability to secrete sweat

Question 38

How does a sweat gland work?

Answer 38

1) innervation by an acetylcholine-secreting sympathetic nerve 2) formation of a primary protein-free secretion by the glandular portion, with reabsorption of most of the electrolytes in the duct, leaving a dilute, watery secretion

Question 39

What does the hypothalamic center do?

Answer 39

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

Question 40

Are there also both heat and cold-sensitive neurons in the hypothalamus?

Answer 40

Yes, with proportionately more heat-sensitive neurons

Question 41

What does “anticipatory” feedback mean?

Answer 41

Changes in skin temperature reflect the environment, and the resulting reflexes prevent corresponding changes in body core temperature. These reflexes are termed “anticipatory”

Question 42

What does “negative” feedback mean?

Answer 42

Changes in core temperature, such as during exercise, result in responses involving “negative” feedback that serve to minimize the change in core temperature.

Question 43

What effect does increased body temperature have in hypothalamic response?

Answer 43

• skin vasodilation - sweating - decreased heat production to reduce body heat

Question 44

What effect does decreased body temperature have in hypothalamic response?

Answer 44

• skin vasoconstriction - piloerection (important in lower animals, not humans) - thermogenesis/heat production (shivering, sympathetic/chemical excitation, thyroid hormone production)

Question 45

What is the critical set point to which temperature control mechanisms continually attempt to bring body temp back?

Answer 45

37.1 C/98.8 F

Question 46

What happens if the preoptic area of the hypothalamus is heated in experimental subjects?

Answer 46

1. heat sensitive neurons/receptors in the hypothalamus are activated 2. the skin sweats profusely 3. the skin vessels vasodilate

Question 47

What are the components of a negative feedback homeostatic reflex arc in the process of thermoregulation?

Answer 47

regulated variable –> body temperature stimulus –> decreased body temperature sensors –> temperature sensitive neurons in the periphery and CNS integrator –> hypothalamic neurons that compare input to set point effectors –> sympathetic nerves regulating blood vessels in the skin & sweat glands *hypothalamic motor centers regulate shivering

Question 48

What happens to body heat balance during exercise?

Answer 48

The steady-state core temp is not “regulated” at the elevated level

Question 49

What causes exercise-induced hyperthermia?

Answer 49

An initial imbalance between rates of heat production and dissipation

Question 50

What happens after the beginning of exercise?

Answer 50

Evaporative heat loss from sweating is normally a much more important means of eliminating heat than convective or radiative loss of heat

Question 51

What happens when body core temperature rises by about 1oC due to the delayed onset of sweating?

Answer 51

Rise in core temperature is an essential feature of homeostatic thermoregulation by providing the error signal that sustains the sweating response during exercise

Question 52

What happens to skin temperature during exercise?

Answer 52

It is maintained nearly constant due to the effect of sweating *it decreases slightly due to increased evaporative cooling of the skin

Question 53

What do fever-producing stimuli (pyrogens) do?

Answer 53

Trigger an increase in the set point of the hypothalamic temperature-regulating center *usually do to pathological process

Question 54

How do endogenous pyrogens increase the temperature set-point?

Answer 54

By crossing the blood-brain barrier the hypothalamic fenestrated capillary endothelium

Question 55

What are pyrogens?

Answer 55

Fever-producing stimuli, such as cytokines (interleukins and TNF)

Question 56

How are endogenous pyrogens sensed by hypothalamic control neurons?

Answer 56

Via local release of prostaglandins

Question 57

Why are aspirin and acetaminophen named antipyretics?

Answer 57

Because they inhibit prostaglandin synthesis and reduce fever *also attenuate muscle and joint pain that often accompany fever

Question 58

Why may fever be beneficial during an infection?

Answer 58

Because immune cells may operate optimally at the higher temp

Question 59

What happens when the pyrogen is removed?

Answer 59

The set point returns to normal