Lecture 1. Human Thermoregulation Exercise in the heat

Question 1

What are the 5 stages of Homeostasis of thermoregulation ?

Answer 1

1. Stimulus produces changes in variable
2. change detected by receptor
3. input: info sent along afferent pathway to control center
4. output: info sent along efferent pathway to effector
5. Response of effector feeds back to influence magnitude of stimulus and returns variable to homeostasis

Question 2

What is the conceptual heat balance equation ?

Answer 2

(M-W)±(K±C±R-Esk)=S

(Rate of heat prod)±(Rate of total heat loss/gain)=Rate of heat storage

Question 3

What does each symbol mean in the equation (M-W)±(K±C±R-Esk)=S ?

Answer 3

M = metabolism
W = work (only small amount of heat is transferred into mechanical work)
K= Conductive
C = Convective
R= Radiative
Esk = Evaporation
S = Storage

Question 4

Rate of total heat loss/gain can be broken down to ?

Answer 4

Dry heat loss and evaporation from the skin

these work down gradients from a high region to a low region
Dry heat loss: works down temp gradient, from high to low between skin and enviro/air
Evaporative heat loss works down a water vapour pressure gradient between skin and air
if we want to increase heat dissipation, need to increase those gradients
body does this through vasomotor control (vasodilation) and sudomotor control (sweating)

Question 5

Dry heat loss works across what temperature gradient ?

Answer 5

down a temp gradient from high to low between skin and ambient air

Question 6

Evaporative heat loss works across what temp gradient ?

Answer 6

down a water vapour pressure gradient from high to low between skin and ambient air

Question 7

If we want to increase heat dissipation we need to ?

Answer 7

increase the temp gradient at the skin and water vapour pressure at skin
the body does this by vasomotor control (vasodilation) and sudomotor control (sweating)

Question 8

Vasodilation works by ?

Answer 8

increases blood flow to the skin, relaxation of the smooth muscle lining of the vessels, happens mainly around the capillaries so we get an increase in the warm blood flow out to the capillaries which are close to skin network so it can be dissipated out to the skin

Question 9

Explain why the by-pass/ shunt vessels (arteriovenous anastomosis) vasoconstrict ?

Answer 9

these are by=pass vessels that connect the vein to the artery usually wanting that quick blood flow to happen between and they by-pass the artery, in this case we don’t want that to happen we want the capillaries to become flushed with blood, so these actually vasoconstrict so we get the warm blood flushing through the capillary network ultimately increasing the gradient making the skin hotter so heat can be dissipated out to the cooler air

Question 10

What is also known as insensible heat loss and why ?

Answer 10

Sweating because of increased dehydration

Question 11

How does sweating increase heat dissipation ?

Answer 11

• ↑P skin - ↑P air gradient
• • increase water vapour at the skin surface
• ↑temp gradient from core to skin
• -cools skin temp aids to dissipation of heat

Question 12

What is the sweat gland controlled by thermoregulation ?

Answer 12

the eccrine sweat gland

Question 13

What role does the Sympathetic nervous system play in sweating ?

Answer 13

• Sweating is stimulation by sympathetic nervous stimulation that releases acetylcholine which causes the fluid in the coil to be released
• the fluid in terms of composition is very like plasma
• as it flows through the duct you reabsorb electrolytes such as sodium, chlorides so the body maintains electrolyte balance
• because of this the sweat on the skin surface is a lot more in terms of fluid compared to your electrolyte ratio so its hypotonic so we lose a lot more fluid than our electrolytes it is variable between individs, gender age fitness level etc and with individ such as clothing etc

Question 14

What is one other way we can reduce heat storage ?

Answer 14

• Metabolism, if you reduce heat production, ultimately you will reduced your heat storage
• To limit metabolism either stop or reduce exercise intensity, remember its driven by vo2
• coined behavioural thermoregulation is a conscious effort to down regulate exercise intensity driven by thermal comfort, thermal sensation even thirst sensation

Question 15

Environmental factors affecting heat loss are ?

Answer 15

• Increased ambient temperature
• Increased absolute humidity
• Decreased wind velocity

• Increased Solar Radiation – - Increase in dry heat gain (R) - Skin Temp.
Dry = evaporated quickly, Humid = accumulates

Question 16

Increased ambient temperature affects heat loss how ?

Answer 16

• Reduces effectiveness of heat loss particularly

by radiation, conduction, and convection

Question 17

Increased absolute humidity affects heat loss how ?

Answer 17

• Reduces effectiveness of heat loss by evaporation

- Dry = evaporated quickly, Humid = accumulates

Question 18

Decreased wind velocity affects heat loss how ?

Answer 18

• Reduces both convective and evaporative effectiveness

Question 19

Increased Solar Radiation affects heat loss how ?

Answer 19

• Increase in dry heat gain (R) - Skin Temp.

Dry = evaporated quickly, Humid = accumulates

Question 20

What factors are impaired by exercise in warm/hot conditions

Answer 20

• Muscle: 1) Temperature, 2) Metabolic, 3) Afferent feedback
• CNS: 1) brain metabolism, 2) Neurotransmitter levels 3) Temperature
• Cardiovascular: 1) Blood pressure, 2) Blood flow, 3) Oxygen delivery, 4) Metabolite removal
• Psychological: 1) Thermal comfort, 2) perceived exertion, 3) Motivation, 4) Expectations
• Respiration: 1) Hypocapnia (hyperventilating, state of reduced co2 in the blood), 2) Alkalosis (excessive alkaline condition of the body leading to weakness or cramp), 3) breathing sensations

Question 21

Fluid balance..

Increased sweat rate leeds to ?

Answer 21

dehydration/ hypohydration

Question 22

Sweat is hypotonic to plasma, further explain this

Answer 22

– ↑ Plasma Osmolality (Hyperosmolality)
• Delays sweating onset and skin vasodilation
• Inhibits fluid availability to the sweat glands (↓ sweating rate)

– Fluid deficits = hypovolemia (↓plasma volume)
• Reduces skin blood flow due to ↓ venous pressure, venous return and CO

Inhibiting thermoregulation and performance

Question 23

what is the splanchnic BF ?

Answer 23

800 - 1000ml of blood flow, mainly the kidneys and liver, blood flow from these are redirected to where its needed (skin and exercising muscles)

Question 24

Circulatory responses to heat are ?

Answer 24

• ↑ Skin blood flow and plasma loss (causes blood pool in extremities)
• ↓Venous pressure and cardiac filling
• ↓diastolic filling time (due to ↑ HR)
• ↓ Stroke Volume
• Compensate: ↑ HR and contractility

Question 25

Circulatory responses if heat stress continues are ? (@extreme levels)

Answer 25

• Baroreceptor stimulation (will shut off {vasoconstrict} blood flow to skin and to muscles, decreases thermoregulatory capacity and muscle function but enables central blood volume to get back to normal
• Muscle and Skin vasoconstriction
• ↓ Heat dissipation
  ↓ Muscle function

- Blood pressure and cardiac
function preserved (temporarily)

Question 26

Performance implications in heat are ?

Answer 26

reduction in fixed intensity - Galloway found exercise time to exhaustion (70% vo2max) also self pace.
11 degrees is optimum temperature for time to exhaustion.

tatterson found 30 min cycling Time Trial at
32oC (HT) and 23oC (NT) 60% RH in elite road cyclists - core body temp was the same, high temp (32oC) was 6.5% lower than 23oC) in power output across 6x5 minute stages