The Effect of Exercise on Cardiovascular Function in a Hot Environment

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The Effect of Exercise on Cardiovascular Function
in a Hot Environment

• By Amber Prince, Bethany Card, Flordeliza
  Lilagan, and Paul Haerling

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The Problem

• Julie notices that when training for a
  half-marathon she tires more quickly when the
  weather is hot. She wants to know why.

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The Problem

• In order to answer Julies question, we run an
  experiment showing the relationship between mean
  arterial pressure (MAP) and oxygen consumption
  (VO2) during two different exercise bouts, one
  performed in a hot environment and one performed
  in a neutral temperature environment.

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The Problem

• We obtained this graph as a result of the
  experiment

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The Problem

• The problem is that weve lost our notes and we
  still need to answer the following questions
• During which exercise bout would the systemic
  vascular resistance (SVR) be higher at a VO2 of 2
  L/min?
• During which exercise bout would central blood
  volume be lower at VO2 of 2 L/min?
• During which exercise bout would heart rate be
  higher at VO2 of 2 L/min?
• Which graph represents exercise performed in a
  hot environment, A or B?

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Importance and Relevance

• Cardiovascular function in relation to mean
  arterial pressure
• Oxygen consumption
• Thermoregulation and homeostasis

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Mean Arterial Pressure

• driving force for blood flow
• diastolic pressure (1/3)(systolic-diastolic)
• Influenced by blood volume, cardiac output,
  resistance to blood flow in the arteries, and
  distribution of blood between arterial and venous
  blood vessels

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Cardiovascular response to exercise

• Extrinsic Control (increased SNS and decreased
  PNS)
• Vasoconstriction in visceral organs
• Increased cardiac output (increased HR and stroke
  volume)
• Increase in Ca2 and Na influx increase in
  rate at SA node and more Ca2 for stronger force
  of contraction
• Increase in venous return (skeletal muscle pump
  and respiratory pump)
• Baroreceptor reflexes that normally control blood
  pressure do not function during exercise so MAP
  increase is maintained

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Cardiovascular response to exercise

• Intrinsic Control (at effector organs)
• Vasodilation in skeletal muscles in response to
  decrease in O2 and increase in temperature and
  CO2
• causes a decrease in vascular resistance
• Increased HR stroke volume (SA stretch and
  Starlings Law)
• MAP is maintained despite peripheral vasodilation

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During which exercise bout would Systemic
Vascular Resistance be higher at VO2 of 2.0 L/min?

• Systemic vascular resistance would be higher at
  neutral temperature than at hotter conditions
  because exercise in a hot environment would lead
  to vasodilation not just in skeletal muscles but
  also in the skin to lose heat through convective
  cooling
• Resistance is inversely proportional to the
  radius of blood vessel, so increasing the radius,
  decreases resistance

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During which exercise bout would central blood
volume be lower at VO2 of 2.0L/min?

• Central blood volume is lower at VO2 of 2 L/min
  when exercise is performed in a hot environment.
• Again, this is because of temperature regulatory
  mechanisms that cause vasodilation of vessels
  near the surface of the skin (in addition to
  vasodilation of the vessels in muscles caused by
  exercise)
• The result is that there is more blood in the
  systemic circuit and less is able to return to
  the heart and lungs per unit time.

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During which exercise bout would heart rate be
higher at VO2 of 2.0 L/min?

• The heart rate would be higher in the hot
  environment.
• Decreased venous return gt increase in heart rate
  to maintain cardiac output
• The heart is pumping blood to the skeletal
  muscles and to the blood vessels in the skin for
  thermoregulation. The longer total vessel
  length and decreased peripheral resistance
  requires higher cardiac output in order to
  maintain a minimum MAP
• Stroke volume is decreased because of decreased
  venous return, so HR must increase

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Which graph represents exercise performed in a
hot environment (A or B)?

• MAP decreases when exercise is performed in a hot
  environment. So line B represents exercise
  performed in a hot environment and line A
  represents exercise performed in neutral temp.
• This is partly due to vasodilation of vessels
  near the surface of the skin and a reduction in
  venous return (thus stroke volume)
• However, another thermoregulatory mechanism may
  contribute to the decreased MAP sweating.
  Sweating causes a drop in blood volume and can
  result in lower blood pressure.
• In extreme cases, too much exercise in a hot
  environment can cause heat stroke

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Why Does Julie Tire?

• As for why Julie tires more rapidly in a hot
  environment, this is because blood is being
  diverted away from muscle tissue to the skin in
  order to cool the body. A decreased MAP means
  less blood to muscle tissue, and therefore less
  nutrient and oxygen availability to aerobically-
  metabolizing muscle cells. The result is more
  rapid fatigue.

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Analogy

• A set of twins gets invited to their best
  friends birthday party. The only problem is,
  that they have a concert recital at the same
  time. Both events are very important to them. So
  they splitone to the party, the other to the
  concert, but neither one has as much fun as they
  would have if they both went to both events.