Cardiovascular Dynamics During Exercise

1
Cardiovascular Dynamics During Exercise

• Chapters 15 16

2
Introduction

• At rest O2 supply O2 demand
• Exercise O2 demand increases
• To the muscles
• To the heart
• To the skin
• Maintain flow to the brain
• How does the heart increase O2 supply to meet the
  O2 demand?

3
Cardiac Output

• Q heart rate times stroke volume

4
Cardiac Output

• Blood flow per minute.
• At rest Q 5-6 liters/min
• Q increases linearly with the demand for more O2
• Indicator of oxygen supply

5
How does cardiac output increase?

• Increase heart rate
• Increase stroke volume

6
Heart Rate

• Resting heart rate
• Anxiety
• Dehydration
• Temperature
• Digestion
• Over-training
• The most important factor for increasing Q during
  acute exercise.

7
Heart Rate

• What causes HR to increase during exercise?
• Decrease parasympathetic (vagal) stimulation
• Increase sympathetic stimulation

8
Heart Rate

• Steady state exercise
• Why does heart rate level off during steady state
  exercise?

9
Heart Rate

• Increases with intensity and levels off at
  maximal effort.
• HRmax 220 age
• ( 12)

10
Stroke Volume

• Volume pumped per beat of the heart
• Influenced by preload and afterload

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13
Stroke Volume

• Increases until about 25-50 of maximum
• After that it may plateau (untrained) or continue
  to increase (trained)
• Decrease at maximum effort?

14
Stroke Volume

• How does stroke volume increase during exercise?
• Increase preload (EDV)
• Increase venous return
• Muscle pump, etc.
• Decrease afterload
• Vasodilation
• Metabolic control and sympathetic stimulation
• Increase contractility (ESV)
• Increase sympathetic stimulation

15
Frank-Starling Mechanism

• Frank-Starling mechanism the ability of the
  heart to alter the force of contraction is
  dependent on changes in preload.
• As the myocardial fibers are stretched, the force
  of contraction is increased.
• Because the length of the fiber is determined
  primarily by the volume of blood in the
  ventricle, EDV is the primary determinant of
  preload

16

• This graph depicts the Frank-Starling mechanism
  of compensation in CHF.
• The black curves represent ventricular function
  in a normal subject and the colored curve is with
  left ventricular dysfunction.
• Line N to A represents the initial reduction in
  cardiac output due to CHF.
• Line A to B represents the Frank-Starling
  mechanism of compensation an increase in left
  ventricular end-diastolic pressure needed to
  maintain cardiac output.

17
Stroke Volume
18
Stroke Volume
Increased sympathetic stimulation
Vasodilation from autoregulation
19
Cardiovascular drift

• Caused by a decrease in venous return
• Cardiac output is maintained by..?

20
Cardiovascular Drift
21
Stroke Volume

• SV greater in trained
• Most significant effect of training

22
Result

• An increase in cardiac output
• Increase HR
• Increase SV
• results in an increase in O2 supply

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24
Hemodynamics
25
Blood Vessels

• Arteries
• Arterioles
• Capillaries
• Venules
• Veins

26
Physical Characteristics of Blood

• Plasma
• Liquid portion of blood
• Contains ions, proteins, hormones
• Cells
• Red blood cells
• Contain hemoglobin to carry oxygen
• White blood cells
• Platelets
• Important in blood clotting

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The Blood
Arterial blood carries 20 ml of oxygen per 100
ml of blood
28
Hematocrit
Percent of blood composed of cells
29
The Blood

• Arterial blood 97-98 saturated with O2
• Venous blood
• Rest 75
• Exercise 25

30
Blood Pressure

• Expressed as systolic/diastolic
• Normal is 120/80 mmHg
• High is 140/90 mmHg
• Systolic pressure (top number)
• Pressure generated during ventricular contraction
  (systole)
• Diastolic pressure
• Pressure in the arteries during cardiac
  relaxation (diastole)

31
Blood Pressure

• Pulse pressure
• Difference between systolic and diastolic
• Mean arterial pressure (MAP)
• Average pressure in the arteries

Pulse Pressure Systolic - Diastolic
MAP Diastolic 1/3(pulse pressure)
32
Mean Arterial Pressure

• Blood pressure of 120/80 mm Hg
• MAP 80 mm Hg .33(120-80)
• 80 mm Hg 13
• 93 mm Hg

33
Hemodynamics

• Based on interrelationships between
• Pressure
• Resistance

34
Hemodynamics Pressure

• Blood flows from high ? low pressure
• Proportional to the difference between MAP and
  right atrial pressure (?P)

35
Blood Flow Through the Systemic Circuit
36
Hemodynamics Resistance

• Resistance depends upon
• Length of the vessel
• Viscosity of the blood
• Radius of the vessel
• A small change in vessel diameter can have a
  dramatic impact on resistance!

37
Hemodynamics Blood Flow

• Directly proportional to the pressure difference
  between the two ends of the system
• Inversely proportional to resistance

38
Sources of Vascular Resistance

• MAP decreases throughout the systemic circulation
• Largest drop occurs across the arterioles
• Arterioles are called resistance vessels

39
Pressure Changes Across the Systemic Circulation
40
Pressure Changes During the Cardiac Cycle
41
Factors That Influence Arterial Blood Pressure
42
Cardiovascular Control
43
How can the blood vessels increase blood flow?

• Vasodilation to increase blood flow to muscles
  and skin
• Waste products (metabolic or local control)
• Sympathetic stimulation (cholinergic)
• Vasoconstriction to maintain blood pressure
• Sympathetic stimulation (adrenergic)
• Maximum muscle blood flow is limited by the
  ability to maintain blood pressure

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Vasodilation
Vasoconstriction
45
Blood Vessels
46
Oxygen Extraction

• Measured as a-v O2 difference
• a O2 in arteries (20 ml/100 ml of blood)
• v O2 in veins (15 ml/100 ml of blood)
• (a-v)O2 5 ml/100 ml of blood

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a-v O2 difference

• No change in O2 content in the blood
• Remains at 20 ml/100 ml of blood
• Decrease in O2 inside the muscle
• Greater pressure difference between the blood and
  the muscles
• Oxygen moves from a HIGH pressure area (blood) to
  a LOW pressure area (muscle)
• Therefore, more O2 is extracted from the blood

High pressure to a Low pressure
High pressure to a Lower pressure
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RESTING
EXERCISE
20 ml or P02 98
20 ml or P02 98
15 ml extracted
5 ml extracted
PO2 40
PO2 20
Lower PO2 due to an increase in O2 consumption
(VO2) during exercise
49
Oxygen Consumption

• VO2
• liters per minute
• milliliters per kilogram per minute
• VO2 oxygen supply x oxygen extraction
• VO2 Q x a-v O2 difference
• VO2 HR x SV x a-v O2 difference

50
Oxygen Consumption

• An increase in oxygen supply leads to an increase
  in oxygen consumption
• Increase in cardiac output
• With help from HR and SV
• Increase in (a-v)O2
• More O2 is supplied and extracted
• Therefore, more O2 can be used by the muscle
  fibers (mito)

51
Oxygen Consumption

• Q and a-v O2 difference each account for 50 of
  the increase in VO2 during exercise
• Near maximal exercise, Q accounts for 75 of the
  increase in VO2

52
Oxygen Consumption

• VO2 increases with intensity
• VO2 rate of blood flow times the O2 extracted
  from a given amount of blood
• VO2 cardiac output x a-vO2 difference
• VO2 can increase by
• A greater blood flow
• Taking more oxygen out of every 100 ml of blood

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What limits aerobic exercise?

• Lack of oxygen supply?
• If so, wouldnt the muscles be more anaerobic?
• And, wouldnt the heart also be more anaerobic?
• But an anaerobic heart produces angina
• Maybe the central nervous system protects the
  heart from ischemia by causing muscle fatigue
  before the heart becomes anaerobic?