Cardiovascular System & Exercise

• Cardiovascular and respiratory systems respond to exercise to increase oxygen, and remove waste
• Limitations in these systems impact the ability to sustain aerobic exercise

Cardiovascular Responses to Acute Exercise

• Aims to increase blood flow to muscles
• Controls tissues and organs
• It is critical to examine heart and peripheral circulation function

Heart Rate (HR)

• Measuring involves taking pulse at radial or carotid artery, a good indicator of exercise intensity
• Resting heart rate (RHR) averages 60-80 bpm in most individuals
• Endurance-trained athletes can have resting rates as low as 28 bpm, due to increased parasympathetic tone
• Environmental factors can affect heart rate, increasing with extremes in temperature and altitude
• Preexercise HR increases above normal, termed the anticipatory response
• Mediated by norepinephrine and epinephrine release, alongside vagal tone decrease
• HR increases proportionally with exercise intensity until near-maximal exercise.
• HR plateaus as maximal exercise intensity is approached.
• Maximum heart rate (HRmax) is the highest value at volitional fatigue and a reliable value
• HRmax declines slightly yearly due to age-related factors.
• Predicted HRmax can be estimated by subtracting age from 220 bpm, or using the equation HRmax = 208 - (0.7 x age)
• Submaximal workload causes rapid HR increase until plateau, called steady-state heart rate
• Steady-state HR indicates optimal circulatory demands and is reached within 3 minutes
• Steady-state heart rate forms the basis for cardiorespiratory fitness tests
• Individuals exercise at standardized intensities on equipment, and those with better endurance have lower steady-state HR
• Lower steady-state HR at fixed intensity suggests better cardiorespiratory fitness

Measuring Heart Rate Variability (HRV)

• Heart rate variability measures the rhythmic fluctuation in HR
• Occurs due to sympathetic-parasympathetic balance controlling sinus rhythm
• HR variability analysis assesses contributions of nervous systems during rest and exercise
• Factors such as body temperature, nerve activity, and respiratory rate increase HR variability during aerobic exercise
• HR variability increases after exercise due to greater vagal tone
• Changes in HR variability assess impact of exercise training, overtraining, and clinical conditions
• HRmax commonly used to prescribe exercise intensity in training and rehab
• HRmax determined via individual exercise test to exhaustion
• It is verified by plateau despite increased exertion

Stroke Volume (SV)

• It also adapts with acute exercise to support exercise demands
• A major determinant of cardiorespiratory endurance at near-maximal intensities
• Stoke volume is determined by four factors:
  • Volume of venous blood (only pumps what returns)
  • Ventricular distensibility (enlarging ventricle capacity)
  • Ventricular contractility (inherent capacity)
  • Aortic/pulmonary artery pressure (pressure to contract against)
• The first two factors determine end-diastolic volume (EDV), or preload
• The last two influence ventricle emptying during systole under pressure, known as the afterload
• SV increases above resting values during exercise, up to 40-60% VO2max, then plateaus
• Other research indicates stroke volume continues past 40%. Some studies indicate that SV continues to maximal exercise intensity
• Upright position allows SV to double from rest to max, untrained SV increase from 60-70 ml/beat --> 110-130 ml/beat during maximal exercise
• Trained SV increase from 80-110 ml/beat --> 160-200 ml/beat at max
• Supine exercise increases SV by 20-40%, blood returns more easily to the heart in supine posture
• Increase might result from differences in exercise testing, 40-60% VO2max range typically used cycle ergometers
• The blood is pooled and decreases venous return from legs.
• A plateau may only apply to cycling
• Subjects in studies where SV continued to maximal exercise intensities were highly trained
• Highly trained athletes can increase SV beyond 40-60% VO2max due to increased venous return
• Frank-Starling mechanism improves ventricular filling and contraction force

Importance of Stroke Volume to VO2 max

• VO2 max is the single best measure of cardiorespiratory endurance
• VO2max establishes the limit of cardiovascular function

How Does Stroke Volume Increase During Exercise?

• SV increases despite less filling time, due to increased preload and ventricle stretch
• EDV increases when ventricle stretches during filling, resulting in forceful contraction known as Frank-Starling mechanism.
• Greater myocardial cell stretch leads to more actin-myosin cross-bridges and force.
• SV increases if ventricular contractility enhances via sympathetic stimulation
• Increased force of contraction can improve SV; low mean arterial pressure improves SV due to less aortic outflow resistance
• HR increase is tied to exercise intensity. High HR reduces filling time which could plateau EDV.
• Diastolic filling time could limit filling with increasing intensities and a reduced venous return
• Redistributing blood flow from inactive or splanchnic areas increases central blood, aiding Frank-Starling.

Reviewing Stroke Volume

• Increased venous return and ventricular contractility increase stroke volume, preload, and contraction.
• Decreased afterload from vasodilation aids SV.
• Decreased TPR in skeletal muscle vasodilation allows more blood to be expelled
• Decreased total peripheral resistance allows greater emptying.

Cardiac Output

• Cardiac output is the product of heart rate and stroke volume
• resting Cardiac output averages 5.0 L/min
• Varies by person and size
• Maximal cardiac output ranges from 20 L/min (sedentary) to 40+ L/min (elite athletes)
• Maximizing cardiac output depends on both body size and endurance training
• Cardiac output is linear with exercise intensity due to muscles' oxygen demand
• Reaches plateau at max intensity

The Fick Equation

• Developed in the 1870s by Adolph Fick
• It states that the oxygen consumption of a tissue depends on blood flow and oxygen extraction
• Can be applied to the whole body or regional circulations
• Oxygen consumption is the product of blood flow and the arterial-venous oxygen concentration difference
• VO₂ = Cardiac Output × (a-v)O₂ difference
• VO₂ = HR × SV × (a-v)O₂ difference

The Cardiac Response to Exercise

• HR is ~50 bpm reclining, ~55 bpm sitting, and ~60 bpm standing
• Gravity-induced blood pooling reduces SV which elevates heart rate
• Heart rate increases with walking, jogging, and reaching 180 bpm
• Withdraw parasympathetic tone to increase from 60 to 90bpm
• Sympathetic nervous system activates and stroke volume increases
• Increases as cardiac output for untrained
• Stroke volume increase plateaus or slows from 40-60% of max, HR contributes
• Stroke volume rises as intensely trained athlete's

Blood Pressure

• Systolic BP increases or proportion with exercise intensity
• Diastolic BP does not change or decreases
• Mean arterial BP increases
• Systolic pressures at 120 mmHg normally at rest exceed 200 mmHg at maximal exercise (240-250 mmHG reported in trained atheletes)
• Increased systolic blood pressure increases rates of work to increase blood flow with vasculature
• Hydrostatic pressure increases plasma release to capillaires to increase supplies needed for muscles
• After the initial increase, mean arterial pressure reaches a steady state
• Systolic blood pressure increases with work intensity
• If exercise is prolonged, systolic pressure decreases gradual and diastolic pressure remains constant
• Decline of systolic is normal vasodilation
• Diastolic varies negligibly with submaximal exercise; it may rise slightly when exercise is at it's max
• Diastolic increases when heart at rest (diastole). Dynamic activates sympathetic increases vascular tone overall vasoconstriction
• Vasoconstriction by releases vasodilatrs

Blood Pressure and the Body

• Upper-body exercise raises BP because if smaller mass in upper compared with lower
• Needs more energy to stabilize upper. Difference if arm than lower-exercise
• SBP are related to product to HR: myocardial oxygen uptake and blood flow rate-pressure product: HRx SBP
• RPP increases with body, exercise, static and exercise.
• RPP is indirect measurement, it's is key to testing.

Blood Pressure Increase with Resistance

• Weightlifting increases intense high.
• Extreme BP occurs as Valsalva heavy
• Mouth, nose,glottis is blocked. Trunks and pressure rises much BP high.

Blood Flow

• Acute increases in cardiac output and blood pressure during exercise = Blood Flow
• Additional blood redirected
• Sympathetic nervous systems to redirect greatest to blood flows.

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