Cardiovascular System & Exercise

• This module focuses on the cardiovascular system and its responses to exercise.

Learning Objectives

• Review the structure and function of the heart, vascular system, and blood.
• Understand how the cardiovascular system responds to increased demands during exercise.
• Explore the role of the cardiovascular system in delivering oxygen and nutrients to active body tissues.
• Calculate training heart rate in athletes and non-athletes.

Major Cardiovascular Functions

• Delivery of oxygen and nutrients
• Removal of carbon dioxide and waste products
• Transportation of hormones
• Maintenance of body temperature and pH
• Prevention of infection and support of immune function

Cardiovascular System Components

• A pump (the heart)
• A system of channels (blood vessels, arteries, veins, capillaries)
• A fluid medium (the blood)

Anatomy of the Heart

• Key structures include the atria, ventricles, aorta, vena cava, pulmonary artery and veins, and various valves.

The Heart

• The apex points down and to the left.
• The average size is 5.5 inches long and 3.5 inches wide.
• Surrounded by a pericardial sac (pericardium) to prevent over-distension.

Wall of the Heart

• Epicardium: outer layer
• Myocardium: muscle layer that forces blood out of the heart chambers
• Endocardium: inner layer

Heart Chambers

• Left ventricle: most powerful chamber; can increase in size due to exercise

Blood Vessels

• Veins: carry blood toward the heart
• Arteries: carry blood away from the heart
• Capillaries: connect arteries to veins and facilitate the exchange of oxygen, carbon dioxide, nutrients, and wastes

Heart Size Adaptations

• The left ventricle changes the most in response to endurance training.
• The internal dimensions of the left ventricle increase due to increased ventricular filling.
• The wall thickness of the left ventricle increases, allowing a more forceful contraction.

Blood Supply to the Heart

• Provided by the right and left coronary arteries, which arise from the base of the aorta and encircle the myocardium
• Blood passing through the capillaries in the heart muscle is drained by cardiac veins.
• Cardiac veins join the coronary sinus.

Electrical Conduction System of the Heart

• Sinoatrial (SA) node: the pacemaker, generates an impulse of 60-80 beats/min (normal sinus rhythm)
• Atrioventricular (AV) node: delays the impulse by 0.13 seconds to allow atria to contract and force blood into the ventricles

Electrical Conduction System of the Heart (continued)

• AV bundle (bundle of His): conducts the impulse from the AV node to the ventricles
• Bundle branches: extensions of the AV bundle (His-Purkinje system) that carry the impulse to the Purkinje fibers
• Purkinje fibers: extensions of the bundle branches that contract the ventricles and transmit the impulse approximately 6 times faster than through the rest of the system

Heart's Regulation of Electrical Impulses

• Parasympathetic nervous system: decreases heart rate and force of contraction; exercise decreases PNS stimulation
• Sympathetic nervous system: increases heart rate and force of contraction; stimulated by stress; exercise increases SNS stimulation
• Endocrine system:
  • Epinephrine and norepinephrine: released due to sympathetic stimulation, increasing heart rate
  • Acetylcholine: released due to parasympathetic influence, decreasing heart rate

Electrocardiogram (ECG)

• ECG complexes reflect the heart’s electrical activity.

Phases of a Resting ECG

• Depolarization phase: membrane potential becomes less negative, reaches zero, then becomes positive
• Repolarization phase: membrane potential is restored to the resting state of -70 mV
• U wave: repolarization of the Purkinje system or the terminal phase of ventricular repolarization

ECG Intervals and Waves

• Atrial contraction follows the P wave.
• Ventricular contraction follows the QRS interval

ECG Reading

• X-axis: time (seconds)
  • One small box = 1 mm = 0.04 seconds
  • Five small boxes = 5 mm = 0.20 seconds
• Y-axis: electrical impulse (mV)
  • 1 mm = 0.1 mV
  • 5 mm = 0.5 mV

Most Accurate Calculation

• The standard ECG paper speed is 25 mm/sec.

HR (bpm) = (25 mm/s x 60 s/min) / mm/beat

• Simplified Formula:

HR = 1500 / # of small boxes (between 2 consecutive beats, 2 R waves)

Estimation by Large Squares

• 1 large square = 300 bpm
• 2 large squares = 150 bpm
• 3 large squares = 100 bpm
• 4 large squares = 75 bpm
• 5 large squares = 60 bpm
• 6 large squares = 50 bpm

Heart Rate Calculation Using the Kochan Method

• More accurately determines heart rate by considering the fractions of large squares between R waves.
• If R waves are between 3 and 4 large squares, HR = 75 + 5 + 5 + 5 = 90 bpm.

Six-Second Strip Method

• Count the number of R waves in a 6-second strip and multiply by 10.
• Least accurate.

ECG Responses During and Post-Exercise

• Altered action potential duration, conduction velocity, and contractile velocity due to increased heart rate result in normal ECG changes in healthy individuals.
  • Interval between P wave and QRS decreases
  • Shortening of QRS complex
  • R wave height may increase slightly from rest to submaximal exercise, however, R wave decreases at maximal exercise
  • RR interval decreases
  • QT interval shortens
  • Superimposition of P waves and T waves on successive beats may be observed
  • ST segment depression with increasing HR
  • Upsloping of ST segment
  • Tall, peaked T waves occur
  • Increased Q wave

Cardiac Cycle

• Events that occur between two consecutive heartbeats (systole to systole)
• Diastole: Relaxation phase during which the chambers fill with blood (T wave to QRS) - 62% of cycle duration
• Systole: Contraction phase during which the chambers expel blood (QRS to T wave) - 38% of cycle duration

Cardiac Cycle Duration

• Rest: 60 sec/cycle / HR = 75 beats/min = 0.8 seconds
• Exercise: 60 sec/cycle / HR = 150 bpm = 0.4 seconds

Phases of the Cardiac Cycle

• Ventricular filling (diastolic relaxation)
• Isovolumetric contraction (systolic contraction)
• Ventricular ejection (systolic contraction)
• Isovolumetric relaxation (diastolic relaxation)

Heart Rate Definitions

• Normal resting heart rate (NSR): 60–100 bpm
• Bradycardia: less than 60 beats/min
• Tachycardia: greater than 100 beats/min

Heart Rate and Endurance Training

• Resting heart rate decreases with endurance training, likely due to more blood returning to the heart.
• Sedentary individuals can decrease resting heart rate by 1 beat/min per week during initial training.
• Resting heart rate prior to an exercise test should be below 100 bpm.

Heart Rate During Exercise

• Submaximal: Decreases proportionately with the amount of training completed; may decrease by 10–30 beats/min after 6 months of moderate training
• Maximal: Remains unchanged

Estimating Maximum Heart Rate (HRmax)

• HRmax = 220 – age (less accurate)
• Tanaka equation: HRmax = 208 – (0.7 x age) (more accurate)
• Safety Limit: 85% HRmax = 0.85 x [208 – (0.7 x age)]

Karvonen Formula

• Used to predict work intensity:
• HRR = (HRmax – HRrest) × %intensity + HRrest

Borg Scale

• Used to measure rate of perceived exertion during exercise.
• Helps determine the intensity of the work.

Heart Rate Recovery Period

• The time it takes for the heart rate to return to its resting rate after exercise
• With exercise training, heart rate returns to resting level more quickly.

Stroke Volume (SV)

• Volume of blood pumped per contraction (beat)
  • SV = EDV – ESV

Cardiac Output (Q)

• Total volume of blood pumped by the ventricle per minute
  • Q = HR × SV

Stroke Volume (SV)

• A determinant of cardiorespiratory endurance capacity at maximal rates of work
• SV increases with increasing intensity up to 40-60% maximal capacity, then plateaus
• Magnitude of changes in SV depends on position of the body during exercises

Factors Affecting Stroke Volume

• Preload: Volume of blood in the heart at the end of diastole
  • In trained individuals it increases from rest to exercise
• Contractility: Force of ventricular contraction
  • Trained individuals have higher contractility
• Afterload: Resistance presented to contracting ventricle
  • Trained individuals would have a lower afterload

Ejection Fraction (EF)

• Proportion of blood pumped out of the left ventricle each beat
• EF = SV (EDV – ESV) / EDV
• EF averages 60% at rest

Typical Strove Volumes at Rest and Maxiimal Exercise

• Untrained: 50-70 ml (rest), 80-110 ml (max)
• Trained: 70-90 ml (rest), 110-150 ml (max)
• Highly trained: 90-110 ml (rest), 150-220 ml (max)

Stroke Volume Increases During Exercise Due To:

• Frank-Starling mechanism: greater stretch leads to greater contraction.
• Increased ventricular contractility
• Decreased total peripheral resistance resulting from increased vasodilation in active muscles

Cardiac Output

• Increases directly with increasing exercise intensity, up to between 20 and 40 Liters per mintue
• The magnitude depends on size
• Above 40-60% intensity, heart rate increases more than Stroke Volume

Relative Distribution of Cardiac Output

• At rest, 15-20% of cardiac output goes to the muscle.
• During exercise, 80-85% goes to the muscle.

Blood Pressure

• Blood pressure is the force exerted by the blood against the walls of the blood vessels.
  • BP = cardiac output x peripheral resistance
  • BP = (stroke volume x HR) x peripheral resistance
  • BP = ((EDV – ESV) x HR) x peripheral resistance

Blood Pressure Measurements

• Systolic blood pressure (SBP): highest pressure
• Diastolic blood pressure (DBP): lowest pressure
• SBP: Provides an estimate of the work of the heart
• DBP: Indicates peripheral resistance

Normative Blood Pressures

• Normal resting BP = 120/80 mmHg
• Minimum BP of 80/50 is required
• Exercise systolic blood pressure should be < 160 mmHg
• Exercise diastolic blood pressure should be < 90 mmHg

Mean Arterial Pressure (MAP)

• MAP = DBP + [0.333 × (SBP – DBP)]
• MAP indicates the relationship between cardiac output and peripheral resistance

Measuring Total Volume Blood Flow

Q= MAP/TPR
TPR = MAP/Q

Total Volume Blood at Rest

MAP = 93
Q = 5 L/min
TPR = 19 mmHg/L/min

Total Volume Blood During Heavy Exercise

MAP = 120
Q = 5 L/min
TPR = 19 mmHg/L/min

Volume Blood Adjustments During Exercise

• Total flow increases by 5x
• Driving pressure increases by 30%
• Resistance decreases by 4x

Active Muscles

• Vasodilation of blood vessels
• Decreased resistance
• More blood flow

Non-Active Muscles

• Vasoconstriction
• Increased resistance
• Less blood flow

Blood pressure During Exercise

• Systolic BP increases in direct proportion to increased exercise intensity during cardiovascular endurance exercises
• Diastolic BP changes little
• Muscle contraction occludes blood vessels, elevates pressure
• Resistance Exercise can go as high as 480/350 mmHG because of this
• The Valsalva maneuver contributes

Additional Notes on Blood Pressure

• Upper body musculature causes increases in BP
• Less total volume of blood in upper muscles also has an effect

Double Product

• Estimating "workload of the heart."
• The product of HR and SBP HR * SBP

Cardiovascular Responses to Acute Exercise

• Heart rate increases.
• Stroke volume increases.
• Cardiac Output increase.
• Blood flow and blood pressure increases

The Blood

• Transports gas, nutrients, and wastes
• Regulates temperature
• Buffers and balances acid base

Whole Blood Composition

• plasma (55%)
• formed elements (45%)

Resting Blood Oxygen

• Arterial: 20 mm
• Venous: 14 mm
• AvO2 difference: 6

Arterial Blood During Exercise

• O2 arterial blood typically doesn't change from rest to exercise
• The increase in a-v O2 difference with exercise means the muscle can extract more O2