Cardiovascular System Responses to Exercise - HR and SV

Major Challenge Posed by Exercise

muscular demand for O2

CV System Puposes

• transport O2 (nutrients & hormones)

• removal of CO2 (+ waste products)

• regulation of body temperature

2 Major Adjustments of Body Function During Exercise

1. increase cardiac output (more blood out of heart per min)

2. blood flow redistribution (to active tissue)

Cardiac Output (CO)

Q = HR x SV

Blood Flow

Flow = pressure/resistance

• while maintaining BP

Important Factors

1. how much blood can the heart pump out? e.g. cardiac output

2. can the vasculature receive that blood? e.g. BP

3. can that blood be delivered to working muscle? e.g. perfusion

The CV System

• closed loop system that circulates blood throughout the body

• heart: muscular pump

• arteries: carry blood away from heart

• veins: return blood to heart

• channels: blood vessels

• fluid medium: blood

Cardiac Cycle

repeated contraction and relaxation of the heart

• SA node influence at rest

Cardiac Cycle Phases

• systole: contraction phase; 0.3 seconds

• diastole: relaxation phase (ventricles relax and fill); 0.5 seconds

Diastole

shortens as HR increases

• allowing the heart to beat faster

Resting Heart Rate (RHR)

• normal: 60-85 bpm

• average: 70 bpm

• elite endurance athlete: 28-40 bpm

HR at Rest

• ventricles eject 2/3 of volume

• typical resting HR 70-75 bpm

• SA node determines rate

HR during exercise

• HRmax can be approx. 180 bpm

• large decrease in cardiac cycle

HR is a Good Indicator Of

execise intensity

• increase steadily with exercise

Decrease HR For a Given Workload Indicate

a more efficient heart (less stress on heart)

Regulation of HR - PNS

decreases HR (“breaking system”)

Regulation of HR - SNS

increases HR

• releases norepinephrine

Initial Increase HR Due To

PNS withdrawl

• rest to 100 bpm

• SA node 100 bpm

Rest of Increase HR Due To

SNS activity

• cardiac accelerator nerves

The PNS Fibres Release

ACh causing hyperpolarization making it harder to depolarize → decreases HR

Maximal HR - Equation

220 - age in years

• decline 1 bpm/year

Untrained Athletes HR

• RHR = 70 bpm

• HRmax = 200 bpm

Trained Athletes HR

• RHR = 60 bpm

• HRmax = 200 bpm

Elite Athletes HR

• RHR = 30-50 bpm

• HRmax = 180 bpm

Heart As A Pump

myocardial cells contract togetjer to generate enough force

• act as a single unit made of many muscle fibres

Larger Force of Contraction

more blood is pumped out

Stroke Volume

EDV - ESV

SV Increases Due To

• increased venous return → increased EDV

• increased sympathetic activity → increased contractility

Stroke Volume Determined By

1. end-diastolic volume

2. vascular resistance

3. contractility

End-Diastolic Volume AKA

pre-load

• discovered by Frank & Starling

End-Diastolic Volume

as the LV stretches, SV increases

• due to enhancing actin-myosin overlap

Frank-Starling Law of the Heart

• increased EDV stretches cardiac muscle fibers

• results in stronger ventricular contraction

What Alters EDV

venous return: amount of blood returning to heart

• increased venous return → increased EDV → increases SV

3 Primary Factors of Venous Return

1. venoconstriction (increases venous pressure)

2. muscle pump (increase amount of blood moved back to heart)

3. respiratory pump

Vascular Resistance is AKA

afterload

Vascular Resistance

• aortic pressure

• ability of vascular system to receive SV

• SV is inversely proportional to afterload

Increased Afterload

decreases SV

• makes it harder to eject blood

Arteriole Vasodilation

reduces afterload

Contractility

• force of heart contraction

• altered by circulating concentrations of SNS catecholamines (NE and Epi)

Contractility Increase

• SNS catecholamines → increase contractility → increase SV

• EDV

Ejection Fraction

portion of blood pumped out of LV each beat

Well Trained Endurance Athletes

may reach a SV of 200 mL/beat

SV During Exercise

• increase lineraly with exercise

• levels off at 40-60% VO2max

Elite Athletes Rely On

HR and SV to increase CO

Cardiac Output

amount of blood pumped by the heart per minute

Cardiac Output Equation

CO = HR x SV

• in L/min

CO Reflects The

functional capacity of the CV system