Cardiac Physiology

Cardiac Output definition

• amount of blood pumped by each ventricle in 1 minute

Heart Rate definition

• number of contractions of the ventricles each minute

Stroke Volume Definition

Amount of blood ejected from each ventricle with each contraction

Cardiac Output Equation

• CO=HR X SV

What factors impact Heart Rate?

• sympathetic nervous system

• epinephrine

What factors impact Stroke Volume

• venous return (preload)

• Blood volume

• Sympathetic nervous system (contractility)

• INOTROPIC FACTORS

• Peripheral Resistance (afterload)

Factors that determine cardiac output

• heart rate

• stroke volume

What are factors that increase heart rate

• sympathetic nervous system

• epinephrine, norephrine

• thyroid hormone

• glucagon

• nicotine, caffeine

• HYPOcalcemia

What are factors that increase stroke volume

• increased preload

• positive inotropic agents

• sympathetic nervous system

• epinephrine, norephrine

• glucagon

• digitalis

• nicotine, caffeine

• HYPERcalcemia

what are factors that reduce heart rate?

• negative chronotropic agents

• parasympathetic nervous systen

• acetylcholine

• HYPERcalcemia

• HYPOkalemia

• beta blockers

what are factors that reduce stroke volume?

• reduced preload

• reduced contractility

• increased afterload

• negative inotropic agents

• HYPOcalcemia

• HYPERkelemia.

Factors that affect heart rate

• age

• autonomic innervation

• hormones

• fitness levels

• age

factors that affect stroke volume

• heart size

• fitness level

• gender

• contractility

• duration of contraction

• preload

• afterload

What does the vagus nerve do

• decrease heart rate

what do sympathetic cardiac nerves do

• increase heart rate and force contraction

How does acute exercise influence cardiac output?

• increases cardiac output

• increases preload because of venous return

How does acute exercise influence Stroke Volume?

• increases it

• increases contractility as well as intracellular Ca2+

How does acute exercise influence heart rate?

• increases it

• increases rate of depolarization

How does acute exercise influence afterload?

• decreases it

• SNS and local factors cause vasodilation

How does chronic exercise influence cardiac output at rest?

• stays the same

How does chronic exercise influence stroke volume at rest?

• increases it

• contractility increases

How does chronic exercise influence heart rate at rest?

• decreases it

• parasympathetic nodal cells hyperpolarize

Preload

• amount of tension in ventricular myocardium immediately before it contracts

Afterload

• sum of all forces a ventricle must overcome before it can eject blood

Contractility

• how hard the myocardium contracts for a given preload

How does increasing preload influence stroke volume?

• higher stroke volume

  • more blood in ventricles

  • stretches heart muscle more

How does decreasing preload influence stroke volume?

• decreases stroke volume

  • less blood in ventricles

  • weaker contraction

How does increasing afterload influence stroke volume?

• reduces stroke volume

  • heart needs to generate more pressure to eject blood

  • prolongs ejection time

  • decreases amount pumped out with each beat

How does decreasing afterload influence stroke volume?

• increases stroke volume

  • heart doesn’t have to work as hard to pump blood

  • allows more blood to be ejected with each beat

How does decreasing contractility influence stroke volume?

• reduces stroke volume

  • heart muscle contracts with less force

  • expels less blood per beat

How does increasing contractility influence stroke volume?

• increases stroke volume

  • heart muscle contracts with more force

  • expels more blood per beat

How does PSNS influence cardiac output

• decreases heart rate which causes lower cardiac output

Why does PNS lower cardiac output?

• vagus nerve releases acetylcholine that slows SA node firing

• heart pumps blood less frequently

How does SNS influence cardiac output?

• increases cardiac output

• releases epinephrine that stimulates heart to beat at greater force

• increases heart rate, contraction, leading to a higher stroke volume

How does taking a Beta blocker influence cardiac output?

• slows the heart rate

• reduces force at which heart contracts

• blocks effects of epinephrine and SNS

Cardiac Output

= output of ventricles/minute

• av. 5-6 L/min

Stoke Volume

= output of ventricles/beat

• 70-80 ml/beat

Heart Rate

=beats/minute

• 60-100 beats/min