Cardiovascular Physiology: Cardiac Output and Stroke Volume

Questions and Answers

Which of the following formulas is used to calculate stroke volume?

Heart rate x End-systolic volume

End-systolic volume - End-diastolic volume

Cardiac output / Heart rate

End-diastolic volume - End-systolic volume (correct)

What is the normal value of ejection fraction?

60% (correct)

40%

80%

100%

What are the two factors that cardiac output depends on?

Stroke volume and end-diastolic volume

Contractility and preload

Stroke volume and heart rate (correct)

End-systolic volume and heart rate

What is the effect of an increase in contractility on ejection fraction?

It increases Signup and view all the answers

How can cardiac output be increased?

By increasing stroke volume, heart rate, or both Signup and view all the answers

What are the three factors that affect stroke volume?

Preload, contractility, and afterload Signup and view all the answers

What is the formula used to calculate left ventricular ejection fraction?

Stroke volume / End-diastolic volume Signup and view all the answers

What is the primary factor that influences end-diastolic volume and initial cardiac fiber length?

Diastolic filling and venous return Signup and view all the answers

According to the Frank-Starling Law, what is the relationship between end-diastolic volume and stroke volume?

Stroke volume increases as end-diastolic volume increases Signup and view all the answers

What is the effect of increased contractility on stroke volume?

Stroke volume increases due to decreased end-systolic volume Signup and view all the answers

What is the term for the intrinsic ability of myocardial cells to develop force at a given cell length?

Contractility Signup and view all the answers

What is the formula to calculate stroke volume?

Stroke volume = End-diastolic volume - End-systolic volume Signup and view all the answers

What is the effect of increased preload on stroke volume, according to the Frank-Starling Law?

Stroke volume increases as preload increases Signup and view all the answers

What is the term for the relationship between end-diastolic volume and stroke volume, as described by Otto Frank and Ernest Starling?

Frank-Starling Law Signup and view all the answers

What is the formula for stroke volume?

End-diastolic volume - End-systolic volume Signup and view all the answers

What determines the preload?

Venous return and diastolic filling Signup and view all the answers

What is the optimal resting length for skeletal muscle?

The normal resting length Signup and view all the answers

How does an increase in fiber length affect the contractile force of the heart?

It increases the contractile force Signup and view all the answers

What is the relationship between fiber length and contractile force in cardiac muscle?

An increase in fiber length increases contractile force Signup and view all the answers

What is the effect of preload on stroke volume?

It increases stroke volume Signup and view all the answers

What is the normal resting fiber length of cardiac muscle compared to the optimal length?

Shorter than optimal Signup and view all the answers

What is the primary effect of an increase in aortic blood pressure on stroke volume?

It decreases stroke volume Signup and view all the answers

What is the Anrep effect?

A mechanism that allows myocardium to compensate for increased end-systolic volume and decreased stroke volume Signup and view all the answers

What is the effect of sympathetic stimulation on contractility?

It has a positive inotropic effect Signup and view all the answers

What is afterload?

The force opposing ejection Signup and view all the answers

What is the effect of an increase in intracellular calcium concentration on contractility?

It increases contractility Signup and view all the answers

What is the effect of epinephrine on cardiomyocyte relaxation?

It speeds up relaxation Signup and view all the answers

What is the effect of increased afterload on end-systolic volume?

It increases end-systolic volume Signup and view all the answers

What is the effect of an increase in aortic pressure on the Frank-Starling mechanism?

It reduces the Frank-Starling mechanism Signup and view all the answers

What is the primary mechanism by which epinephrine increases contractility?

Through stimulation of beta 1 adrenergic receptors Signup and view all the answers

Study Notes

Cardiac Output

Cardiac output is the total volume of blood ejected by the left ventricle per unit time (mL/min).
It depends on stroke volume and heart rate.

Stroke Volume

Stroke volume is the volume of blood ejected out of the left ventricle during each systolic cardiac contraction.
It is the difference between end-diastolic volume and end-systolic volume.
Stroke volume (ml) = End-diastolic volume – End-systolic volume.

Ejection Fraction

Ejection fraction is the fraction (%) of the end-diastolic volume that is ejected in one stroke volume.
It describes the effectiveness of the ventricles in ejecting blood.
Normally around 60% (0.6).
It is an indicator of contractility.

Factors Affecting Stroke Volume

Preload (End-Diastolic Volume/Fiber Length)
- Influenced by diastolic filling and venous return.
- The greater the extent of diastolic filling, the larger the end-diastolic volume, and the more the heart is stretched.
- The more the heart is stretched, the longer the initial cardiac fiber length before contraction.
- The increased length leads to a greater force of contraction and a greater stroke volume.
- Frank-Starling mechanism: The greater the heart muscle is stretched during filling, the greater is the force of contraction, and the greater the quantity of blood pumped into the aorta.
Contractility or Inotropism
- Refers to the pumping ability of a ventricle.
- Intrinsic ability of myocardial cells to develop force at a given cell length.
- An increase in contractility leads to a more complete emptying of the ventricle during systole = DECREASE IN END SYSTOLIC VOLUME.
- Consequently, INCREASE IN STROKE VOLUME.
- Without the need to increase end-diastolic volume.
Afterload
- Is the resistance that the ventricles must overcome to empty its content – The force opposing ejection.
- The afterload for the left ventricle is the aortic pressure.
- When aortic blood pressure increases, stroke volume decreases, and end systolic volume/pressure increases.
- Can also be raised by obstructions to flow (i.e. semilunar valve stenosis).
- Stroke volume decreases as afterload increases.
- The Anrep effect allows myocardium to compensate for an increased end-systolic volume and decreased stroke volume that occurs when aortic blood pressure increases.

Control of Cardiac Output

Cardiac output can be increased only if stroke volume increases, heart rate increases, or both increase.
To understand how the body controls cardiac output, we must understand how the body controls stroke volume and heart rate.

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Description

Understand the concepts of cardiac output, stroke volume, and ejection fraction in the context of cardiovascular physiology. Learn how to calculate stroke volume and its relationship with heart rate and end-diastolic and end-systolic volumes.