This document discusses the mechanical activity of the heart, including cardiac output, stroke volume, and factors that affect them. Cardiac output is the volume of blood pumped per unit time, equal to heart rate multiplied by stroke volume. Stroke volume is the difference between the ventricle volume before and after contraction. Factors like preload (ventricular stretch), afterload (resistance against ejection), contractility (strength of contraction), and heart rate influence stroke volume and thereby cardiac output. Changes in these factors can increase or decrease the blood the heart pumps with each beat.

1. MECHANICAL ACTIVITY OF
THE HEART
Abuzar Tabassum

2. INTRODUCTION
• Each action potential (electrical activity) in heart
muscle followed by contraction (mechanical
activity).
• The mechanical aspects of heart function relate
to the
• Changes in cardiac pressure and
• Volume that lead to ejection of blood during each
heartbeat.

3. CARDIAC OUTPUT, STROKE
VOLUME, AND HEART RATE
• Cardiac output is the volume of blood pumped per
unit time from a ventricle.
• In mammals it is defined as the volume ejected from
the right or left ventricle, not the combined volume
from both ventricles.
• The volume of blood ejected by each beat of the heart
is termed the stroke volume.
• The mean stroke volume can be determined by
dividing cardiac output by heart rate.

4. • Stroke volume is the difference between the
volume of the ventricle just before contraction
(end-diastolic volume) and the volume of the
ventricle at the end of a contraction (end-systolic
volume).
• Changes in stroke volume may result from changes
in either end-diastolic or end-systolic volume.

5. • The end-diastolic volume is determined by four parameters:
• Venous filling pressure
• Pressures generated during atrial contraction
• Distensibility of the ventricular wall
• The time available for filling the ventricle

6. CARDIAC OUTPUT
• Cardiac output is defined as the amount of blood
pumped per ventricle per unit time.
Cardiac output = heart rate x stroke volume

7. FACTORS THAT AFFECT
CARDIAC OUTPUT
• Contractility
• Heart rate
• Preload
• Afterload

8. FACTORS THAT AFFECT CARDIAC OUTPUT
CONTRACTILITY
STROKE VOLUME
CARDIAC OUTPUT
PRELOAD AFTERLOAD
HEART RATE•Synergism of contraction
•Ventricular integrity
•Valve competent

9. THE EFFECTS OF CONTRACTILITY
• Contractility depend on the rate of ca2+ to enter
myocytes
• Contractility increase cardiac output.
• Changes in heart rate affect myocardial contractility.
• The ejection fraction (a measurement, expressed as a
percentage, of how much blood the left ventricle pumps out with each
contraction) is a good measure of contractility.
• Contractility of the heart is the heart’s performance
independent of loads.

10. THE EFFECTS OF HEART RATE
• Heart rate is the speed of the heartbeat measured by the
number of contractions (beats) of the heart per minute
(bpm).
• The relationship between heart rate and cardiac output is
complex.
• Heart rate affected cardiac output by changes ventricular
filling.
• Cardiac output is varied depend on heart rate.
• Heart rate more than 150 times/min decreased cardiac
output

11. THE EFFECTS OF PRELOAD
• Preload, also known as the left ventricular end-diastolic
pressure (LVEDP), is the amount of ventricular stretch
at the end of diastole.
• Preload is ventricular pressure at the end of diastolic
phase.
• Preload depends on:
• Filling pressure,
• Filling time,
• Distensibility of ventricular wall (the capacity to swell as a result of
pressure from inside.)

12. Cardiac Muscle Length-Tension Curve
Cardiac muscle has similar
length-tension properties to
skeletal muscle.
Cardiac muscle normally
operates well below optimum
length.
Increasing ventricular
volume stretches the
ventricular muscle towards
optimum length.
Stretching the ventricles
increases the pressure they
can generate.

13. Cardiac Function Curve and Pre-load
Increasing venous return
increases the ventricular
end diastolic volume and
stretches the ventricles.
Venous return determines
the pre-load (cardiac end
diastolic pressure) on the
heart.
Normally increasing venous
return increases the force of
contraction.

14. THE EFFECTS OF AFTERLOAD
• Afterload is the force or load against which the heart has
to contract to eject the blood.
• Also known as the systemic vascular resistance (SVR)
• Afterload is the aortic pressure during ejection phase.
• The most realistic indicator for afterload is arterial
blood pressure
• Complex relationship exist between afterload and cardiac
output

15. The Arterial Pressure After-loads the Heart

16. Filling time Filling pressure Contractility
Distensibility
End distolic
volume
End systolic
volume
Heart
rate
Stroke
volume
Pheripheral
Resistant
Cardiac
output
Blood pressure Afterload

17. STROKE VOLUME
• Stroke volume (SV) is the volume of blood in
millilitres ejected from the each ventricle due to the
contraction of the heart muscle which compresses
these ventricles.
• Sv is the difference between end diastolic volume
(edv) and end systolic volume (esv).

18. FACTORS AFFECTING THE
STROKE VOLUME
• Multiple factors can affect SV
• eg. Factors that change either EDV or ESV will
change SV.
• The three primary factors that regulate SV are
preload, afterload and contractility.