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Chapter 20 Physiology (cardiac cycle)

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What is the cardiac cycle?
All of the events that amke up one heart beat (lub-dub)
What is the average duration of the cardiac cycle?
*~0.75 - 1 second in length

***This time frame can vary widely
How is heart rate calculated?
*Heart rate is dictated by the cardiac cycle

*Divide cardiac cycle time into 60

EXAMPLE:
-Cardiac cycle is 0.75 seconds long
-60 seconds in a minute
-Divide 60 by 0.75 to get heart rate

60/0.75= 80 beats/minute
Approximately how much of the cardiac cycle does the heart spend in systole?
~ 1/3 of its time
Approximately how much of the cardiac cycle does the heart spend in diastole?
~ 2/3 of its time
What does systole mean?
Contracting
What does diastole mean?
Relaxing
What is atrial systole?
Time the atria are contracting
What is ventricular systole?
Time the ventricles are contracting

***If the term systole is used without specifying atrial or ventricular, ventricular systole is ALWAYS implied
What is atriole diastole?
Time the atria are relaxing
What is ventricular diastole?
Time the ventricles are relaxing

******If the term diastole is used without specifying atrial or ventricular, ventricular diastole is ALWAYS implied
What are the five periods of the cardiac cycle?
1. Isovolumic contraction (systole)
2. Period of ejection (systole)
3.Isovolumic relaxation (diastole)
4. Passive ventricular filling (diastole)
5. Active ventricular filling (diastole)

***The cardiac cycle only focuses on events occurring in the ventricles, NOT atria)
What occurs during the isovolumic contraction period?
*Ventricles begin to contract, causing vetricular pressure to increase

*AV valves snap shut

*ALL HEART VALVES ARE NOW CLOSED so no blood flow out of the ventricle

***Blood volume does not change (isovolumic)

***First heart sound is heard (S₁)

***AV insufficiency can be heard at this time
What occurs during the period of ejection?
*First heart sound (S₁) still evident

*Further contraction of ventricles causes further increase in ventricular pressure

*Increase in pressure forces the pulmonary valve and aortic valve to open

*Blood empties from ventricles into the pulmonary trunk and aorta

*AV insufficiency can still be heard

*Semilunar stenosis is now heard
What is stroke volume?
*Stroke volume (SV) is the volume of blood pumped from one ventricle of the heart with each beat
Do the ventricles pump the same or different quantities of blood with each beat?
*Approximately the same amount of blood is pumped by each ventricle, even though the ventricles apply vastly different amounts of pressure when contracting
What is the end-diastolic volume?
*End-diastolic volume (EDV) is the volume of blood present in each ventricle at the end of diastole (relaxing) and beginning of systole (contracting)
What is the average value of end-diastolic volume?
~ 120 ml
What is the average stroke volume?
~ 70 ml is ejected by EACH ventricle with each heart beat
What is the ejection fraction?
*Percent of end-diastolic volume (EDV) left in each ventricle after the stroke volume (SV) has been ejected

*Based on normal EDV and SV values, the ejection fraction is 55%
How is the ejection fraction calculated?
Stroke volume (SV) = 70 ml
End-diastolic volume (EDV) = 120 ml

Divide SV by EDV x 100

(70ml / 120ml) x 100 = 55%
What is end-systolic volume?
*End-systolic volume is the volume of blood left in the ventricle at the end of systole (contacting) and the beginning of diastole (relaxing)
What is the average value of end-systolic volume?
~ 50ml
How is end-systolic volume calculated?
(End-diastolic volume) - (Stroke Volume) = End-systolic volume

(120 ml) - (70 ml) = 50 ml
What occurs during the isovolumic relaxation period?
*Relaxation of the ventricles

*Ventricular pressure decreases below pulmonary trunk and aortic pressures

*Semilunar valves snap shut

*ALL HEART VALVES ARE NOW CLOSED (isovolumic)

*Second heart sound (S₂) is heard ("dub")

*Semilunar insufficiency is heard at this time
What occurs during the passive ventricular filling period?
*Pressure within relaxed ventricles is lower than atrial pressure (because the atria are currently filling with blood)

*AV valves open (but atria is not yet contracting)

*Blood flows passively from the atria to the ventricle

*Semilunar insufficiency can still be heard

*AV stenosis heard at this time
What occurs during the active ventricular filling period?
*Atria contract and actively fill ventricles with blood

*At the end of this period, ventricles usually contain ~ 120 ml of blood (end-diastolic volume)