1102 objective 2 pbl 2 cardiac cycle . (1)

8/19/2019 1102 OBJECTIVE 2 PBL 2 Cardiac Cycle . (1)

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Heart rate is the number of times the heart beats in one minute. The average adult resting heart

rate is 75 beats per minute (bpm). Stroke volume is the amount of blood pumped by each

ventricle with each heartbeat. The average adult resting stroke volume is 7 ml per beat. !ardiacoutput is the amount of blood pumped out by each ventricle in one minute. "t is directly related to

heart rate and stroke volume

Heart Rate x Stroke Volume = Cardiac Output

The S# node of the heart is enervated by both sympathetic and parasympathetic nerve fibers.

$nder conditions of rest the parasympathetic fibers release acetylcholine% which acts to slow the

pacemaker potential of the S# node and thus reduce heart rate. $nder conditions of physical oremotional activity sympathetic nerve fibers release norepinephrine% which acts to speed up the

pacemaker potential of the S# node thus increasing heart rate. Sympathetic nervous system

activity also causes the release of epinephrine from the adrenal medulla. &pinephrine enters the blood stream% and is delivered to the heart where it binds with S# node receptors. 'inding of

epinephrine leads to further increase in heart rate.

End-diastolic Volume

#n increase in venous return of blood to the heart will result in greater filling of the

ventricles during diastole. !onseuently the volume of blood in the ventricles at the end of diastole% called end-diastolic volume% will be increased. # larger enddiastolic volume will

stretch the heart. Stretching the muscles of the heart optimi*es the lengthstrength

relationship of the cardiac muscle fibers% resulting in stronger contractility and greaterstroke volume.


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"ncreased sympathetic stimulation due to fright or anger increases heart rate. The cardiac muscle

cells of the ventricular myocardium are richly enervated by sympathetic nerve fibers. +elease of

norepinephrine by these fibers causes an increase in the strength of myocardiall contraction% thusincreasing stroke volume. ,orepinephrine is thought to increase the intracellular concentration of

calcium in myocardial cells% thus facilitating faster actin-myosin cross bridging. #lso% a general

sympathetic response by the body will induce the release of epinephrine from the adrenal

medulla. &pinephrine% like norepinephrine will stimulate an increase in the strength of

myocardial contraction and thus increase stroke volume.

• Age

#n infant has a higher heart rate than adults% usually at 5 beats per minute. This gradually

decreases and stabili*es to around bpm as the person matures.

• Gender

/emales have a usually higher resting heart rate than male adults.

• P!sical "itness

# physically active person usually has a resting heart rate at around 5 bpm.

• #od! temperature

"ncrease in temperature will also increase your cardiac output.

"t is therefore important that a person should be aware of these factors% to avoid applying unduestress and pressure on the heart muscles.

!hronic high blood pressure% smoking% drug use% kidney disease% high cholesterol% lack of

e0ercise% poor diet and other genetic and lifestyle factors can affect the heart1s ability to contract

$% &nvestigate te "actors tat contri'ute to cardiomegal! and

o( te! a""ect cardiac output%

$nder conditions of rest% the heart does not fill to its ma0imum capacity. "f the heart were

to fill more per beat then it could pump out more blood per beat% thus increasing strokevolume. #lso% the ventricles of the heart empty only about 52 of their volume during

systole. "f the heart were to contract more strongly then the heart could pump out more

blood per beat. "n other words% a stronger contraction would lead to a larger strokevolume. 3uring periods of e0ercise% the stroke volume increases because of both these

mechanisms4 the heart fills up with more blood and the heart contracts more strongly.

Stroke volume is increased by mechanisms6

1. increase in enddiastolic volume

2. increase in sympathetic system activity

http://www.livestrong.com/blood-pressure/http://www.livestrong.com/high-cholesterol/http://www.livestrong.com/high-cholesterol/http://www.livestrong.com/blood-pressure/


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and pump blood.

1102 objective 2 pbl 2 cardiac cycle . (1)

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