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Cardiac Cycle All events related to the flow of blood through the heartduring one heart beat is called cardiac cycle. When heart rate is 75 beat/min then a cardiac cyclelasts 0.8 second. The event taking place during the cardiac cycle are thefollowing.Atrial systole: Duration .1 sec Atria are contracted and at the same time ventriclerelax. Atrial systole contribute 25ml of blood to the volumealready in each ventricle(125 ml). The end of atrial systole is also the end of ventriculardiastole. Thus each ventricle contain about 130 ml of blood. It is marked by the P wave of ECG.

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Ventricular systole: Duration 0.3 second Ventricle are contracted at the same time the

atrium relax It is marked by the QRS complex of the ECG. The left ventricle eject about 70 ml of blood

into aorta and the right ventricle eject thesame volume into the pulmonary trunk.

The volume remaining in each ventricle is 60ml and this volume is called end systolicvolume.

Relaxation Period:

During this period the atria and ventricle bothare relax. As the heart beat faster and faster the

relaxation period become shorter and shorter

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Heart Sound: The sound of heart beat comes from blood

turbulence caused by closing of the heart valve.

During each cardiac cycle there are four heartsounds but in normal heart only the 1st and 2ndsound (S1 and S2) are loud enough to be heardwith stethoscope.

S1 lubb sound is caused by blood turbulenceassociated with closure of AV valve.

S2 dupp sound is caused by blood turbulenceassociated with closure of SL valve.

S3 is due to blood turbulence due to rapid

ventricular filling. S4 is due to blood turbulence during atrial

systole.

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Heart at rest Blood flows from large veins into atria Passive flow from atria into ventricles

Atria (R & L) contract simultaneously Blood forced into ventricles

Ventricles (R & L) contractsimultaneously Atrioventricular valves close lubb

sound Blood forced into large arteries

Ventricles relax Semilunar valves close dub sound

Heart at rest

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Voltage-gatedfast Na+ channelsopen Rapid influx of

Na+ anddepolarization

Release of Ca2+from SR andextracellularspace Plateau phase:

Depolarization

prolonged Opening K+

channels &repolarization

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Heart isautorhythmic

Depolarizationbegins in sinoatrial(SA) node

Spread through

atrial myocardium Delay in

atrioventricular (AV)node

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Spread fromatrioventricular(AV) node AV bundle

Bundle branches

Purkinje fibers

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Depolarization inSA node precedesdepolarization in

atria, AV node,ventricles

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Method developed by Wilhelm Einthoven Dutch Elektrokardiogram (EKG)

Now usually ECG.

Records electrical events (movements ofions) in heart. Variations in electrical potential radiate from

heart; detectable at wrists, ankles.

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P wave Depolarization of atria Followed by contraction

QRS complex 3 waves (Q, R, & S)

Depolarization ofventricles Followed by contraction

T wave Repolarization of

ventricles

P-Q interval Q-T interval

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P-Q interval (or P-R)

Time atriadepolarize &remain depolarized

Q-T interval Time ventricles

depolarize &remain depolarized

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Cardiac Output = volume of blood pumped perminute Product of Heart Rate (beats/ min.) X Stroke

Volume (vol./ beat) At rest:

75 b/ min. X 70 ml/ b = 5250 mL/ min. = 5.25 L/ min. During exercise:

100 b/ min. X 110 ml/ b = 11000 mL/ min. = 11.00 L/min.

Extreme exercise:

120 b/ min. X 125 ml/ b = 15000 mL/ min. = 15.00 L/min.

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Regulation of Heart Rate Electrolytes

Na+, K+

Ca+

Hormonal regulation

Epinephrine Thyroxine

Other factors

Gender, age, size, health,

physical fitness

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Regulation of HeartRateAutonomic regulation

O2, CO2, H+ in blood

stimulate cardiovascularcenters in medulla

Sympathetic

CO2, H+ products of

exercise, stress

Activatecardioacceleratory center

Sympathetic pathway

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Regulation of HeartRate Autonomic regulation

Parasympathetic More O2 in blood during

rest

Activatecardioinhibitory center

Branch of vagus nerves

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Regulation of Stroke Volume Change in volume during contraction

SV = EDV ESV

EDV = end-diastolic volume

ESV = end-systolic volume

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Regulation of StrokeVolume Preload

Contractility Afterload

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Regulation ofStroke Volume Preload

Contractility

Afterload

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Regulation ofStroke Volume Preload

Stretch of musclefibers at EDV

Venous return

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Regulation ofStroke Volume Contractility

Force of contractionat any given preload.

Sympatheticstimulation

thyroxine

digitalis

extracellular Ca2+

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Regulation ofStroke Volume Afterload

Force of blood

pressure in largearteries,

resistance to moreblood pumped intothose arteries.

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