Heart physiology - mechanical events - cardiac cycle

cardiac cycle

blood flow through heart during one complete heartbeat: atrial systole & diastole followed by ventricular systole & diastole; series of pressure & blood volume changes

systole

period of contraction

diastole

period of relaxation

2 phenomena control blood floe through heart

1. contraction & relaxation sequences of chambers

2. opening & closing of AV & SL valves

blood flow

controlled entirely by pressure changes; flows from an area of higher pressure to an area of lower pressure

pressure developed w/in heart chamber is related to

1. volume of blood within chamber

2. size of chamber

pressures to keep in mind

1. atrial pressure

2. ventricular pressure

3. arterial pressure (aorta & pulmonary trunk)

normal cardiac cycle

2 atria contract while 2 ventricles relax, 2 ventricles contract while 2 atria relax, & then all chambers relax until next P wave (SA node fires again)

4 phases of cardiac cycle

1. ventricular filling (mid-late diastole)

2. isovolumetric contraction (ventricular systole)

3. ventricular ejection (ventricular systole)

4. isovolumetric relaxation (early diastole)

isovolumetric

all valves are closed; ventricles are completely closed chambers, so volume remains constant

ventricular filling

mid-to-late diastole; AV valves are open; active & passive; EDV = 120mL

Ventricular filling passive

80% of blood passively flows into ventricles; atrial pressure greater than ventricular pressure; AV valves forced open

ventricular flowing passive status of structures

1. ventricles: relaxed

2. atria: relaxed

3. AV valves: opened

4. SL valves: closed

ventricular filling active

atrial systole occurs, delivering remaining 20%; atria contract to complete filling; ventricular diastole ends, & end diastolic volume of ventricles achieved

ventricular filling active status of structures

1. ventricles: relaxed

2. atria: contracted

3. AV valves: opened

4. SL valves: closed

AV valves open when

atrial pressure > ventricular pressure

end diastolic volume (EDV)

volume of blood in each ventricle at then end of ventricular diastole

ventricular systole

atria relax; ventricles begin to contract; rising ventricular pressure > closing AV valves

2 phases of ventricular systole

1. isovolumetric contraction phase (all valves closed)

2. ventricular ejection phase

end systolic volume (ESV)

volume of blood remaining in each ventricle after systole; =50ml

AV valves close when

ventricular pressure > atrial pressure

SL valves open when

ventricular pressure > arterial pressure

stroke volume (SV) =

70 ml

isovolumetric contraction phase

contraction of ventricles begins & ventricular pressure increases closing AV valves

Isovolumetric contractions status of structures

1. ventricles: contracted

2. atria: relaxed

3. AV valves: closed

4. SL valves: closed

ventricular ejection phase

ventricular pressure exceeds pressure in large arteries, forcing Sl valves open; pressure exceeds pressure in great vessels

ventricular ejection phase status of structures

1. ventricles: contracted

2. atria: relaxed

3. AV valves: closed

4. SL valves: opened

isovolumetric relaxtion

early diastole; ventricles relax, atria relax & filling; ventricular pressure drops, backflow of blood in aorta & pulmonary trunk closes SL valves; atrial pressure exceeds that in ventricles > AV valved open; cycle begins again at step 1

SL valves close when

arterial pressure > ventricular pressure

quiescent period begins when

atrial pressure > ventricular pressure

After isovolumetric relaxation

AV valves open & period of ventricular filling begins for next cycle

ventricular pressure dropping during isovolumetric relaxation

causes dicrotic notch; ventricles totally closed chambers (all valves closed)

dicrotic notch

brief rise in aortic pressure as blood rebounds off closed valve

Isovolumetric relaxation status of structures

1. ventricles: relaxed

2. atria: relaxed

3. AV valves: closed

4. SL valves: closed

overview of volume changes pt.1

end systolic volume (left from previous heartbeat) 50ml + 70ml ventricular filling (passive & due to atrial systole) = 120ml end diastolic volume

overview of volume changes pt.2

end diastolic volume 120ml - 70ml stroke volume (ejected by ventricular systole) = 50ml leaves the end systolic volume

timing of 1 cardiac cycle

requires 0.8 seconds; when heart rate changes quiescent period altered

time requirements of cardiac cycle

1. atrial & ventricular systoles = 0.4 seconds

2. quiescent period (total heart relaxation) = 0.4 seconds

average resting heart rate

75 beats/min

heart sounds

two sounds (lub-dup) associated with closing of heart valves

2 heart sounds

first as AV valves close, beginning of ventricular systole; second as SL valves close, beginning of ventricular diastole

heart murmurs

abnormal heart sounds; usually indicate valve issues; incompetent ('leaky') valves & stenotic valves

incompetent 'leaky' valves

don't close completely, backflow of blood; Lub-Dup-Swish

stenotic valves

fail to open completely; narrow opening restricts flow; described as high-pitched screeching