The Cardiac Cycle And Cardiac Output

Terms used throughout this page:

LV- Left Ventricle ♦ (LV)EDV- (Left Ventricle) End Diastolic Volume ♦ ESV- End Systolic Volume ♦ MV- Mitral Valve ♦ AV- Aortic Valve ♦ PV- Pulmonary Valve ♦ TV- Tricuspid Valve ♦ SV- Stroke Volume

The cardiovascular system’s base job is to supply the required volume of blood to peripheral tissue to ensure adequate perfusion under a range of different conditions.

In essence, this means that whether one is exercising, sleeping or sitting etcetera, the cardiovascular system has to supply a sufficient quantity of blood to all the present tissues and organs so they can function properly.

In order for this to be achieved, certain factors have to remain in a state of homeostasis;

• Myocardial networks
• Neural and hormonal influence
• The blood volume in circulation

and

• The compliance and resistance of the peripheral vascular system

The cardiac cycle has three defined phases:

• LV contraction
• LV relaxation
• LV filling

*Whilst similar phases are in occurrence on the right side of the heart, the left side is the one that is described

In greater detail however, it can be split into 7 phases:

1. Atrial Systole
2. Isovolumetric Contraction
3. Rapid Ejection
4. Reduced Ejection
5. Isovolumetric Relaxation
6. Rapid Filling
7. Reduced Filling

Atrial Systole

• MV, TV open
• AV, PV closed
• At rest: atrial contraction responsible for 10% of LV filling
• During exercise: 4x increase in LV filling

Isovolumetric Contraction

• MV, TV, AV, PV closed
• LV volume is fixed

Rapid Ejection

• AV, PV open
• MV, TV closed
• LV pressure peaks, then begins to descend
• LVEDV quickly declines

Reduced Ejection

• AV, PV open
• MV, TV closed
• Reduced ejection rate due to ventricular repolarisation

Isovolumetric Relaxation

• MV, TV, AV, PV closed
• Aortic pressure rises slightly, giving dicrotic notch
• No change in blood volume
• EDV 50ml approx
• Lusitopy of LV- pressure drops until it’s lower than LA, then MV opens, leading to:

Rapid Filling

• MV, TV open
• AV, PV closed
• Accounts for most of LV filling with active diastolic relaxation

Reduced Filling

• MV, TV open
• AV, PV closed
• LV becomes 90% full
• Filling virtually stops when equilibrium between atria and ventricles is reached
• Active atrial systole required to overcome pressure and start new cycle

Cardiac Output (CO) is the volume of blood pumped by the heart in one minute.

It is given by the equation:

CO (mL/min) = SV (mL/beat) * HR (BPM)

So if the average human has a HR of 70bpm and an average SV of 70mL/beat, we have a cardiac output of:

70 * 70 = CO = 4900mL/min

Stroke Volume is the volume of blood ejected from the ventricle in a single beat and is controlled by

• Preload
• Afterload
• Inotropy

The relationship can be seen here:

Preload denotes the initial stretching of cardiomyocytes prior to contraction, so the ‘load’ is the stretch of the cardiac muscle at the end of diastole.

An increase in preload increases EDV and subsequently, an increased SV.

Afterload is the term for the ‘load’ the heart must contract again to eject blood; the lower the afterload, the greater the volume of blood ejected with each contraction. An increased afterload increases ESV and reduced SV

Inotropy (contractility): the force of the cardiac muscle contraction. A decreased inotropic effect lowers both ESV and SV.