Heart activity

• Changes in pressure and volume during the heart cycle

The heart cycle

• Period from the beginning of one heart beat to the beginning of the next heart beat

• Two phases:– Diastole, period of cardiac muscle relaxation– Systole, period of cardiac muscle contraction

• The atria and ventricles do not contract and relax at the same time.

5 phases of the heart cycle

1. Heart at rest (atrial and ventricular diastole)

2. Completion of ventricular filling (atrial systole)

3. Early ventricular contraction (first heart sound)

4. The heart pumps (ventricular ejection)5. Ventricular relaxation (second heart

sound)

1. Heart at rest (atrial and ventricular diastole)

• Atria and ventricles are relaxing• blood flows into the atria from veins• AV valves are open• Blood flows into the ventricles from the

atria• Relaxed ventricles accept blood

2. Completion of ventricular filling (atrial systole)

• Depolarization from the SA node• Contraction of atria• Blood pushed into the ventricles• Pressure increase accompanies

contraction• Increase in pressure pushes some blood

back to the veins

3. Early ventricular contraction (first heart sound)

• Depolarization to AV node, down bundle of His, up Purkinje fibers

• Ventricular systole begins at apex • Blood pushing up on AV pushes them shut• Blood does not flow back to atria• First heart sound when AV valves close

(lub)

3. Early ventricular contraction (first heart sound)

• The semilunar valves are also shut• Blood stays in the ventricle while it

contracts• High pressure on the heart walls during

this contraction ISOMETRIC CONTRATION

3. Early ventricular contraction (first heart sound)

• Atria begin to repolarize and relaxAtrial pressure falls below venous

pressureBlood flows from veins to atria Blood stays in atria because the AV

valves are closed

4. The heart pumps (ventricular ejection)

• Ventricles contractSemilunar valves openVentricular blood is pushed into the

arteries

5. Ventricular relaxation (second heart sound)

Ventricular pressure fallsBlood starts to flow from arteries into

ventriclesThis backflow shuts the semilunar valves

(dup of lub-dup)Ventricles become closedAV valves open when ventricular pressure

is lower than atrial pressure

End Diastole Volume (EDV)

• Ventricles are maximally filled at the end of ventricular relaxation (diastole)

• When heart rate is very high, the ventricles may not have enough time to fill as much as when the heart rate is slow

End-systole volume (ESV)

• The amount of blood left in the heart at the end of each contraction