MCB Module 19 Cardiac Cycle

1. Filling phase

During diastole: the atria and ventricles passively fill. AV valves are open so ventricles are getting partially filled and expanding (semilunar valves are closed, so blood doesn’t leave heart).

At the end of diastole, the SA node and atria depolarize and the atria contract (atrial systole). This creates high atrial pressure, driving the last of the blood from the atria into the ventricles (atria contraction is sometimes considered a separate phase).

2. Isovolumetric contraction

Isovolumetric contraction is the first part of systole: the depolarization spreads to the ventricles and they start to contract. 

This creates high pressure in the ventricles, causing the AV valves to close. As the AV valves close it generates the first heart sound “lub” (sometimes called S1). Sound is created by blood rushing through the narrowing valves as they close.

Since all the valves are closed, blood doesn’t flow, so the volume of blood in ventricles remains the same (= isovolumetric).

3. Ventricular ejection

Ventricular ejection is the second part of systole. The pressure in the ventricles increases enough to open semilunar valves and blood goes from the ventricles into the arteries. This causes an increase in the pressure in the arteries.

4. Isovolumetric relaxation

The ventricles repolarize and relax (diastole), causing the pressure in the ventricles to drop below the pressure in the arteries. The semilunar valves close, causing the second heart sound “dub” (sometimes called S2). All valves are closed briefly at this time, so this is isovolumetric relaxation.

Then AV valves open and the ventricles start to fill again (go back to the first part of the process)

There are two main components of the cardiac cycle: Diastole – when the ventricle muscles are relaxed. Systole – when the ventricle muscles contract

The volume of blood in the ventricle at the end of diastole is called the end-diastolic volume (EDV), which is the maximum ventricular volume in the cardiac cycle.

The volume of blood in the ventricle at the end of systole is called the end-systolic volume (ESV) and is the minimum ventricular volume.

EDV – ESV = stroke volume (SV) = the amount of blood ejected from the heart during one beat (Fig 13.21). The stroke volume can be regulated by hormones and the sympathetic nervous system (more about this in module 20).

During diastole, the volume of blood in the aorta decreases as blood flows through the rest of the blood vessels in the body. This causes decreased aortic pressure, which is known as the diastolic pressure

Once the semilunar valves open, blood enters the aorta, causing an increase in blood pressure known as the systolic pressure (SP).