The cardiac cycle involves events, patterns of contraction and relaxation of the heart to complete one complete heartbeat. The cardiac output is the measure of the rate of flow of blood through the heart involving blood vessels. The change in pressure enables the flow of blood through the cardiac cycle. This is regulated by the cardiac conduction system and controlled by the medulla through the autonomic nervous system.
What is Systole and Diastole? – Define Systole and Diastole
The contraction of the muscles of the heart is referred to as systole, while the relaxation of the heart muscles is referred to as diastole. Systole and diastole are two phases of the cardiac cycle that occur as the heartbeats, which pump blood through the system of blood vessels carrying blood to each part of the body. Systole occurs when the heart contracts, pumping blood out, while diastole takes place when the heart relaxes after contraction.
The cardiac cycle is the time period comprising all the events from one heart contraction to the start of the next heart contraction. Each of these cycles starts with the depolarization of the SA node which is then followed by the atrial systole [0.1 seconds], then ventricular systole [0.3 seconds], followed by the diastole of the complete heart [0.4 seconds].
The electrical events taking place are action potential while the mechanical events include volume, pressure and sound changes.
Duration of the cardiac cycle
Calculation of the duration of one complete cardiac cycle: duration – 60/HR
For instance, if the HR of a person is 72 bpm, then the duration of the cardiac cycle is given by: 60/75 = 0.83 s
The duration is inversely proportional to the HR
The two phases of the cardiac cycle are –
|Systole (0.1 seconds)||Diastole (0.7 seconds)|
|Systole (0.3 seconds)||Diastole (0.5 seconds)|
Typically, the total duration of the cardiac cycle with HR = 75 bpm is 0.8 seconds
Fundamentals of Cardiac Cycle
- The heart is mainly involved in pumping blood through the body through the pulmonary and systemic circulations
- The contraction of the heart creates fluctuations in the pressure, allowing movement of blood
- The flow of blood is from the region of high pressure to a region of low pressure
- Events in left and right ventricles are the same, however, with lower pressure gradients in the right side
- Even though the relaxations occur at the same time at the time of diastole of complete heart lasting for 0.4 seconds, the atrial and ventricular systole does not occur at the same time
Mechanism of Cardiac Cycle
Physiologically, the contraction of both the atria precedes that of both the ventricles. This contraction sequence enables the separation of the right and the left heart (functionally at least) as two distinct circuits. The cardiac cycle events can be split into diastole and systole. The diastole indicates ventricular filling, while the systole indicates ventricular ejection or contraction. Though with varying pressures, the systole and diastole occur in both the right and left heart.
The diastole starts with the closing of the aortic valve and terminates with the closing of the tricuspid or mitral valve. This period includes ventricular filling and relaxation. The diastole indicates when blood vessels revert blood to the heart, preparing for the following ventricular contraction.
On the other hand, systole starts when the tricuspid or mitral valve closes and ends with the closing of the aortic valve. This phase of the cardiac cycle indicates ventricular contraction, which forces blood into the arteries. During the contraction of the ventricles, the pressure in the ventricles starts becoming greater compared to that of the adjacent blood vessels, valves allow the blood to flow out.
Phases of Cardiac Cycle
The cardiac cycle occurs through these stages –
The Atrial and Ventricular diastole – relaxed chambers filling with blood, the Atrial systole – contraction of atria, remaining blood is pushed into ventricles, the Ventricular systole – contraction of ventricles forcing the blood out through the aorta and pulmonary artery.
The different phases hence are –
Stage 1 – Early joint diastole, late joint diastole
Stage 2 – Atrial systole
Stage 3 – Early ventricular systole, late ventricular systole
The following table gives a brief description of the phases –
|Atrial Systole||Atria contract pushes the last 25% of blood to the
|0.11 seconds||Open||Closed||It rises to EDV (120-130 ml) at the end of this phase||Slight ↑ first (blood entry) Then slight ↓ (ventricular dilation)||↑ first (by systole)
Then ↓ (blood exit)
|Ventricle builds up tension with no change in its length, opening the semilunar valves. Occurs at the beginning of ventricular systole||0.05 seconds||Closed||Closed||Maximum EDV = 120-130 ml||Sudden rise up to 80
the aortic valve
|Increases because of doming of
closed A-V cusps into
(Rapid & Reduced)
|Ventricle shortens its wall ejecting blood
through the aorta and pulmonary artery in two phases, marks
the end of systole:
Rapid (ejecting 70%) & Reduced (ejecting 30%)
|Rapid: 0.10 seconds
Reduced: 0.15 seconds
|Closed||Open||Drops to ESV = 50 ml||Rapid: increase to 120
|First decreases due to contraction of ventricles, pulling the AV fibrous ring &
valves down then increase.
|Isovolumetric relaxation||Ventricles relax, causes a decrease in pressure leading to
backflow of blood from the aorta
|0.06 -0.04 seconds||Closed||Closed||Volume unchanged ESV= 50 ml||Rapid drop to diastolic levels (2-10 mmHg)
as the valves are closed & the relaxation is
|Gradually increases because of accumulation
of venous blood
|Filling phase (Rapid & Slow)||Ventricle fills in 3 phases due to opening of AV – Slow/
Diastasis (5%), Atrial systole (25%), Rapid (70%)
|Rapid: 0.11 seconds, Slow (diastasis): 0.22 seconds||Open||Closed||Ventricles – volume increases, Atria – decreases||Slight increase due to an increase in
volume, still lesser than
|First sudden decrease due
to rush of blood into
Then increase due to
venous blood entry.
*EDV: End-diastolic volume, ESV: End-systolic volume,
Systole and Diastole Pressure
It is dynamic and not constant when pressure is exerted by the blood flow through the arteries. When there is an active heartbeat (systole), it ejects blood into arteries. Such a dynamic ejection of the blood into the arteries results in an increase in the pressure in the arteries. This peak blood pressure which is reached at the time of active cardiac contraction is referred to as the systolic blood pressure.
Normally, the systolic blood pressure in an individual quietly sitting is 120 mmHg or below.
This pressure is exerted by blood in the arteries between heartbeats – when the heart is not ejecting blood into the arteries actively.
Once the heart contracts, the cardiac ventricles momentarily relax for it to be refilled with blood preparing for the next contraction. This phase of ventricular relaxation is referred to as diastole, the blood pressure at the time of diastole is referred to as diastolic blood pressure.
Normally, the diastolic blood pressure when sitting quietly is 80 mmHg or below. This blood pressure often increases even when at quiet rest, when there is hypertension.
Difference Between Systolic and Diastolic Blood Pressure
In systolic pressure, the pressure builds on the walls of the arteries at the time of heartbeat when muscles contract, pumping blood from the chambers into arteries. On the other hand, diastolic pressure builds on the walls of the arteries when the muscles relax, allowing chambers to fill with blood.
Let us take a look at some other differences –
|Systolic Pressure||Diastolic Pressure|
|Muscles of the heart contracts||Muscles of the heart relax|
|Blood is pumped from chambers to arteries||Chambers filled with blood|
|Typically, the systolic pressure is 120mmHg||Typically, the diastolic pressure is 80mmHg|
|Pressure is exerted when blood is ejected into arteries||Pressure exerts in arteries between heartbeats|
This was a brief on Systole and Diastole. Learn other related NEET concepts at BYJU’S.