A single cycle (cardiac cycle ) of cardiac activity can be divided into two basic phases - diastole and systole
Diastole represents the period of time when the ventricles are relaxed (not contracting).Throughout most of this period, blood is passively flowing from the left atrium (LA) and right atrium (RA) into the left ventricle (LV) and right ventricle (RV), respectively (see figure at right). The blood flows through atrioventricular valves (mitral and tricuspid) that separate the atria from the ventricles. The RA receives venous blood from the body through the superior vena cava (SVC) and inferior vena cava (IVC). The LA receives oxygenated blood from lungs through four pulmonary veins that enter the LA. At the end of diastole, both atria contract, which propels an additional amount of blood into the ventricles.
Systole represents the time during which the left and right ventricles contract and eject blood into the aorta and pulmonary artery, respectively. During systole, the aortic and pulmonic valves open to permit ejection into the aorta and pulmonary artery. The atrioventricular valves are closed during systole, therefore no blood is entering the ventricles; however, blood continues to enter the atria though the vena cavae and pulmonary veins.
To study cardiac cycle in detail,systole and diastole are further subdivided into phases which are as follows:
- phase of atrial contraction (Atrial systole)
- isovolumetric contraction
- rapid ejection
- reduced ejection
- isovolumetric relaxation
- rapid ventricular filling
- reduced ventricular filling
- pressure and volume changes
- ECG changes
- Heart sounds
Atrial systole
 | Heart:Prior to atrial systole, blood has been flowing passively from the atrium into the ventricle through the open AV valve. During atrial systole the atrium contracts and tops off the volume in the ventricle with only a small amount of blood. Atrial contraction is complete before the ventricle begins to contract (because of the delay in conduction of nerve impulses from SA node to AV node and as SA node is in the superolateral wall of atria contract before ventricles do.To appreciate this have a look at the animation at the start of this page.Nerve signals are shown in yellow) |
 | Atrial pressure:The "a" wave occurs when the atrium contracts, increasing atrial pressure (yellow). Blood arriving at the heart cannot enter the atrium so it flows back up the jugular vein, causing the first discernible wave in the jugular venous pulse. Atrial pressure drops when the atria stop contracting. |
 | ECG:An impulse arising from the SA node results in depolarization and contraction of the atria (the right atrium contracts slightly before the left atrium). The P wave is due to this atrial depolarization. The PR segment is electrically quiet as the depolarization proceeds to the AV node. This brief pause before contraction allows the ventricles to fill completely with blood. |
 | Heart sounds:A fourth heart sound (S4) is abnormal and is associated with the end of atrial emptying after atrial contraction. It occurs with hypertrophic congestive heart failure, massive pulmonary embolism, tricuspid incompetence, or cor pulmonale. |
ISOVOLUMETRIC CONTRACTION
The beginning of systole
 | Heart:The atrioventricular (AV) valves close at the beginning of this phase. Electrically, ventricular systole is defined as the interval between the QRS complex and the end of the T wave (the Q-T interval). Mechanically, the isovolumic phase of ventricular systole is defined as the interval between the closing of the AV valves and the opening of the semilunar valves (aortic and pulmonary valves). |
 | Pressures & Volume:The AV valves close when the pressure in the ventricles (red) exceeds the pressure in the atria (yellow). As the ventricles contract isovolumetrically -- their volume does not change (white) -- the pressure inside increases, approaching the pressure in the aorta and pulmonary arteries (green). |
 | ECG:The electrical impulse propagates from the AV node through the His bundle and Purkinje system to allow the ventricles to contract from the apex of the heart towards the base. The QRS complex is due to ventricular depolarization, and it marks the beginning of ventricular systole. It is so large that it masks the underlying atrial repolarization signal. |
 | Heart sounds:The first heart sound (S1, "lub") is due to the closing AV valves and associated blood turbulence. |
RAPID EJECTION
 | Heart:The semilunar (aortic and pulmonary) valves open at the beginning of this phase of ventricular systole. |
 | Pressures & Volume:While the ventricles continue contracting, the pressure in the ventricles (red) exceeds the pressure in the aorta and pulmonary arteries (green); the semilunar valves open, blood exits the ventricles, and the volume in the ventricles decreases rapidly (white). As more blood enters the arteries, pressure there builds until the flow of blood reaches a peak. The "c" wave of atrial pressure is not normally discernible in the jugular venous pulse. Right ventricular contraction pushes the tricuspid valve into the atrium and increases atrial pressure, creating a small wave into the jugular vein. It is normally simultaneous with the carotid pulse. |
 | ECG: |
 | Heart sounds: |
REDUCED EJECTION
The end of systole
 | Heart:At the end of this phase the semilunar (aortic and pulmonary) valves close. |
 | Pressures & Volume:After the peak in ventricular and arterial pressures (red and green), blood flow out of the ventricles decreases and ventricular volume decreases more slowly (white). When the pressure in the ventricles falls below the pressure in the arteries, blood in the arteries begins to flow back toward the ventricles and causes the semilunar valves to close. This marks the end of ventricular systole mechanically. |
 | ECG:The T wave is due to ventricular repolarization. The end of the T wave marks the end of ventricular systole electrically. |
 | Heart sounds: |
ISOVOLUMETRIC RELAXATION
The beginning of diastole
 | Heart:At the beginning of this phase the AV valves are closed. |
 | Pressures & Volume:Throughout this and the previous two phases, the atrium in diastole has been filling with blood on top of the closed AV valve, causing atrial pressure to rise gradually (yellow). The "v" wave is due to the back flow of blood after it hits the closed AV valve. It is the second discernible wave of the jugular venous pulse. The pressure in the ventricles (red) continues to drop. Ventricular volume (white) is at a minimum and is ready to be filled again with blood. |
 | ECG: |
 | Heart sounds:The second heart sound (S2, "dup") occurs when the semilunar (aortic and pulmonary) valves close. S2 is normally split because the aortic valve closes slightly earlier than the pulmonary valve. |
RAPID VENTRICULAR FILLING
 | Heart:Once the AV valves open, blood that has accumulated in the atria flows rapidly into the ventricles. |
 | Pressures & Volume:Ventricular volume (white) increases rapidly as blood flows from the atria into the ventricles. |
 | ECG: |
 | Heart sounds:A third heart sound (S3) is usually abnormal and is due to rapid passive ventricular filling. It occurs in dilated congestive heart failure, severe hypertension, myocardial infarction, or mitral incompetence. |
REDUCED VENTRICULAR FILLING
(DIASTASIS)
 | Heart: |
 | Pressures & Volume:Ventricular volume (white) increases more slowly now. The ventricles continue to fill with blood until they are nearly full. |
 | ECG: |
 | Heart sounds: |
TEST YOUR UNDERSTANDING
Having learned this i expect you to interpret the following animation with regards to all the phases of cardiac and also note ECG changes,heart sounds,pressure and volume changes (this is a test of your understanding).
YOU can watch each step in detail by pressing on the button "STEP" beloow
Note that in the yellow is shown the electrical activity generated in the SA node,conducted to the AV node and then through he bundle of His to the Purkinje fibers.Black arrows indicate the direction of blood flow.
Also appreciate the opening and closing of the valves.
YOU CAN ALSO WATCH CARDIAC CYCLE TUTORIAL HERE CARDIAC CYCLE TUTORIAL
