The Cardiac Cycle
Lesson Goals:
Describe the major events of the cardiac cycle.
Key Vocabulary:
Diastole: Relaxation of the heart chambers.
Systole: Contraction of the heart chambers.
Introduction:
The cardiac cycle is the sequence of events occuring in the heart during one complete heartbeat. It is important to know the steps of the cardiac cycle to better understand how the heart works so this can be related to heart rhythm problems and diseases.
The Cardiac Cycle
During one heartbeat, the heart must receive blood in the atria and then move this blood to the ventricles and out to the lungs or the body. The cardiac cycle is the sequence of events that makes this possible.
There are three phases in the cardiac cycle. In phase one (called the rest phase) the heart is relaxed or in diastole. Blood passively flows into the atria and subsequently into the ventricles. Actually, about 70% of blood flows into the ventricles without any contraction of the atria.
As we stated earlier the valves operate by changes in pressure. During phase one the pressure is greater in the atria than in the ventricles. This causes both atrioventricular valves (bicuspid and tricuspid) to open. The relaxation of the ventricles (ventricular diastole) creates a lower pressure in the ventricles than in the pulmonary trunk and aorta. This causes the semilunar valves (pulmonary and aortic) to close.
The next phase in the cycle is characterized by systole of the atria. The ventricles are still in diastole in this phase. The contraction of the atria pushes the remaining 30% of the blood into the ventricles. The atrioventricular valves remain open and the semilunar valves remain closed.
The final phase is characterized by atrial diastole and ventricular systole. Contraction of the ventricles causes increased pressure in the ventricles. This increase in pressure causes the atrioventricular valves to close and the semilunar valves to open.
The heart again enters the rest phase and the cycle repeats.
Heart Sounds
A good deal of information can be obtained from listening to the heart with a stethoscope. This procedure is called auscultation. The heart sounds are produced by changes in blood flow during different parts of the cardiac cycle.
There are four heart sounds that repeat continuously but only two are usually heard in healthy adults. The first sound or S1 (lubb) is produced by turbulent flow of blood resulting from closure of the atrioventricular valves (bicuspid, tricuspid) in ventricular systole. It is louder and longer than the second sound (S2). The S2 sound (dub) results from closure of the semilunar valves (pulmonary, aortic) during ventricular diastole.
A third sound (S3) can sometimes be heard between S2 and S1. This sound is the result of ventricular filling and can be heard in children and athletes. It also arises in congestive heart failure.
Examine the difference between systole and diastole in the images below:
