Mikie H. answered • 05/02/19
Physiology tutor 2+ years
C. Heart sound 1 occurs
P wave triggers atrial depolarization. Depolarization of atria cause atrial walls to contract. This is atrial systole. Notice, this is ventricular diastole, and is thus, still considered diastole of the heart. Atrial contraction forces the last 20% of blood into the ventricles.
In order for this to happen, the atrial pressure must exceed ventricular pressure. When this happens, the AV valves are open. They remain open until the ventricular pressure exceeds atrial pressure. When this happens, the AV valves close. This starts the first phase of ventricular systole (systole of the heart), called isovolumic contraction. Isovolumic contraction is when the ventriclular pressure is increasing rapidly, but ventricular pressure has not yet exceeded arterial pressure. Therefore, during isovolumic contraction, both AV valves and SL valves are closed.
Heart sound 1, lub, does not occur immediately following the P wave. It occurs during the QRS complex. QRS complex triggers ventricular depolarization. Ventricular depolarization induces ventricular contraction. Once ventricular pressure exceeds atrial pressure, AV valves close. Heart sound 1 occurs after the AV valves close because blood hits them with a turbulent force. Therefore, the first heart sound correlates with ventricular systole. Clinical workers use a sphygmomanometer to measure blood pressure. The systolic pressure is measured when they hear heart sound 1 and the diastolic pressure is measured when they no longer hear any sound.
For technicality, cardiologists used to consider the sound of atrial contraction pushing blood actively into the ventricles as heart sound 4. It is extremely difficult to hear heart sound. A comparable analogy would be a sound heard when squirting a squirt gun or condiment bottle.
Heart sound 1 (lub) and heart sound 2 (dub) are the most clinically significant heart sounds to access health.
