USMLE Step 1 Physiology Review 55 13 Cardiac Cycle (3_4)

USMLE Step 1 Physiology Review 55 13 Cardiac Cycle (3_4)

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Play USMLE Audio MP3 55 13 Cardiac Cycle (3_4) Below

Begin 55 13 Cardiac Cycle (3_4) Transcription

Okay now for some more questions. Once the AV valves close the ventricular volume remains constant until the ventricular pressure exceeds the pressure in the respective artery, at which point ventricular ejection begins. In which ventricle does the ejection begin?

  • The right ventricle

Why does right ventricle ejection occur before left ventricle ejection?

  • Because the pressure in the pulmonary artery is lower than in the aorta.

What bioelectrical event corresponds with the peaks of aortic and ventricular pressure curves?

  • Think about it, we are at the peak of the ventricular contraction and therefore the peak of the effect of the action potential, ventricular relaxation is about to begin. 

Okay what is the bioelectrical event?

  • Ventricular repolarization

Where does the hump that is caused by ventricular repolarization?

  • Just after the QRS complex

What is the hump in the EKG called?

  • The T wave

Upon ventricular contraction ejection is initially rapid with two-thirds of the blood volume ejected during the first third of systole, the change in ventricular volume levels out, and this leveling out corresponds with what part of the aortic and ventricular pressure curves?

  • The peaks

Student doctor please pause the tape and summarize the information about systole discussed since the last summary.

–       The first question was about in which ventricle ejection occurs. Once the AV valves close the ventricular volume remains constant until the ventricular pressure exceeds the pressure in the respective artery, at which point ventricular ejection begins. The ejection begins in the right ventricle because the pressure in the pulmonary artery is lower than in the aorta. Ventricular repolarization corresponds with the peaks of aortic and ventricular pressure curves. The hump that is caused by ventricular repolarization occurs just after the QRS complex. This hump is called the T wave. Upon ventricular contraction ejection is initially rapid with two-thirds of the blood volume ejected during the first third of systole. The change in ventricular volume levels out and this leveling out corresponds with the peaks of the aortic and ventricular pressure curves.

Okay back to more on systole. The ventricles relax, the ventricle and arterial pressure decrease as the what event prevents blood from following back into the ventricles?

  • The closure of semilunar valves

Which valves are the semilunar valves?

  • The pulmonic and the aortic valves

What sound is produced by this valve closure?

  • The sound heart sound, S2.

Relative to the S1, is S2 high or low?

  • High

S2 can be subdivided into two different components, what is the first component of S2?

  • A2

What causes the A2 heart sound?

  •  The closure of the aortic valve.

The second sound is referred to as the P2 heart sound and is caused by what?

  • The closure of the pulmonic valve

A2 occurs before P2 because the aortic valve occurs before the pulmonic valve. Why does the aortic valve close before the pulmonic valve?

  • Because the left ventricle ejection is faster than the right ventricle ejection

Sometimes P2 occurs before A2 due to the delayed closure of the aortic valve, what is this an indication of?

  • The delayed closure of the aortic valve is an indication of the disease process affecting the ventricle

Student doctor please pause the tape and summarize the information about systole discussed since the last summary.

The first question was about the closure of semilunar valves. During the last two thirds of systole the rate of ejection declines, the ventricles relax, the ventricle and arterial pressure decrease as the rate of flow through the peripheral vessels begins to the rate of blood flow from the ventricles. The closure of semilunar valves event prevents blood from following back into the ventricles.  The semilunar valves are the pulmonic and the aortic valves and their closure makes the sound heart sound, S2. Relative to the S1, S2 is a high sound. S2 can be subdivided into two different components, A2 and P2. The closure of the aortic valve causes the A2 heart sound which is the first component of S2. The second part is referred to as the P2 heart sound and is caused by the closure of the pulmonic valve. A2 occurs before P2 because the aortic valve closes before the pulmonic valve because the left ventricle ejection is faster than the right ventricle ejection. Sometimes, P2 occurs before A2 because of delayed closure of the aortic valve. This is an indication of the disease process affecting the left ventricle.

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