Module 5 Section 4

Question 1

Within the context of the electrical activity of the heart, describe the mechanical activity of the cardiac cycle.

Answer 1

Consists of alternating periods of contraction (systole) and relaxation (diastole).

1) Just before the start of the P-wave, blood is flowing from the pulmonary veins into the left atrium & b/c the AV valve is currently open, blood is passively flowing into the LV.
- When the atria contract (the P wave) pressure is generated in the LA and blood is forcefully squeezing into the LV to increase both LV blood volume and pressure. This continues until the AV nodal delay

2) While the atria are depolarizing, there’s no net movement so the ECG remains flat until the AV node delay has occurred. Following this, the wave of excitation travels down bundle of His and purkinje fibres to depolarize the ventricles. The current is the QRS complex.
3) At end of AV nodal delay, wave of excitation -> entire LV and contraction begins. Contraction is enough pressure to close the AV valve to prevent back flow. B/c both valves are close, period = isovolumetric (same volume) contraction. Eventually, pressure generated in LV > aorta, which allows the aortic valve to open. This result is the squeezing of blood from the LV into the aorta.
4) When the ventricle relaxes enough, its pressure drops below that of the atria and the AV valve reopens to begin the cycle again.

Question 2

Describe why there are periods of isovolumetric contraction and relaxation.

Answer 2

1) Passive filling during ventricular and atrial diastole.
2) Atrial contraction.
3) Isovolumetric ventricular contraction.
4) Ventricular ejection.
5) Isovolumetric ventricular relaxation.

Question 3

Describe why heart sounds can be heard.

Answer 3

Valves open = quiet. Valves close = vibrations within the walls of the ventricles and arteries which can be heard.

When the valves open the pressure allows for smooth or laminar flow, whereas the closing of the blood causes turbulent flow that causes vibrations.

1) The first heart sound heard occurs when the AV valves close (“lub”). This signals the beginning of ventricular systole.
2) The second heart sound occurs when the semilunar valves close (“dub”). This signals the onset of ventricular diastole.

Stenosis = whistling sound (valve doesn’t fully open).

Regurgitation = swishing/gurgling sound (valve doesn’t fully close).