1. Does depolarization cause the muscle to become more or less likely to produce an action...

1. Does depolarization cause the muscle to become more or less likely to produce an action potential?

2. If blood travels from the left atrium to the left ventricle during ventricular diastole, why do you need a stage of the cardiac cycle known as atrial systole? What would happen to cardiac output if no atrial systole occurred?

Expert Solution

1. Depolarization cause the muscle to become more likely to produce an action potential.

Explanation: - During depolarization phase in the muscle, there is opening of voltage gated sodium channels which causes the membrane potential to rise from -90mV to +35mV.

- During initial phase of depolarization, there is opening of few voltage gated sodium channels that changes the membrane potential from -90mV to threshold level of -55mV. If the membrane potential reaches the threshold level, it causes further opening of voltage gated sodium channels resulting in sodium influx which causes the depolarization and subsequent generation of action potential.

2. - During the beginning of cardiac cycle, both atria and ventricles are relaxing and blood flows from the vena cavae into the right atrium and via the two pairs of pulmonary veins into the left atrium from the lungs.

- During this diastolic phase, when both the atria and ventricles are relaxing, the atrioventricular valves (mitral valve and tricuspid valve) are open and blood flows continuously from the left atrium into the left ventricle and from the right atrium into the right ventricle. Almost 70-80% of ventricular filling occurs during this phase by passive filling of ventricles by the atria.

- During the atrial systole, atrial contraction occurs and remaining 20-30% blood ventricular filling occurs as the atrioventricular valves are opened at this stage. Thus, by active contraction of atria, the ventricles are filled completely with blood just prior to the beginning of ventricular systole.

- Thus, the atrial systole makes sure that the ventricular filling is completed prior to ventricular systole as 20-30% of ventricular filling occurs due to atrial contraction during atrial systole.

- If no atrial systole occurred then the ventricular filling during diastolic phase would remain incomplete which would decrease the cardiac output as stroke volume would be lowered.

Sydney Mullin answered 1 year ago

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