Which of the following produces the first heart sound?
the closing of the valves between the ventricles and the body
the opening of the valves between the atria and the ventricles
the closing of the valves between the atria and ventricles
the opening of the valves between the ventricles and the body

The closing of the valves between the atria and ventricles.

Explanation:

Heart sounds are defined as noises made by the beating heart and blood flow through heart. The S1 or first heart sound is generated by turbulence due to closing of two components, mitral valve and tricupsid valve.

The mitral valve and tricupsid valve are generally known as atrioventricular valves as they present between the atria and ventricles.

S1 sound is has low frequency, which is described as 'lub' and generates at the beginning of systole

Thus, the correct answer is third option.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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