Cardiac muscle cells are connected to one another by intercalated discs. These discs are composed of gap junctions, which allow for communication between the neighboring cells. These gap junctions allow for an action potential to spread throughout the entire muscle by electrical synapses, and result in a unified contraction.

In order to beat, the heart has a conduction system that works independently of the nervous system. The sinoatrial node, found in the right atrium, is composed of autorhythmic cells that can spontaneously depolarize. This results in an action potential that then spreads throughout the entire heart.

From the sinoatrial node, this action potential spreads through gap junctions to the atria to cause atrial systole, and to the atrioventricular node. The action potential is delayed at the atrioventricular node before being distributed to the bundle of His and Purkinje fibers to cause coordinated ventricular systole. The vagus nerve sends parasympathetic signals to the sinoatrial node to decrease heart rate. The natural rhythm of the heart is roughly 80-100 beats per minute, but the vagus nerve reduces resting heart rate to around 60 beats per minute.

The left ventricle needs to pump blood to all areas of the body. As a result, the left ventricle has a very powerful muscle wall, which allows for the blood to be pumped with a great deal of force throughout the body. The left ventricle typically has a muscle wall three times as thick as the right ventricle, but both ventricles have the same stroke volume. Otherwise, there would eventually be a backup of blood on one side of the heart. Both ventricles also contract with the same frequency, coordinated by the purkinje fibers.

Cardiac output tells us the volume of blood pumped by each ventricle every minute. As a result, the two factors that we need to consider are the number of times that the heart beats every minute (heart rate), and the amount of blood pumped by each ventricle every beat (stroke volume). These two factors are multiplied in order to determine the cardiac output of the heart.

The vagus nerve is a parasympathetic nerve. The parasympathetic nervous system is responsible for "rest and digest" functions in the body, and helps slow down the heart rate. When the vagus nerve is detached from the heart, the heart will still beat, but the loss of parasympathetic innervation will result in an increased heart rate.

Autorhythmic contraction of the heart begins in the sinoatrial node with spontaneous depolarization. The sinoatrial node will generate an action potential roughly 80-100 times per minute. Innervation by the vagus nerve mediates these depolarizations to reduce resting heart rate to around 60 beats per minute.

Cardiac output is the volume of blood that the heart pumps out per minute. The equation for cardiac output is:

Stroke volume (blood pumped out in one beat) x Heart rate = Cardiac output

We are given the stroke volume and beats per minute; it is simply a matter of multiplying our values.

Ejection fraction is the percentage of blood pumped out of the left ventricle, in comparison to its volume when completely filled. Ejection fraction can be found by dividing stroke volume by end distolic volume. The other information given is excess and unnecessary.

In a normal, healthy heart, electrical conduction begins in the pacemaker cells of the sinoatrial node. This signal first spreads across the left and right atria before being directed to the atrioventricular node. The signal slows for an instant to allow the ventricles to fill with blood. This signal is then released and moves along the bundle of His, located within the interventricular septum. From here, the signal divides into the left and right bundle branches, and terminates in the Purkinje fibers on the ventricles. 

In order to determine the total volume of blood the heart pumps out during each cycle, we must first determine Joe's stroke volume, given his cardiac output and heart rate. Rearrange the cardiac output formula, to solve for stroke volume. 

The question is asking us for the total volume that the heart pumps out per cycle. Stroke volume is defined as the amount of blood pumped out of the left ventricle per heart beat, so we must multiply our answer by 2 in order to determine the total volume of blood pumped during each cardiac cycle. 

The mitral valve is also known as the bicuspid valve, and/or the left atrioventricular valve. Closure of the mitral valve is intended to maintain forward, uni-directional flow of blood within the heart. During ventricular contraction, the mitral valve closes, preventing backflow of blood into the left atrium and instead out the aorta through the aortic semilunar valve.