The vagus nerve is responsible for slowing down the heart rate, and is able to "override" the faster, natural pace of the sinoatrial node. When the vagus nerve is severed from the heart, the heart will pump at the pace of the SA node.

Note that innervation is not necessary for the heart to continue beating; it is self-sustaining, but can be affected by innervation from the vagus nerve.

The chordae tendinae (tendinous chords or heart strings) are physical structures located in the heart lumen that connect the muscular wall of the heart to the tricuspid and mitral valves via papillary muscles.

The other answer options are examples of cell bundles and tissues that orchestrate the electrical conduction through the heart. Signals begin at the sinoatrial node and transition to the atrioventricular node. They then pass through the atrioventricular bundle (or bundle of His) to the purkinje fibers, which coordinate simultaneous ventricular contraction.

The sinoatrial node is responsible for initiating the contraction of the heart. Depolarization of the sinoatrial node coincides with atrial contraction. The depolarization travels very quickly to the atrioventricular node during this period. The atrioventricular node delays the spread of the impulse, preventing it from triggering ventricular contraction. This time delay allows the atria to fill the ventricles with blood before the impulse causes the ventricles to contract. Without this delay, an inadequate amount of blood would be pumped from the ventricles.

The circulatory system is composed of two primary regions: the systemic circuit and the pulmonary circuit.

The systemic circuit allows blood to travel from the heart to the tissues of the body, delivering nutrients and oxygen, and then returns the deoxygenated blood to the heart. Most arteries, arterioles, capillaries, venules, and veins are part of the systemic circuit. The systemic circuit essentially starts with the aorta and ends with the vena cavae.

The pulmonary circuit receives dexoygenated blood from the body and carries it to the lungs for reoxygentation, before returning it to the heart to enter the systemic circuit through the aorta.

In general circulation, the vena cavae empty into the right atrium. Blood then enters the right ventricle and enters the pulmonary circuit. It travels to the lungs through the pulmonary arteries, becomes reoxygenated in the capillaries of the lungs, then returns to the left atrium via the pulmonary veins. From the left atrium, blood enters the left ventricle and is transferred to the systemic circuit.

Multiplying stroke volume by heart rate gives another measure called cardiac output. Stroke volume will be influenced by variables such as resistance in the arteries and contractility of the heart muscle cells, while heart rate will be influenced by variables such as emotional state and age. Both stroke volume and heart rate increase with exercise.

The left ventricle supplies oxygenated blood from the heart to the aorta so it can be distributed to the body. The left ventricle must overcome the forces in the aorta and arteries (e.g., blood pressure). The higher the blood pressure, the harder the left ventricle must contract to force blood out of the aortic valve. This leads to muscle growth and enlargement of the left ventricle over time.