During which phase of the cardiac cycle are the semilunar valves open?

A. To contract and generate force for blood circulation:
The myocardium is the muscular layer of the heart wall responsible for contracting and generating pressure to circulate blood through the systemic and pulmonary circuits. Its thickness varies by chamber, with the left ventricle being the thickest due to the higher pressure required to pump blood systemically. Dysfunction in the myocardium leads to heart failure or arrhythmias.

B. To produce pericardial fluid:
The pericardium, not the myocardium, secretes pericardial fluid to reduce friction during cardiac movement. Confusing these structures may lead to misunderstanding cardiac physiology.

C. To protect the heart from external injury:
Protection is primarily provided by the pericardium and thoracic cage. The myocardium’s main role is mechanical contraction, not external protection.

D. To facilitate the electrical conduction in the heart:
Electrical conduction is carried out by specialized nodes and fibers (SA node, AV node, bundle of His, Purkinje fibers). The myocardium responds to these signals but does not generate the primary conduction itself.

A. To facilitate blood flow from ventricles to the lungs:
Blood flow to the lungs occurs via the pulmonic valve and pulmonary artery, not the AV valves.

B. To prevent blood from flowing into the coronary arteries:
Coronary arteries receive blood from the aortic sinuses, independent of AV valves. AV valves do not regulate coronary perfusion.

C. To regulate blood flow between the two ventricles:
Blood does not flow directly between ventricles. AV valves regulate flow from atria to ventricles, not between ventricles.

D. To prevent backflow of blood into the atria:
The mitral and tricuspid valves prevent backflow of blood into the atria during ventricular contraction (systole), ensuring unidirectional blood flow and efficient cardiac output.