The endocardium lines the internal chambers of the heart.

A. All cells contract independently unless stimulated by an external force: Cardiac myocytes possess intrinsic automaticity, allowing them to generate action potentials spontaneously. However, when coupled electrically, they do not remain completely independent; they influence each other’s activity through gap junctions.

B. The fastest cell controls the rhythm by spreading its impulse to slower cells: In a network of electrically coupled cardiac cells, the cell with the highest intrinsic firing rate (typically pacemaker-like cells) sets the rhythm. Its action potential propagates through gap junctions to slower cells, synchronizing contractions across the tissue. This phenomenon demonstrates the principle of overdrive suppression and explains how the sinoatrial node establishes the dominant heart rate in vivo.

C. Slower cells override faster cells to establish a uniform contraction rate: Slower cells cannot override faster pacemaking cells because electrical impulses flow from faster to slower cells, not the reverse. Slower cells follow the rhythm set by the fastest pacemaker.

D. The presence of electrical impulses forces all cells to contract at different rates: Coupled cardiac cells do not maintain different contraction rates when electrically connected; instead, the fastest cell entrains the others, producing a synchronized contraction, which is essential for coordinated cardiac pumping.

N: Pulmonary valve- The pulmonary valve is a semilunar valve between the right ventricle and pulmonary artery. It prevents backflow into the ventricle during diastole, facilitating blood flow toward the lungs for oxygenation.

E: Fossa ovalis - The fossa ovalis is a shallow, thumb-sized depression located in the interatrial septum (the wall separating the right and left atria). In a developing fetus, the foramen ovale is an open "tunnel" that allows blood to bypass the lungs by flowing directly from the right atrium to the left atrium. Once a baby takes their first breath, the pressure changes in the heart cause a flap of tissue to close over this opening. Over time, it fuses shut, leaving behind the shallow indensee fossa ovalis.

O: chordae tendinae: Chordae tendineae are fibrous cords connecting the atrioventricular valve leaflets (mitral and tricuspid) to papillary muscles. They prevent valve prolapse during ventricular contraction, ensuring unidirectional blood flow.

J: Aortic arch- The aortic arch is the curved portion of the aorta that distributes oxygenated blood from the left ventricle to systemic arteries. It contains baroreceptors and helps regulate blood pressure.