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valve prevents backflow of blood into the right atrium when the right ventricle contracts.

A. Z-line to Z-line: A sarcomere, the functional contractile unit of a cardiomyocyte, is defined as the region between two adjacent Z-lines (or Z-discs). The Z-lines anchor the thin filaments (actin) and mark the lateral boundaries of the sarcomere. During contraction, the sarcomere shortens as actin and myosin filaments slide past each other, generating tension and ultimately producing myocardial contraction.

B. A-line to A-line: The A-band corresponds to the length of the thick filaments (myosin) within a sarcomere and does not represent the full functional unit. It includes both overlapping regions with thin filaments and the central H-zone, but A-band boundaries do not define sarcomere length.

C. I-band to I-band: The I-band contains only thin filaments and is bisected by the Z-line. It shortens during contraction, but its boundaries alone do not encompass the entire sarcomere, making it an incomplete reference for the sarcomere’s limits.

D. M-line to M-line: The M-line lies at the center of the sarcomere, anchoring thick filaments. While it is important for structural integrity, using M-line to M-line does not define the full sarcomere; it represents only the midpoint rather than the full functional contractile unit.

A. Higher pressure in the atria than in the veins: While venous return contributes to atrial filling, blood flow from the atria into the ventricles during diastole is primarily driven by pressure gradients between the atria and ventricles, not just between veins and atria. Venous pressure alone does not account for the efficient filling of the ventricles.

B. Contraction of the atria: During atrial systole, the atria contract, increasing atrial pressure above ventricular pressure. This pressure gradient forces blood through the open atrioventricular valves (tricuspid and mitral) into the ventricles, completing the final phase of ventricular filling known as the “atrial kick,” which contributes approximately 20–30% of ventricular end-diastolic volume.

C. Higher pressure in the veins than in the atria: Although venous pressure drives passive filling of the atria, the actual movement of blood into the ventricles requires the atria to generate pressure higher than that in the relaxed ventricles. Without atrial contraction, ventricular filling would rely solely on passive flow, which is less efficient.

D. Contraction of the ventricles: Ventricular contraction occurs during systole, when ventricular pressure rises above atrial pressure. This causes the atrioventricular valves to close, preventing backflow into the atria. Ventricular contraction does not facilitate blood flow from atria to ventricles; it actually temporarily halts it.