Atrial systole marks the end of ventricular filling with a small volume increase

Steps:

• Identify cardiac cycle phases on the ventricular volume graph: passive filling during early diastole, followed by atrial systole, then ventricular systole with volume decrease.
• Locate the point after passive filling where volume begins a slight, rapid rise due to atrial contraction.
• This rise precedes isovolumetric contraction and ejection.
• Match this to point C, which aligns with the onset of atrial systole.

Why C is correct:

• Atrial systole starts when atria contract, adding 20-30% more blood to the ventricle via the open AV valve, causing a distinct volume uptick per standard cardiac physiology.

Why the others are wrong:

• A: Marks the start of passive diastolic filling, before atrial contraction.
• B: Indicates the end of atrial systole or start of ventricular contraction.
• D: Represents the beginning of ventricular ejection, during systole.

Final answer: C