Which of the following factors is NOT associated with afterload?

Afterload refers to the resistance that the heart must overcome to eject blood during ventricular contraction, particularly from the left ventricle. It is intimately related to the pressures and resistances presented by the systemic circulation.

Resistance plays a critical role in determining afterload, as it includes the systemic vascular resistance that the heart must work against. Pressure is also a key factor in afterload, as it reflects the amount of force the ventricles must generate to overcome the systemic vascular pressures during systole.

Arterial contractility, while more commonly related to the heart's contractile function, does have a relationship with afterload in terms of how well the arteries can accommodate and facilitate blood flow against resistance.

Volume, however, is not a direct determinant of afterload. While increased vascular volume can lead to elevated blood pressure, which might influence afterload indirectly, volume itself does not define the resistance or pressure the heart faces during contraction. Therefore, it is not associated with afterload in the same way that pressure, resistance, and contractility are.