What is the primary physiological activity during mid-diastole?

During mid-diastole, the primary physiological activity is passive ventricular filling. This phase occurs after the mitral and tricuspid valves open, allowing blood to flow from the atria into the ventricles without any additional effort from the atria. The ventricular pressure is low at this time, facilitating the passive flow of blood due to the pressure gradient between the atria and ventricles.

As the heart continues through its cycle, the atria will contract later, helping to fill the ventricles more completely, but this occurs after the mid-diastole phase. Moreover, during mid-diastole, the ventricles are not contracting; in fact, they are in a state of relaxation, which allows for this passive filling to occur. The semilunar valves remain closed during this period, preventing blood from entering the aorta and pulmonary arteries, further supporting the idea that passive ventricular filling is the dominant physiological activity at this stage.