What causes the high aortic pressure during mid-diastole?

High aortic pressure during mid-diastole is primarily attributed to passive filling of the ventricles. During this phase, the aortic valve is closed, and blood is not being actively pumped into the aorta from the ventricles. Instead, the aortic pressure remains relatively high due to the elastic properties of the aortic walls, which allows them to accommodate the blood pressure generated during previous ventricular systole. The pressure in the aorta is maintained as the blood continues to flow back from the peripheral circulation and as the elastic recoil of the aorta helps maintain pressure.

The mechanism of passive filling involves the early part of ventricular diastole where the ventricles relax, and blood flows from the atria into the ventricles. While the atrial contraction does contribute to the filling of the ventricles, it occurs later in diastole and has less impact on mid-diastolic pressure. Ventricular contraction, on the other hand, occurs during systole and does not play a role in maintaining aortic pressure during mid-diastole. Similarly, a blood volume increase can raise overall pressures but is not the direct cause of the high aortic pressure specifically during mid-diastole.