ANATOMY

Detailed explanation-1: -The atrial systole is the last phase of a diastole during which the ventricular filling is completed. The atrioventricular valves are open; the semilunar valves are closed (fig. 6.1). The atria contract to eject blood into the ventricles.

Detailed explanation-2: -The P wave of the ECG represents atrial depolarization, which is followed by contraction and an increase in pressure in the atria (atrial systole). The AV valves are open, and there is no valve between the atria and veins, so this small increase in pressure is also evident in the ventricle (a wave) and in the veins.

Detailed explanation-3: -When the ventricles relax, atrial pressure exceeds ventricular pressure, the AV valves are pushed open and Page 2 blood flows into the ventricles. However, when the ventricles contract, ventricular pressure exceeds atrial pressure causing the AV valves to snap shut.

Detailed explanation-4: -During diastole, the build up of blood in the atria creates a pressure gradient that forces open the AV valves allowing for about 75% of this blood to pass into the ventricle causing a gradual increase in ventricular diastolic pressure (point A).