Heart:
Prior to atrial systole, blood has been flowing passively from the atrium into the ventricle through the open AV valve. During atrial systole the atrium contracts and tops off the volume in the ventricle with only a small amount of blood. Atrial contraction is complete before the ventricle begins to contract.
Atrial pressure:
The "a" wave occurs when the atrium contracts, increasing atrial pressure (yellow). Blood arriving at the heart cannot enter the atrium so it flows back up the jugular vein, causing the first discernible wave in the jugular venous pulse. Atrial pressure drops when the atria stop contracting.
ECG:
An impulse arising from the SA node results in depolarization and contraction of the atria (the right atrium contracts slightly before the left atrium). The P wave is due to this atrial depolarization.
The PR segment is electrically quiet as the depolarization proceeds to the AV node. This brief pause before contraction allows the ventricles to fill completely with blood.
Heart sounds:
A fourth heart sound (S4) is abnormal and is associated with the end of atrial emptying after atrial contraction. It occurs with hypertrophic congestive heart failure, massive pulmonary embolism, tricuspid incompetence, or cor pulmonale.