The a wave coincides with right atrial contraction, the v wave with
right atrial filling when the tricuspid valve is closed in systole. It
represents a passive rise in venous pressure (venous filling) as
venous return continues against a closed tricuspid valve.
The c wave is so small in a normal subject that it is usually not visible
to the naked eye. It is due to tricuspid valve closure, indicating
onset of ventricular contraction.