"a" wave - peak resulting from atrial systole

"c" wave - smaller peak following, due to bulging of the tricuspid valve into the right atrium

"x" descent - fall in pressure as AV valve moves downward with the contraction of the right ventricle

"v" wave - atrial diastole (ie. filling); ventricles relaxing

"y" descent - atrial blood begins flowing into ventricle

Note: similar events occur in the pulmonary arteries