'a'wave in JVP is seen in-
The a wave is due to atrial systole.some blood regurgitates into great veins when the atria contract.In addition,venous flow stops, the resultant rise in venous pressure contributes to a wave. (Ganong&;s Reviw of medical physiology,23rd edition,pg no.512)
Will be large when atria contracts against resistance
Filling of ventricles
It is followed by "V" waves
in inspiration
C.V.S