Draw a standard $ECG$ and explain the different segments in it.

(N/A) An Electrocardiogram $(ECG)$ is a graphical representation of the electrical activity of the heart during a cardiac cycle.
$A$ standard $ECG$ consists of a series of waves: $P, Q, R, S,$ and $T$.
$(a)$ The $P$-wave represents the electrical excitation (or depolarisation) of the atria,which leads to the contraction of both atria.
$(b)$ The $QRS$ complex represents the depolarisation of the ventricles,which initiates ventricular contraction. The contraction starts shortly after $Q$ and marks the beginning of the systole.
$(c)$ The $T$-wave represents the return of the ventricles from an excited to a normal state (repolarisation). The end of the $T$-wave marks the end of systole.
By counting the number of $QRS$ complexes that occur in a given time period,one can determine the heart rate of an individual.