Figure 7.

The sinuatrial junction of reptiles and mammals. Reptiles have a sinuatrial junction which harbors the dominant pacemaker and establishes a delay between the electrical activation of the sinus venosus and the atria. This enables the sinus venosus to function as a chamber that aids the filling of the right atrium. In eutherian mammals, the sinuatrial junction is remodeled and the sinuatrial delay is lost as revealed on the ECG by the absence of a sinus venosus wave. The sinus venosus of embryonic mammals may function much like in reptiles, and the mammalian sinus node develop from tissues with a similar phenotype as the reptilian sinuatrial junction.