Speciation is not a precise division: overlap between closely related species can occur in areas called hybrid zones. Organisms
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The labels beside the heart (2):
Sinoatrial Node (SA node)
Atrioventricular Node (AV node)
The labels inside the wave image(5):
From LEFT to RIGHT (top to bottom):
Atrial depolarization
P wave
Ventricular depolarization
Ventricular repolarization
T Wave
Please look at the attached picture for clarification.
Here is more about your question:
The images depict the electrical conduction system of the heart.
The sinoatrial node (SA node) is located in the right atrium. It works as a pacemaker of the heart. It starts all the heartbeats and the heart rate. It spreads electrical impulses throughout both atrias, to stimulate them to contract.
The AV node is like a gateway for electrical impulses. It delays the electrical impulses going towards the ventricles to make sure that all the blood from the atria is emptied into the ventricles before contraction.
The next image is an ECG reading. It shows the action potentials that are produced by the nodes and cells of the heart.
Each wave and segment of the image shows an event that occurred in the heart as it was recorded.
P-wave occurs when the atria depolarizes. This happens when the atria fills up with blood, which initiates the SA node. The SA node then spreads electrical signals and causes them to depolarize.
QRS complex is the point where ventricular depolarization occurs.
- Q-wave, the interventricular septum depolarizes;
- R-wave, the main mass of the ventricles depolarizes;
- S-wave, the rest of the parts left near the base of the heart of the ventricles depolarizes.
T-wave indicate ventricular repolarization which happens right before the relaxation of the ventricles. At this point, the cycle will repeat.
This cycle will repeat in every heartbeat.
