Is Punnett Square what you are looking for?
The answer is the platelets
In a typical ECG (Electrocardiogram) the first wave obtained is P wave; The P wave represents the depolarization of the atria. After this, the QRS complex is formed. The QRS wave represents the depolarization of the ventricles. During the depolarization phase, the ventricles contracts and pushes the blood into the atria. Hence, the correct answer is depolarization of the ventricles.
<em>They are exothermic as they take temperature from the surrounding and then maintain their internal body temperature according to the amount of heat absorbed :)</em>