Why does the electrical impulse that simulates heart contraction first travel down to the bottom of the heart, and then stimulates contraction from the bottom of the ventricles upwards?

This happens to squeeze out blood completely, else the heart may not function properly

Explanation:

The electrical impulses are carried by bundle of His and send to the bottom of the heart via purkyne fibers. This happens in order to spread the impulse from the atrio-ventricular node to the sides of the ventricles. Now in the next round of impulse causes the ventricles to contract and squeeze out all the blood from the ventricles. If this does not happen then the heart will not be able to perform to its full capacity. Atria may also not release its entire blood into the ventricles.  

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before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.