Question

Why contraction of ventricles starts at the base.

Answer 1

Since we evolved from early tetrapods, then early mammals, the heart actually evolved to pump forward (rostrally) rather than "up" (superiorly) as we observe in humans.Evolution could have just as easily found the opposite solution worked as well, but it didn't.  This is similar to asking "why our proteins are made up of only L-amoino acids, or we use D-sugars?"  It was probably a 50-50 chance at the very beginning of self replication, and these just happened to be the stereoisomers that become incorporated.  After that, the pattern was set.  Could evolution have just as well chosen D-AAs?  Absolutely; proteins could be formed just the same. Now, since the heart pumps towards the brain (up in humans, forward in dogs), the most efficient way to squeeze the left ventricle to pump blood, is from its "apex" at it's "base," up and out the aorta.  The contraction also has a helical component, not too different than how you twist a rag to wring out the water.Hope that helps.