MISCONCEPTION: A Blue Whales heart is extremely voluminous, the same size as a car . Also its main artery the Aorta is so wide that a thin person or child could swim down it.
FACT: The heart of a fully grown is actually that massive, in fact it is the size of a small car, such as a Volkswagen Beetle. Tha Aorta is a different story. The main artery has no need to be that large and it indeed is not. The only child that could fit in would have to be a newborn baby, and that would actually be a rather sadistic thing to do.