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Here ~ one more thing on those square hearts, they are (as noted) associated with Bradycardia ~ a type of arrhythmia in the upper Atrial chamber.

That, by itself, is frequently associated with an extra sinus ~ or nerve nexus that fires an electrical charge that makes the heart beat.

So, with a square heart, enlarged Atrial chambers, and an extra sinus you can end up with arrhythmia so severe that you are almost an invalid and will usually not get enough sleep time.

Fortunately modern science has given us a solution ~ a simple procedure where they insert a tube up through your groin into your heart. Another tube is inserted into your main artery and down to about the same spot. The physicians then send an electrical charge into the extra sinus and ablate it (which means to coagulate the cells). That ends the problem. They pull out the equipment. Put a bandage on your crotch, and you go home about 2 hours after you arrived, Cured!

One of the symptoms of having this condition involves an inability to get your heart pumping more than about 120 beats per minute except when it's involved in one of it's spells of arrhythmia.

You'll be pumping along doing what you want and your heart is chucking faster and faster and it hits 120, then it starts slowing down ~ usually as low as 40 beats per minute.

Now that's some serious stuff. On the other hand each beat is much more powerful than it normally would since the atrial chambers are actually helping to move blood through the body, and you are sucking in more oxygen since there's more blood being pushed through the lungs faster.

Been thinking about this one for many years. It has something to do with being able to get dunked in cold Arctic waters and survive ~ and to live around glaciers all the time ~ and survive that.

Remember, if you just move more air in and out of your lungs, you may get a bit more oxygen but at the same time you'll dry out your bronchial tubes and mouth. You'll lose a lot of heat too. So, if you can extract just a little bit more oxygen from the blood, and more efficiently move more CO2 into the air in the lungs on each breath, that more powerful heart can be your friend. Instead of moving air past the blood, you move the blood past the air, reduce your breathing rate, retain more body heat, and ski uphill with ease!

Much of a bird's breathing apparatus does exactly this ~ finding something similar in humans shouldn't be a surprise.