Why are a giraffe's and a blue whale's hearts so different from ours?

A giraffe's heart is built to fight gravity, and a blue whale's heart is built to save oxygen. Both are pumps like the one in our own chests, but each is tuned to a very different problem. A giraffe must push blood up a neck that can be up to 2.4 metres long. A whale must make a single breath last through a deep dive.

Start with the giraffe. Its heart weighs about 11 kg and is about 60 cm long, and it must generate roughly double the blood pressure a human needs to keep blood flowing to the brain. The giraffe has a distinctly high arterial pressure of about 190 mmHg. Its heart wall can be as thick as 7.5 cm.

High pressure creates the opposite problems at the other end. When a giraffe lowers its head to drink, blood rushes down fairly unopposed, and a network of vessels called a rete mirabile in the upper neck prevents excess blood flow to the brain. When it raises its head again, the vessels constrict and push blood into the brain so the animal does not faint. The jugular veins have several valves, most commonly seven, to stop blood flowing back into the head. In the lower legs, thick, tight skin prevents too much blood pouring into them.

The blue whale does the reverse. Its heart is the largest known in any animal, at 180 kg in one stranded whale. But on a deep dive its heart rate can drop to just 2 beats per minute, then rise to 37 on surfacing. That slowdown is part of the diving reflex, which preferentially sends oxygen stores to the heart and brain so the animal can stay submerged longer.