A human could swim through a blue whale's veins.
The blue whale is the world's largest living creature, dwarfing even the largest dinosaurs. Blue whales can grow to be over 100 feet long and weigh more than 100 tonnes. Their hearts are the size of a small car and can weigh up to 1,300 pounds. Blue whales have massive arteries that pump blood through their massive hearts and into their vital organs, which is unsurprising. These arteries are so large that a fully grown human could swim through them; however, you should not attempt this.