Death's Circular Heart

First on page 492, and then later on p. 493 Death says to have a circular heartbeat, and humans with linear ones.

When Death first mentions his circular heart, the importance of the sentence focuses on the crime that he assigns himself due to his job; the (in his mind) taking of peoples lives. He then says that he has a "cursed circular heartbeat" that "revolves" (492).

Later, on page 493 Death mentions again that he has a circular heartbeat, and adds that humans have linear ones, but again, he changes subjects and starts admiring that humans have the sense to die.

Puzzled, I arrived at the following conclusion: Death's "circular heartbeat" grants him immortality, since his life is a never-ending circle. Humans, like a line that goes from point A to point B, have a linear hearts or heartbeats: they are born, and then at the end of the line they die.

This cryptic statement (how unusual, coming from Death!) left me wondering what other ways this statement could be interpreted and if human's "linear" heatbeats were metaphors for anything else...