Binary vascular system

A binary vascular system was a type of circulatory system in which all the blood vessels lead to two hearts. Time Lords were perhaps the most notable species known to possess them (DW: Spearhead from Space, Robot, The Christmas Invasion).

Others included the near-Human people of the planet Dulkis (DW: The Dominators) and, throughout the galaxy of Mutter's Spiral, a few monopod species, and a drone species (NA: Tragedy Day).

The full implications of a binary vascular system were unclear. However, the Dulcian Rago once told Jamie that beings with single hearts were physically weaker than those with two hearts. Similarly, when Henry van Statten performed an invasive scan of the Ninth Doctor, he marveled at the design and enthusiastically announced he was going to patent it. Given van Statten's obsession with superior technology, this suggested that two hearts were, literally, better than one. (DW: Dalek). Immediately after his regeneration, the Eighth Doctor was able to tear a heavy metal door from its hinges, perhaps indicating that Time Lords' vascular systems provide greater strength than a human's (Doctor Who (1996)).