Binary vascular system
A binary vascular system was a type of circulatory system in which all the blood vessels lead to two hearts.
Species with a Binary Vascular System
In general, binary vascular systems were not common amongst near-Humans. In the Mutter's Spiral galaxy, a few monopod species and a drone species had two hearts. (NA: Tragedy Day)
The Doctor once encountered another double-hearted near-Human species called the Dulcians. (DW: The Dominators) Later, he met the Kulan, another race whose overall stamina seemed to be improved over Humans by their two hearts. (EDA: Escape Velocity) The Kirithons were genetically engineered by the Panjistri to have two hearts. (NA: Timewyrm: Apocalypse)
Gallifreyans were perhaps the most notable species known to possess a binary vascular systems. (DW: Spearhead from Space, Robot, The Christmas Invasion)
Benefits
The full implications of a binary vascular system were unclear. However, the Dulcian Rago once told Jamie McCrimmon that beings with single hearts were physically weaker than those with two hearts. (DW: The Dominators)
Similarly, when Henry van Statten performed an invasive scan of the Ninth Doctor, he marvelled 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)
A Gallifreyan could survive with only one heart, should the other be punctured or destroyed, though he or she would be weakened greatly. (MA: Managra, EDA: The Adventuress of Henrietta Street, DW:The Shakespeare Code)
A binary vascular system provided some level of protection from attack, in that single-hearted near-Humans failed to aim correctly when attempting a close-range attack. The Fifth Doctor, for instance, was once stabbed in the chest, but survived because his Human attacker aimed for the spot where the Human heart would be. Because the Doctor's two hearts were in a slightly different location, the attack was ineffectual. (BFA: Son of the Dragon)
