The Eridian circulatory system differs significantly from those we see in humans and other terrestrial life, having evolved under the high pressure, high temperature, ammonia-heavy conditions on Erid.

Eridians have two separate circulatory systems, “ambient” and “hot”. Both contain vessels that can expand or contract for heat regulation purposes, with the ambient system working in conjunction with what Grace refers to as Rocky’s “radiator” organ 14.184 and the hot system defending the body from pathogens 24.098 (as Eridians lack anything that we would recognize as a separate immune system).

Rocky’s blood is also said to be “mostly liquid mercury” 14.178 which is pumped by his ambient circulatory system; Ryland concludes that this must be a major factor in how heavy Eridians are.

While the specifics of Eridian physiology exist only in the realm of Andy Weir’s imagination, there are some organisms on Earth who have fascinatingly odd circulatory systems.

The phylum Mollusca certainly puts forward strong contenders in our unofficial Weird Circulatory System Contest. Octopuses, squid, and cuttlefish all have three hearts. Multiple hearts allow these aquatic creatures to circulate blood around their bodies as well as specifically to their gills for oxygenation. Earthworms are sometimes said to have five hearts, though they technically have multiple pairs of aortic arches rather than a true heart. Other organisms such as Platyhelminthes (flatworms) have no circulatory system at all.

We humans and most other vertebrates have red blood thanks to iron-containing hemoglobin serving as our internal oxygen transportation mechanism. However, hemoglobin isn’t the only oxygen-transporting protein in the animal kingdom. Copper-containing hemocynanin, found in mollusks and arthropods, turns green when oxygenated. Interestingly, Star Trek’s Vulcans, who hail from the same star system as Rocky (see #4), also have green copper-based blood.

Mercury, being a neurotoxic heavy metal unfriendly to our squishy terrestrial bodies, is not a known respiratory pigment outside of science fiction.