Circulation — hearts, chambers and blue blood
Reference: Circulatory system
Heart count runs from none to three, chamber count from two to four, and the blood is not necessarily red — an octopus runs on copper, and bleeds blue.
The range
- Fish — one heart, two chambers. One circuit: heart to gills to body and back. Simple, and it means blood reaches the body after already losing pressure at the gills.
- Amphibians and most reptiles — three chambers. Two circuits, but oxygenated and deoxygenated blood mix somewhat in the single ventricle. Crocodilians are the exception with four.
- Birds and mammals — four chambers, fully separated, arrived at independently in the two groups. Complete separation is what makes a high, sustained metabolic rate possible — and it is another case of the same answer reached twice.
- Octopus — three hearts. Two pump blood through the gills, one pumps it round the body; the systemic heart stops when the animal swims, which is part of why octopuses prefer to crawl.
- Insects — open circulation and no oxygen-carrying blood at all. Haemolymph bathes the organs directly, and oxygen arrives separately by air tube.
And the pigment differs. Vertebrates use iron-based haemoglobin, which is red. Octopuses, squid and horseshoe crabs use copper-based haemocyanin, which is blue, and which works better in cold, low-oxygen water.