Blood is known for transporting substances around the body. The fluid has many functions, from fighting infection to generating hydraulic force in an erect penis, but its main role is to supply oxygen and nutrients so that an animal’s cells can fuel respiration and power metabolic reactions.
Do all animals have blood?
No. Many invertebrates have open circulation: instead of true blood, they use haemolymph, a fluid that empties into the body cavity and bathes cells before it reenters blood vessels. Insect haemolymph doesn’t even carry oxygen! Instead, gases move in and out of the body through air-filled tubes known as tracheae.
This tracheal system depends on diffusion (the movement of molecules from areas of high to low concentration) for gas exchange between cells and surrounding fluid. Diffusion only works over short distances, less than 1mm, which is why insects are small.
All vertebrates have a closed circulatory system: blood doesn’t normally leave vessels and its cargo diffuses to and from cells via a separate fluid that replaces the oxygen which cells use, and removes carbon dioxide and other waste products. This extracellular fluid is recycled back to circulation while oxygen is then replenished by breathing, via gas exchange between the air and lungs (or water and gills).
How does blood circulate?
Blood flows through a vascular system, a network of vessels such as arteries, veins and tiny capillaries. The fluid is propelled by the pressure created by squeezing the whole body, contracting vessels or pumping a dedicated organ, the heart. Octopus and other cephalopods often have multiple hearts and closed circulation, which may explain how these complex molluscs can have active lifestyles and grow so large compared to other invertebrates (one colossal squid weighed in at 495kg).
What does blood contain?
On average in vertebrates, half of the volume is plasma, a yellow liquid that’s mostly water. Blood also contains three kinds of cell: fragments of ‘platelets’ that form clots to prevent bleeding; the immune system’s white blood cells; and red blood cells filled with the famous oxygen-carrying pigment haemoglobin.
More red blood cells mean more oxygen, but more cells also means thicker blood and slower transport. So there’s a trade-off and each species seems to have an optimum proportion of red cells, the ‘haematocrit’.
The world’s largest living reptile, the saltwater crocodile, is lethargic but needs sudden bursts of speed, which is consistent with having fast-flowing blood and a haematocrit of only 19 per cent. At the other extreme, the Weddell seal stores oxygen to dive hundreds of metres for several minutes, and 63 per cent of its blood consists of red blood cells.
Why is blood red?
Blood's red colour comes from iron at the core of the respiratory pigment, haemoglobin. Other pigments can have different metal cores. Haemocyanin contains copper so cephalopods are blue-blooded, like royalty! Though haemoglobin is packed inside red blood cells, other pigments (including haemocyanin) float freely in the plasma.
Which animals drink blood?
Sanguivory or haemoatophagy (blood eating) has evolved at least 29 times and created parasites ranging from ticks and tsetse flies to leeches and lampreys. The anatomy and metabolism of sanguivores is adapted to blood-feeding: a female mosquito’s mouthparts have been modified from a nectar-sucking proboscis, while a vampire bat’s saliva has anticoagulants to stop a host’s blood from clotting.
Vampire bats are the only mammals that drink blood – two species feed on birds, one on mammals. Like the vampires of folklore, bats sneak up on sleeping hosts. But bats also do something that mythical creatures don’t: they will urinate on their victims to lose water (weight) so they’re able to fly away more easily.
Blood is low in fat and bats can’t store energy efficiently, so they depend on regurgitated blood from friends and family to avoid starving to death after three days without food. Sanguivory is a challenging diet: though blood delivers nutrients, it’s not very nutritious!
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