The colour of blood

By Isabelle Hall

“For the life of the flesh is in the blood” (Leviticus 17:11). Through mediation of numerous processes including oxygen transport and waste removal, blood sustains us. Much can be learned about a species through examination of this tissue, and one revealing feature is its hue. Across the animal kingdom, wide variation is found in the colour of blood.

Within the human body, blood runs red. This is due to the presence of iron in the haemoglobin molecules carried by erythrocytes, enabling provision of oxygen to cells. Venous blood is darker as a result of altered haemoglobin conformation, following the release of oxygen. It has been misconceived that human blood can exist in a blue form, based on the appearance of veins. However, this is thought to be related to factors such as Rayleigh scattering and varying penetration of different wavelengths of light through the skin (Van Leeuwen & Baranoski, 2018).

However, animals with blue blood do in fact exist. Examples include the horseshoe crab and many species of octopus. In these cases, haemocyanin gives rise to the bluish hue observed. This molecule contains two copper ions, which act as inorganic cofactors. Upon binding of molecular oxygen, the ions are oxidised to Cu(II), resulting in formation of a blue complex (Hoogenboom, 2019). It appears that haemocyanin plays a role in temperature compensation among those species which utilise it (Oellermann et al., 2015).

Further variation in such oxygen-carrying molecules is responsible for other blood colours, such as that observed in many brachiopods. Here, a violet shade is produced from the oxygenated form of a non-haem iron-containing protein known as haemerythrin (Klippenstein, 1980). Among certain annelids, green blood is seen due to the presence of chlorocruorin. This is a dichroic haem protein, appearing red when highly concentrated, and green in more dilute solutions (Fox, 1949).  

The absence of respiratory pigments such as haemoglobin can also lead to clear blood, as seen in the Antarctic blackfin icefish. In this case, oxygen is only transported through circulation in physical solution. Consequently, oxygen-carrying capacity is significantly lower, and compensatory adaptations have developed in response. These include increased heart size and stroke volume, and alterations to mitochondrial density. A plentiful supply of oxygen in frigid Antarctic waters (higher compared to the levels present in warmer waters) also helps to counteract the effect of lower oxygen-carrying capacity within the blood, enabling survival of this species. It is thought that the lack of haemoglobin may provide an advantage by reducing the viscosity of the blood, allowing easier circulation through vasculature in the cold environment (Castañón, 2019).  

The shade of an animal’s blood may also vary as a result of processes involved in catabolism of respiratory pigment components. Among species of the skink genus Prasinohaema, green blood is observed. These lizards do not rely on the aforementioned chlorocruorin for oxygen transport; instead, they are dependent on haemoglobin. The breakdown of haem produces biliverdin – it is the high concentration of this bile pigment in the plasma which gives rise to the green hue, obscuring red colouration from the haemoglobin.  

The levels of biliverdin in the plasma of these skinks are the highest recorded for any organism, though it appears they have not suffered any negative consequences. In vertebrates, accumulation of biliverdin in tissues or circulation generally leads to jaundice (Austin & Jessing, 1994). This pigment exhibits toxic effects, and is associated with DNA damage. However, the maintenance of this feature across multiple species suggests a selective advantage. One theory is that the high levels of biliverdin provide protection against malaria: previous research has indicated that biliverdin delays the life cycle of the common malarial parasite Plasmodium falciparum. The mechanism responsible for this may involve binding to enolase, an enzyme in the glycolytic pathway of the parasite. A decline in the activity of enolase can impair cell cycle progression (Alves et al., 2016).

From these cases, it is apparent that observation of blood colour can help identify numerous characteristics across species, such as preference for a specific respiratory pigment. It can also be indicative of defence mechanisms used to combat infection. Further exploration within this field may enhance understanding of certain adaptations and the evolutionary pathways through which they have arisen.


Alves, E., Maluf, F.V., Bueno, V.B., Guido, R.V., Oliva, G., Singh, M., Scarpelli, P., Costa, F., Sartorello, R., Catalani, L.H. and Brady, D. (2016). Biliverdin targets enolase and eukaryotic initiation factor 2 (eIF2α) to reduce the growth of intraerythrocytic development of the malaria parasite Plasmodium falciparum. Scientific Reports. 6 (1), 1-12. Available from: doi:  10.1038/srep22093

Austin, C.C. & Jessing, K.W. (1994). Green-blood pigmentation in lizards. Comparative Biochemistry and Physiology Part A: Physiology. 109 (3), 619-626. Available from: doi: 10.1016/0300-9629(94)90201-1 

Castañón, L. (2019). Icefish have antifreeze in their blood, anemia, and osteoporosis. And they feel just fine. Available from: [Accessed 2nd March 2021]

Fox, H.M. (1949). On chlorocruorin and haemoglobin. Proceedings of the Royal Society of London. Series B-Biological Sciences. 136 (884), 378-388. Available from: doi:  10.1098/rspb.1949.0031

Hoogenboom, R. (2019). Copper Curiosity: From Blue Blood to Click Chemistry. Australian Journal of Chemistry. 72 (7), 490-491. Available from: doi: 10.1071/CH19144 

Klippenstein, G.L. (1980). Structural aspects of hemerythrin and myohemerythrin. American Zoologist. 20 (1), 39-51. Available from: doi: 10.1093/icb/20.1.39

Oellermann, M., Lieb, B., Pörtner, H.O., Semmens, J.M. & Mark, F.C. (2015). Blue blood on ice: modulated blood oxygen transport facilitates cold compensation and eurythermy in an Antarctic octopod. Frontiers in Zoology. 12 (1), 1-17. Available from: doi: 10.1186/s12983-015-0097-x 

Van Leeuwen, S.R. & Baranoski, G.V. (2018) Elucidating the contribution of Rayleigh scattering to the bluish appearance of veins. Journal of Biomedical Optics. 23 (2), 025001. Available from: doi: 10.1117/1.JBO.23.2.025001

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