Different colours of blood

When you think of blood, chances are you think of the colour red. But blood actually comes in a variety of colors, including red, blue, green, and purple. This rainbow of colors can be traced to the protein molecules that carry oxygen in the blood. Different proteins produce different colors.

Humans, along with most other animals, birds, reptiles, and fish, have red blood. We all use an oxygen-carrying blood protein, known as hemoglobin, that contains iron. It’s the iron that gives blood its dark red color in the body.  When blood comes into contact with air, it turns the classic scarlet red. Some people appear to have blue blood in their veins. That’s just an optical illusion caused by the way the skin filters light.

However, 

on June 8, 2007, Canadian anesthesiologists Dr. Stephan Schwarz, Dr. Giuseppe Del Vicario, and Dr. Alana Flexman presented an unusual case in The Lancet.[1] A 42-year-old male patient was brought into Vancouver's St. Paul's Hospital after falling asleep in a kneeling position, which caused compartment syndrome and a buildup of pressure in his legs.

When doctors drew the man's blood prior to performing the surgery to relieve the pressure from the man's legs, they noted his blood was green. A sample of the blood was immediately sent to a lab. In this case, sulfhemoglobinemia was possibly caused by the patient taking higher-than-prescribed doses of sumatriptan (2,3). He took it for migraines.

Sulfhemoglobinemia is a rare condition in which there is excess sulfhemoglobin (SulfHb) in the blood. The pigment is a greenish derivative of hemoglobin which cannot be converted back to normal, functional hemoglobin. It causes cyanosis even at low blood levels.

It is a rare blood condition in which the β-pyrrole ring of the hemoglobin molecule has the ability to bind irreversibly to any substance containing a sulfur atom.[1][2] When hydrogen sulfide (H2S) (or sulfide ions) and ferrous ions combine in the heme of hemoglobin, the blood is thus incapable of transporting oxygen to the tissues (4,5).

Symptoms include a blueish or greenish coloration of the blood (cyanosis), skin, and mucous membranes, even though a blood count test may not show any abnormalities in the blood. This discoloration is caused by greater than 5 grams per cent of deoxyhemoglobin, or 1.5 grams per cent of methemoglobin, or 0.5 grams per cent of sulfhemoglobin, all serious medical abnormalities. Sulfhemoglobinemia is usually drug induced, with drugs associated with it including sulphonamides, such as sulfasalazine or sumatriptan. Another possible cause is occupational exposure to sulfur compounds.

This is called sulfhemoglobinemia.

The condition generally resolves itself with erythrocyte (red blood cell) turnover, although blood transfusions can be necessary in extreme cases.

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 It is a myth that deoxygenated blood is blue; all blood in the human body is red. Human blood contains hemoglobin, which is a complex protein molecule in red blood cells. Hemoglobin contains iron. The iron reacts with oxygen, giving blood its red color.

The level or amount of oxygen in the blood determines the hue of red. As blood leaves the heart and is oxygen-rich, it is bright red.

When the blood returns to the heart, it has less oxygen. It is still red but will be darker. This darker red appears blue because of how light travels through the skin.

Similar to people, most animals also have red blood. However, there are a few exceptions (6).

 They include snails, spiders, slugs, octopuses, and squid. The protein that carries oxygen in these creatures is called hemocyanin. Instead of iron, this protein contains copper. The blood appears clear when it’s not carrying oxygen. It turns blue when it picks up oxygen. Some types of octopus, squid, and crustaceans have blue blood. Their blood contains a high concentration of copper. When copper mixes with oxygen, it gives their blood its blue color.

Blue and red are not the only possible colors of blood — some animals bleed green.

The skink, which is a type of lizard, has green blood due to a buildup of biliverdin. Biliverdin and bilirubin are by-products of the liver. Humans also produce these two by-products.

Other creatures with green blood include fantastically shaped sea worms, some leeches, and earthworms. These animals have a blood protein called chlorocruorin. It’s similar to hemoglobin but doesn’t hold oxygen as tightly. Also, it floats free in the bloodstream instead of being inside a blood cell.

However, the human body sends biliverdin and bilirubin to the intestines and through the digestive system to be excreted. Skinks do not excrete biliverdin, so it builds up in their body, making the blood green.

Peanut worms, duck leeches, and bristle worms, all of which live in the ocean, use the protein hemerythrin to carry oxygen in the blood. Without oxygen, their blood is clear in colour. When it carries oxygen, it turns purple .

Footnotes:

1. Flexman AM, Del Vicario G, Schwarz SK (June 2007). "Dark green blood in the operating theatre". Lancet. 369 (9577): 1972. doi:10.1016/S0140-6736(07)60918-0. PMID 17560450. S2CID 39437785.

2. Patient bleeds dark green blood", BBC News, 8 June 2007

3.  "Dark Green Blood In The Operating Theatre" Archived 2007-07-11 at the Wayback Machine, Medical News Today, June 8, 2007

4.  Gharahbaghian, Laleh; Massoudian, Bobby; DiMassa, Giancarlo (August 2009). "Methemoglobinemia and Sulfhemoglobinemia in Two Pediatric Patients.... Western Journal of Emergency Medicine. 10 (3): 197–201. ISSN 1936-900X. PMC 2729224. PMID 19718385.

5.  Curry, Steven (June 6, 2007). "14". In Shannon, Michael; Borron, Stephen; Burns, Michael (eds.). Haddad and Winchester's Clinical Management of Poisoning and Drug Overdose (4 ed.). Saunders. p. 289. doi:10.1016/B978-0-7216-0693-4.50019-0. ISBN 9780721606934. Archived from the original on July 28, 2021. Retrieved July 29, 2021.

6. https://biobeat.nigms.nih.gov/2019/02/roses-are-red-and-so-is-blood....