Oxygenated blood is bright red. Once blood has dropped off its oxygen, it turns a deeper, darker red.1Kauvery Blog. Arterial Blood vs. Venous Blood – What is Arterial and Venous Blood? It is never blue, even though the veins on your wrist can look that way. The blue tint is a trick of light and skin rather than a property of blood, and the color difference between oxygen-rich and oxygen-poor blood is what a hospital fingertip clip reads to check your oxygen.
Why Oxygen Turns Blood Bright Red
Blood gets its color from hemoglobin, an iron-containing protein packed into red blood cells that carries oxygen. When hemoglobin grabs oxygen in the lungs, its shape shifts slightly and it reflects a vivid bright red.2Acibadem Hospitals Group. Is Blood Blue? The Vein Illusion, Explained When it releases that oxygen to your tissues, the shape shifts back and the color deepens, like a fresh strawberry next to a black cherry.3Acibadem Hospitals Group. Is Blood Blue? The Vein Illusion, Explained The iron at the heart of hemoglobin, and the way it holds oxygen, changes which colors of light the molecule absorbs.4Questions and Answers ​in MRI. Color of blood
The red comes entirely from hemoglobin inside the red cells. Plasma, the liquid part of blood, is a pale straw yellow. Each red cell holds a huge number of hemoglobin molecules, which is why the color is so intense.
Because the color shifts gradually with how loaded the hemoglobin is, blood is better thought of as a range of reds than as two fixed colors. Blood in the arteries, fresh from the lungs, is bright red because its hemoglobin is loaded with oxygen.5Kauvery Blog. Arterial Blood vs. Venous Blood – What is Arterial and Venous Blood? Blood in the veins, returning to the heart, has given up much of its oxygen and is dark red, almost blackish.6Kauvery Blog. Arterial Blood vs. Venous Blood – What is Arterial and Venous Blood? Some people describe it as dark reddish-purple, but it is still a red.7Questions and Answers ​in MRI. Color of blood
Blood Without Oxygen Is Dark Red, Not Blue
The idea of blue blood comes from drawing conventions. Many textbooks and anatomy diagrams colored oxygen-poor blood blue purely as visual shorthand.8Private Blood Tests London. What Colour Is Blood Inside Your Body Before Oxygen? Blue helped students tell the two circulation paths apart at a glance.9Private Blood Tests London. What Colour Is Blood Inside Your Body Before Oxygen? The idea then stuck as fact.10Private Blood Tests London. What Colour Is Blood Inside Your Body Before Oxygen? Even the most oxygen-poor human blood stays a dark red.11Acibadem Hospitals Group. Is Blood Blue? The Vein Illusion, Explained
You can see the proof yourself. Blood drawn from a vein into a tube is dark red the moment it appears, before air touches it.
People often claim that blood is blue inside the body and turns red on contact with air. Blood from a cut can brighten slightly as it meets air, which feeds the myth, but that change is between shades of red, not from blue.
Why Your Veins Look Blue
How a vein looks depends on four things: how skin scatters and absorbs different colors of light, how much oxygen the blood holds, how wide and deep the vessel is, and how your eyes and brain judge color.12PubMed. Why do veins appear blue? A new look at an old question A layer of fine protein fibers just under the skin’s surface scatters blue light back toward your eyes.13PubMed. Elucidating the contribution of Rayleigh scattering to the bluish appearance of veins Red light travels deeper into skin and gets absorbed by the blood in a vein, so less red comes back from that spot. The vein looks bluish compared with the skin around it, even though the blood inside is red.
Depth and size matter too. Vessel diameter and depth help set how blue a vein appears.14PubMed. Why do veins appear blue? A new look at an old question Tiny vessels near the surface tend to look red or pink, while larger, deeper veins look blue or green.
Try it on your inner wrist. On lighter skin the veins look blue or green; on darker skin they may look greener or be harder to see.
How a Fingertip Oxygen Clip Uses Blood Color
A pulse oximeter, the clip a nurse puts on your finger, relies on oxygen-rich and oxygen-poor hemoglobin absorbing light differently.15Analog Devices. Guidelines for SpO2 Measurement It shines red and infrared light, the two colors where that difference is largest.16PubMed Central. Pulse oximetry: fundamentals and technology update It reads only the throbbing part of the signal, which comes from artery blood with each heartbeat.17Analog Devices. Guidelines for SpO2 Measurement
One side of the clip has small lights that flash red and infrared in turn, and the other side has a sensor that measures how much of each passes through your finger. Oxygen-rich blood absorbs more infrared and lets more red light through, while oxygen-poor blood does the opposite, so the ratio between the two points to oxygen level. Because the device focuses on the part of the signal that pulses with each heartbeat, it ignores steady light absorbed by skin, bone and other tissue.
Healthy readings are usually 95 to 100 percent, and readings below about 92 percent are worth raising with a doctor. Cold fingers, nail polish and movement can throw readings off, and home clips are less precise than hospital equipment.
Labs do the same with a blood sample. Hospital blood-gas machines shine many colors of light through it to sort out each form of hemoglobin, including the kind bound to carbon monoxide.18Diamond Diagnostics. CO-Oximetry Explained: Indications, Parameters & Analyzer Options That matters because carbon monoxide-bound hemoglobin looks bright red like oxygen-rich blood, so a two-color clip can read it as healthy oxygen. Suspected poisoning needs a lab blood test instead.
Animals With Blue, Violet, or Clear Blood
Many animals, such as octopuses, crabs and snails, use a copper-based oxygen carrier instead of hemoglobin. Their blood turns blue when it carries oxygen and is nearly colorless without it.19European Bioinformatics Institute. What Color Is Oxygenated Blood? The copper changes its chemical state when oxygen binds, which creates the blue.20Wikipedia. Hemocyanin In these animals, unlike us, oxygen-rich blood really is blue.
Horseshoe crab blood is famously pale blue. It is collected to test vaccines and injectable drugs for contamination, and the blue comes from its copper carrier.
Other animals show other colors. Some marine worms use an iron-based carrier that turns deep violet-pink with oxygen and colorless without it.21Scholars Bank. Hemerythrin Bohr shift in the sipunculid, Themiste pyroides Certain tube worms carry oxygen with a green-tinted pigment.22ASBMB / JBC (Core repository). The primary structure of globin and linker chains from the chlorocruorin of the polychaete Sabella spallanzanii Antarctic icefish are the only backboned animals living with no hemoglobin, and their blood is whitish.23PubMed Central. Cold-Driven Hemoglobin Evolution in Antarctic Notothenioid Fishes Prior to Hemoglobin Gene Loss in White-Blooded Icefishes Their freezing, oxygen-rich water lets them survive on oxygen dissolved directly in their blood, with large hearts and wide vessels to compensate.24PubMed. The Antarctic hemoglobinless icefish, fifty five years later: a unique cardiocirculatory interplay of disaptation and phenotypic plasticity
When Human Blood Changes Color
In carbon monoxide poisoning, blood, organs and muscle take on a cherry-red color because the gas locks onto hemoglobin.25PubMed Central. Carbon monoxide poisoning
With carbon monoxide, headache, dizziness and nausea in several people indoors is a warning sign. Get everyone into fresh air right away and call emergency services, since skin color is not a reliable guide. Carbon monoxide alarms near sleeping areas give the earliest warning.
A cherry-red look is described in textbooks for carbon monoxide poisoning but often does not show up in living patients, who may look pale or normal. Doctors rely on a blood test that measures carbon monoxide-bound hemoglobin directly, since appearance and a finger clip can both mislead. Cyanide stops cells from using oxygen, so vein blood stays bright red like artery blood.26Picmonic. Cyanide Poisoning: Pathophysiology and Clinical Manifestation It does this by blocking the cell machinery that burns oxygen for energy.27PubMed Central. Cyanide Toxicity!! Colour of Blood Says It All
Blood can also turn brown or greenish. In methemoglobinemia, the iron in hemoglobin is changed into a form that cannot carry oxygen.28Clinical Chemistry. Laboratory Assessment of Oxygenation in Methemoglobinemia Artery blood turns chocolate brown instead of red.29Wikipedia. Methemoglobinemia Tissues can be starved of oxygen, which can be fatal in severe cases.30Clinical Chemistry. Laboratory Assessment of Oxygenation in Methemoglobinemia A rarer condition, sulfhemoglobinemia, has been reported after misuse of the urinary pain drug phenazopyridine and can tint blood greenish-dark.31PubMed Central. Sulfhemoglobinemia in a 53-Year-Old With a History of Phenazopyridine Misuse
Cold can even redden blood after death. Pooled blood in the skin normally looks bluish-purple,32PubMed. Spectrophotometric evaluation of postmortem lividity but in cold conditions it can turn cherry red as hemoglobin picks oxygen back up.33PubMed. Re-oxygenation of haemoglobin in livores after post-mortem exposure to a cold environment