Human blood: red pigment, blue-looking veins and the measurements between them
Human blood remains red as its oxygen content changes: oxygen-rich blood is brighter, while oxygen-poor blood is a deeper red. Hemoglobin supplies the pigment and binds oxygen. A blue-looking vein beneath skin therefore presents an optical puzzle rather than a sample of blue liquid. Laboratory spectra, measurements inside vessels and computer-generated skin images examine different parts of that puzzle. Their results distinguish the properties of blood itself from the light that eventually reaches an observer through surrounding tissue.
The route of human blood also prevents a simple equation between veins and low oxygen. NHLBI's description of circulation follows oxygen-poor blood through the right side of the heart to the lungs. Oxygen-rich blood then returns through the pulmonary veins before being pumped from the left side to the body. The pulmonary artery carries oxygen-poor blood in the opposite part of that circuit. Thus the vessel names alone cannot specify the pigment's oxygenation. The lungs change the blood's oxygen content, while the return vessels still carry the anatomical name veins.
That circulation supplies a useful distinction for optical measurements. Blood returning from body tissues and blood returning from the lungs occupy different stages in oxygen delivery. NHLBI also describes coronary veins returning oxygen-low blood from the heart muscle itself. The circulation contains multiple routes, rather than one oxygen condition shared by every vein. An instrument aimed at a particular vessel consequently measures a local part of the system.
One original investigation published in PLOS ONE on 20 November 2012 measured that local variation in a rabbit model. Jaime and colleagues used a hyperspectral imaging spectrometer coupled to a camera to examine retinal vessels. The study involved 30 rabbits and compared oxygen measurements before and after an experimentally induced retinal vein occlusion. The measurements concerned absorption across wavelengths, not a visual decision that a vessel looked red or blue. The researchers could compare affected branches with nearby branches serving as controls.
An hour after the occlusion, the affected retinal arteries' mean oxygen saturation had fallen from 85.1% to 80.6%, and the veins' from 71.4% to 64.0%. The nearby control vessels showed no statistically significant change. In animals whose obstructed veins reopened spontaneously, the decrease was reversible. These were findings in rabbits, with particular experimentally altered vessels. Their relevance to human blood optics lies in the measurement principle: spectral data can register changes in oxygen content inside a vessel, separately from the vessel's ordinary apparent hue.
In January 2019, Christian Zoller and Alwin Kienle published a different kind of primary result: software calculating how blood vessels look through skin. Their model represented the epidermis and dermis as separate layers. Vessel depth and diameter, blood oxygenation and tissue absorption and scattering were among the adjustable properties. Monte Carlo simulations tracked light transport, and a stored collection of calculations made image generation possible in about a second. The team checked its lookup-table results against independent simulations. This was a numerical model of the appearance of human blood beneath tissue, rather than a new measurement of the liquid's intrinsic color.
The optical explanation required comparison with surrounding skin. As Zoller and Kienle described it, blue-looking veins did not simply reflect more blue light than red. Red light penetrated farther, so a sufficiently deep vessel reduced returning red light more strongly relative to adjacent skin. Vessel size and depth could therefore change the apparent hue. The calculations also allowed some arteries to look bluish. Geometry and surrounding tissue mattered alongside the oxygenation of human blood; apparent blueness did not uniquely identify either a vein or oxygen-poor pigment.
Mitchell Weigand and colleagues examined the pigment more directly in a PLOS ONE study published on 3 September 2021. They compared spectral and magnetic measurements of hemoglobin obtained from human red blood cells. Their work distinguished oxygenated hemoglobin, deoxygenated hemoglobin and methemoglobin, an oxidized form unable to bind oxygen. Preparations intended to remove oxygen could also produce unwanted chemical changes. The team used absorption spectra to determine whether the resulting samples contained the intended pigment form or additional derivatives. For human blood research, a change in oxygen binding and a change in the pigment's oxidation state were experimentally separable events.
The investigators reported cleaner deoxygenated spectra with their enzyme-based preparation than with one established chemical preparation, which showed an additional spectral shoulder. They also measured oxygen-binding curves after reoxygenation and found that healthy red cells retained their oxygen-binding function. Those controls went beyond assigning a color name to a tube. The record contained a reversible change in bound oxygen, measurements identifying the pigment and a separate test of its continued oxygen-binding function.
- Zoller and Kienle, Fast and precise image generation of blood vessels embedded in skin, 2019 — pmc.ncbi.nlm.nih.gov
- Weigand and colleagues, Magnetophoretic and spectral characterization of oxyhemoglobin and deoxyhemoglobin, 2021 — journals.plos.org
- Jaime and colleagues, Acute Variations in Retinal Vascular Oxygen Content in a Rabbit Model of Retinal Venous Occlusion, 2012 — journals.plos.org
- NHLBI, How the Heart Works: How Blood Flows through the Heart — www.nhlbi.nih.gov
- Wikipedia, Venous blood: color and oxygenation — en.wikipedia.org
- Harvard Health Publishing, Veins are a key player in the body: Here’s why, 29 November 2023 — www.health.harvard.edu
- NHLBI, Your Guide to Anemia, September 2011: blood components and hemoglobin, printed page 3 — www.nhlbi.nih.gov
- Cleveland Clinic, How Varicose and Spider Veins Are Different, 14 August 2023: documented blue-green appearance — health.clevelandclinic.org