Why the larger arteries appear to be white rather than red but veins are blue?

Larger arteries appear white rather than red primarily due to the optical phenomenon known as Rayleigh scattering. This scattering effect causes blue and violet light waves to be dispersed more than other colors of the visible spectrum when light interacts with particles or molecules in the atmosphere or other media.

Here's an explanation of why this happens:

1. Light Scattering: When light, including sunlight, passes through any medium, it can interact with particles or molecules present in that medium. This interaction can cause the light to be scattered or redirected in various directions. The amount of scattering depends on the wavelength of light and the size of the particles or molecules.

2. Rayleigh Scattering: Rayleigh scattering is a specific type of light scattering that occurs when light interacts with particles or molecules that are much smaller than the wavelength of light. This type of scattering is most effective at shorter wavelengths, such as blue and violet light, and less effective at longer wavelengths, like red and orange light.

3. Artery Walls: The walls of larger arteries are relatively thick and contain many connective tissue fibers, including elastin and collagen. These fibers are arranged in a regular pattern and act as scattering centers for light. When white light, which contains all colors of the visible spectrum, passes through the artery wall, the blue and violet light components are scattered more effectively due to Rayleigh scattering.

4. Red Absorption: In addition to scattering, some of the light is absorbed by the tissues and hemoglobin within the artery walls. Hemoglobin, the oxygen-carrying protein in red blood cells, absorbs light in the blue-green region of the spectrum. However, the amount of absorption is not significant enough to completely counteract the scattering effect.

As a result of the combined effects of Rayleigh scattering and absorption, a greater proportion of blue and violet light is scattered back towards the observer, giving the larger arteries a whitish appearance. On the other hand, veins appear blue because the deoxygenated hemoglobin within them absorbs more red and yellow light, allowing more blue light to be reflected and reach our eyes.