Physics behind CYANOSIS

Let's learn the physics behind cyanosis and link it with the basis for white clouds and blue sky

1. DID you ever wonder why the sky is blue & cloud white ?

2. Basis for Blue color of the sky White light from the sunlight (which basically consists of 7 primary colors-VIBGYOR) before striking the earth’s surface passes through earth’s atmosphere (which mostly consists of atmospheric gases like N 2 , O 2 that are smaller than the wavelength of visible light ). When sunlight passes through particles smaller than the wavelength of light, scattering of light occurs which is dependent on the wavelength of light ( Rayleigh scattering ). And the intensity of scattered light is inversely proportional to the 4 th power of wavelength of light. i.e I α 1/ λ 4 So, blue light with lesser wavelength scatters more, giving sky blue color. But wait, isn’t violet the one with smallest wavelength & shouldn’t the sky appear violet ? Well, the answer for that is : Human eyes are more sensitive to blue color than violet, so we see sky blue rather than violet.

3. Basis for white color of the clouds Besides atmospheric gases like N2,O2, our atmosphere also contains particles of size similar or larger than the wavelength of light (like dust particles, water droplets). But what happens when the sunlight passes through this particles? Well, when sunlight passes through particles of size similar or larger than the wavelength of light, then scattering of light occurs equally in all direction ( Mie scattering ) So, white light entering through clouds (water droplets) continues to remain white, that’s why clouds are perceived as white to eyes.

4. Similar is the mechanism for bluish discoloration of skin & mucous membrane in CYANOSIS . 1. Dermal collagen fibers play a major role in scattering of light when light strikes human skin. Since the size of collagen fibrils are smaller than the wavelength of visible light, Rayleigh scattering of light occurs. i.e. blue light due to smaller wavelength scatters more, giving blue skin color. Well you might think then, why is the skin not blue normally? Its because other factors also play a role in skin color besides scattering of light (pigments like melanin, oxyhemoglobin, deoxyhemoglobin , carotene) 2. Another reason for blue skin color in cyanosis could be that : Deoxyhemoglobin is less red than oxyhemoglobin. So deoxyhemoglobin absorbs more red spectrum of light, causing blue spectrum to predominate in reflected light. Since cyanosis is most frequently due to increased deoxyhemoglobin concentration in vasculature, this might be one reason for blue skin color in cyanosis.

5. References https://www.ncbi.nlm.nih.gov/books/NBK367 / https://content.sciendo.com/view/journals/sjdv/2/4/article-p131.xml?lang=en