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I have learned that Cyanosis presents with blue skin and nails. What is the reason for the blue coloration of the skin and nails? Why wont the skin and nails turn another color in the visible spectrum such as green for instance? How is this disease diagnosed?

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The skin appears blue due to the diffraction of light through the skin. Blood is either bright red (sufficiently oxygenated) or dark red. "The blue appearance of surface veins is caused mostly by the scattering of blue light away from the outside of venous tissue if the vein is at 0.5 mm deep or more. Veins and arteries appear similar when skin is removed and are seen directly" 1. The color of blood is due to the difference in color between deoxyhemoglobin and oxyhemoglobin.

Let's consider the "blue fugates" of Kentucky. The ancestors of this family have different shades of blue skin tones. Dr. Madison Cawein discovered that the family had a high concentration of FE$^{3+}$ brown methemoglobin. This form is useless to the body so the enzyme diaphorase reduces the methemoglobin to FE$^{2+}$ (hemoglobin that can carry oxygen) [2].

Basically the skin color comes down to light diffraction not the blood changing from red to blue. enter image description here

Cyanosis is the appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface having low oxygen saturation [3]. Cyanosis is a clinical sign diagnosed visually. Cyanosis is an indication for blood gas testing.

References:

  1. http://en.wikipedia.org/wiki/Venous_blood
  2. http://courses.chem.indiana.edu/c122/documents/Experiment10_BloodandHemoglobin.pdf
  3. http://en.wikipedia.org/wiki/Cyanosis
  4. http://www.webmd.com/lung/arterial-blood-gases
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When haemoglobin takes on oxygen the colour will shift from blue to red. the reason for this is a change in orientation of the Nitrogen ligans of the haemoglobin molecule.

The red cells pass through the veins in the lung(s) to the aevioli; the blue haem (pH~7.2) exchanges CO2 for O2 (now pH~7.4) 98% saturation and goes from blue to red.

We see the colour(s) because the haemoglobin's Fe in the centre of the ring goes in and out of plane to the ring. The colour comes for this phenomenon due to the 4 Nitrogen atoms and the electron states that exist without and with oxygen. This takes place within about 12milliseconds.

Hope this helps.

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