why single erythrocite is yellow-green, but blood is red?

Answer:
Erythrcytes are actually more of a pink.Ruta, these weren't fixed, just a simple blood smear. Blood plasma is kind of a yellow color, so if erythrocytes were yellow-green, our blood would be too. It is the color of the hemoglobin (red when oxygenated, dark red/maroon when deoxygenated) that confers the red color onto the whole blood.
If you are so adamant to contradict everyone, I suggest you cite your source for these yellow-green erythrocytes.

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blood is red only when exposed to oxygen, because of the oxidation of iron in blood

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To Troy and Megs. Could you please read the question before answering?
Question
why single erythrocite is yellow-green, but blood is red? We know how arterial and venous blood look like and have seen it under microscope. As you know cells change while fixation of the preparation, so the fact that you see it "pink" under microscope doesn't mean it's pink naturally. I can asure that a SINGLE erythrocite is not pink or red. It is yellow-green. The question again, why a SINGLE erythrocite (not chemically affected) is yellow-green while a GROUP of erythrocites is red?

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You don't need to fix a blood cell to see that it is red. A fresh sample of blood under a 40x objective will show you a single red/pink erythrocyte. They are not yellow or green. They are red/pink. That is why blood is red. They are red because hemoglobin is red, and RBCs contain 35% hemoglobin.

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