Question

Why does blood turn reddish brown?

Answer 1

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The oxygen carriage of haemoglobin is not a simple oxidation/reduction reaction. Dried haemoglobin is brown mainly due to the loss of water from the structure.

Answer 2

Menstrual flow also can look different over the course of the same period. Blood may start out bright red and turn darker red or brown toward the end of the period. Or blood may start out brown and turn red. And differences in texture, like blood clots and tissue, can happen as the uterus sheds its extra lining.