What determine whether blood is bright red or dull brick red?

Answer 1

The color comes from hemoglobin and the hemoglobin changes color depending upon what is bound to it. Oxyhemoglobin (hemoglobin with oxygen bound to it), deoxyhemoglobin (hemoglobin with no oxygen bound to it) is somewhat bluish. Arterial blood, which has been to the lungs and is on its way to the tissues, is usually fairly saturated with oxygen (anywhere from 95-100% of the hemoglobin is bound to oxygen), and so it looks bright red. Blood in the veins, coming back from the tissues, is less saturated (perhaps 60-80%), and so it appears darker. If there is enough unbound hemoglobin in the blood, it may actually take on a bluish hue, and that is why someone may appear blue if they are not getting oxygen. Carbon monoxide also binds to hemoglobin and results in an even brighter red color than when oxygen binds. Someone with carbon monoxide poisoning may have even brighter red blood and may have a bright red coloration to the lips and fingernails. It is also possible for the iron in hemoglobin to be changed from the ferrous (2+) to ferric (3+) state and the result is methemoglobin, which is a more brownish compound. There is normally a very small amount of methemoglobin circulating in the blood and this can be increased by exposure to certain chemicals (in particular, certain nitrogen compounds). Interestingly, methemoglobin is capable of binding cyanide, so the "antidote" for cyanide poisoning is a nitrogen compound that converts hemoglobin to methemoglobin, which then binds up free cyanide in the blood.

Under normal circumstances, the blood contains primarily oxyhemoglobin and deoxyhemoglobin.