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methaemoglobin Oxidized form of haemoglobin (unlike oxyhaemoglobin, which is a loose and reversible combination with oxygen) which cannot transport oxygen to the tissues. Present in small quantities in normal blood, increased after certain drugs and after smoking; found rarely as a congenital abnormality (methaemoglobinaemia).

It can be formed in the blood of babies after consumption of the small amounts of nitrate found naturally in vegetables and in some drinking water, since the lack of acidity in the stomach permits reduction of nitrate to nitrite.