Oxidised 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.
A dark brown substance formed from haemoglobin which develops during illness, following treatment with some drugs. Methae moglobin cannot transport oxygen round the body, and so causes cyanosis.
A derivative of haemoglobin in which the iron has been oxidized from ferrous to ferric form. It does not combine with oxygen and therefore plays no part in oxygen transport from lungs to blood to tissues. Normal concentration of methaemoglobin in red blood cells is less than 1 per cent of the total haemoglobin.
An anomalous substance created through the oxidation of the iron within hemoglobin, the pigment found in red blood cells, differs from oxyhemoglobin in its inability to absorb and convey oxygen.
Ordinary blood usually contains a small quantity of methemoglobin. Elevated levels of methemoglobin in the blood, a condition called methemoglobinemia, can stem from poisoning by substances like nitrates and chlorates. In certain instances, the surplus arises due to an inherited abnormality in hemoglobin. This results in symptoms such as cyanosis (bluish-grey discoloration of extremities and lips), headaches, drowsiness, and breathlessness. Excessively high methemoglobin levels can lead to unconsciousness and even death. Treatment involves the use of a substance known as methylene blue.