BLOOD PIGMENTS The red pigment of the .corpuscles, which is responsible for their capacity to carry oxygen, is called haemoglobin. It consists of a compound of a protein "globin" with a crystalline substance haematin.
Haematin has the formula C3,H.N404Fe0H. Its principal chemical characteristics are (I) that it contains iron; (2) that it contains four pyrrol groupings— a fact which gives it a superficial relationship to chlorophyll; (3) it is easily reduced and reoxidized by chemical reagents. This oxidation does not seem actually to involve the valency of the iron, which is in the ferrous condition throughout. In haemoglobin the ratio of the iron by weight to the detachable oxygen (labile) is by weight as 56:32. In small quantities haematin is widely distrib uted both in the animal and vegetable kingdoms, as has recently been shown by Keilin. It forms compounds, other than haemo globin, with proteins; a conjunction of three such is frequently found both in animals and plants giving the spectroscopic appear ance of a substance with four absorption bands. To their spec trum the name cytochrome was given. In reality it consists of six bands, three of which overlap and is really three spectra, each of two bands superposed. The three spectra correspond of course to the three haematin-protein compounds mentioned above. The function of cytochrome appears to be that of a catalyst.
Unlike cytochrome, haemo globin can easily be crystallized, the corpuscles of different animals yielding crystals of different forms. They present specific differ ences in solubility, in the power of uniting with gases and in the position of the bands in their absorption spectra. Such differences are believed to be due to the specific characters of the globin. Haemoglobin unites with oxygen to form a compound of so loose a nature that (I) it can be completely broken down by a vacuum and (2) proportion of the whole haemoglobin oxidized is a func tion of the partial pressure of oxygen to which it is exposed. On this property depends the value of the pigment as a carrier of oxygen (see RESPIRATION).
Haemoglobin unites with carbon monoxide in a manner quite similar to that of its union with oxygen, but the affinity of the pigment for carbon monoxide is much greater than for oxygen; at the body temperature in man it is about 25o times as great. (For carbon monoxide poisoning, see ANOXAEMIA.) The spectrum also of carboxyhaemoglobin is very similar to that of oxyhaemoglobin, having two absorption bands in the green as well as a considerable absorption of the ends of the spectrum, especially the blue end. It also has an absorption band in the ultra violet. Haemoglobin also unites with nitrous oxide; their compound does not occur in blood.
In all the above compounds the iron is in the ferrous state; a compound sometimes found in the blood in certain cases of poisoning (see ANOXAEMIA) is methaemo globin, which differs from ordinary haemoglobin by containing its iron in the ferric condition. Its absorption spectrum contains a conspicuous band in the red. It is formed by the action of potassium ferricyanide, nitrites, nitro-bodies, etc., on haemoglobin in the blood.
There are some other important deriva tives of haemoglobin which do not occur in blood—of these per haps the most important is haemochromogen, a body in which haematin, in the reduced condition, is united not with native globin but with another protein formed by the action of strong alkali on globin and known as denatured globin, principally because it is insoluble at the point of neutrality. The practical importance of haemochromogen lies in two facts: (I) that it is very easily made from old blood by the action of alkali in com bination with a reducing agent; (2) of all the blood pigments it is the one whose spectrum can be recognized in the greatest dilu tion. Therefore in medico-legal cases stains suspected of being due to blood can be tested by attempting the conversion of the pigment into haemochromogen and the observation of the material obtained with the spectroscope. (J. BAR.) BIBLIOGRAPHY.-E. H. Starling, Principles of Human Physiology Bibliography.-E. H. Starling, Principles of Human Physiology (19o7); Macleod, Physiology and Biochemistry in Modern Medicine (192o) ; J. S. Haldane, Respiration (New Haven, 1922) ; Sir W. M. Bayliss, Principles of General Physiology (London, 1924) ; E. Ponder, The Erythrocyte and the Action of Simpler Haemolysins (Edinburgh, 1924) ; J. Barcroft, The Respiratory Function of the Blood (1925— 28) ; Olv. de Wesselow, Chemistry of Blood in Clinical Medicine (London, 1924) ; A. Mathews, Physiological Chemistry (1925); H. Woolard, Recent Research in Anatomy (London, 1927) ; J. Pryde, Recent Research in Biochemistry (London, 1928) ; C. L. Evans, Recent Research in Physiology (1928). Also articles in the following periodicals: Harvey Lectures, series i.—xxi. (Baltimore), and Physio logical Review (Baltimore).
