484 KESPIRATION It has been determined that 1 gramme of haemoglobin can link to itself 1-671 cc. of oxygen gas (at C. and 760 mm. pressure) in the above-described loose manner, that is, in such a manner that when admitted to a vacuous space at a moderate temperature the two become again dissociated. When blood or a solution of oxyhaemoglobin is shaken up with carbon monoxide, the " dissociable " or "respiratory" oxygen is displaced, and a new compound of CO and haemoglobin is formed which has a spectrum very like that of oxyhaemoglobin, but is incapable of reduction by the means which are sufficient to reduce oxyhjemoglobin. In like manner (with certain precautions) a com- pound of haemoglobin with NO may be formed. Various products of the destructive decomposition of haemoglobin are known, possessing characteristic spectra and properties ; these need not be further described. Proteid Substances. Besides oxyhaemoglobin, the red corpuscles contain other proteid substances, probably for the most part para- globulin. Lecithin. Cholesterin. Inorganic Salts. Potassium and phosphates are the main con- stituent ; sodium, calcium, and magnesium are also found as chlorides and sulphates. Gases. See below. Constituents of the IVTiile Corpuscles. Proteid Substances. Several varieties have been separated. Lecithin. Certain extractives, including glycogen. Inorganic Salts. Especially potassium and phosphates. The Gases of the Blood. The blood when admitted into a vacuous space readily gives up more than half its volume of mixed gases, consisting of oxygen, carbon dioxide, and nitrogen. The oxygen is present in much larger quantities than could be held in simple solution by the water of the blood ; it is in fact mainly held in feeble combination by the haemoglobin of the coloured corpuscles ; only a trace of it is, under ordinary circumstances, held in true solution. The carbon dioxide, whilst not existing in larger quantity in blood than would be possible if it were simply dissolved by the water of that fluid, is From 100 vols. of Blood. O. C0 2 . N. Arterial 22 vols. 8-12 vols. 36 vols. 40-50 vols. 1-2 vols. 1-2 vols. Venous FIG. 11. Pump for extracting gases of the blood, a, filling-globe ; 6, barometer bulb ; d, three-way stopcock (B C D, the same enlarged, A, its plug) ; e, gas- deliyery tube ; /, mercurial gauge ; g, drying apparatus, containing sulphuric acid ; ft, froth-chamber ; t, bulb in which the blood is boiled. nevertheless to a small extent in a state of loose chemical combina- tion in which both plasma and corpuscles share. The nitrogen is held in a state of simple solution in the liquor sanguinis. These gases are yielded in different proportions by arterial and venous blood, as is shown in the following table : The method of extracting the gases of the blood which lias been found to be most convenient is one in which the blood is introduced into the Torricellian vacuum existing above the column of mercury in a barometer. Blood is collected over mercury in such a way as to avoid all access of air ; it is then introduced by an appropriate mechanism into the space above the barometric mercury. At once the gases froth up and fill the space ; the aqueous vapour which at the same time arises is absorbed by a special drying apparatus com- municating with the vacuum. Means exist of warming the vacu- ous space up to about 45 C. , which is a temperature sufficient to cause the escape of nearly all the gases of the blood ; the last por- tions of carbonic acid are, however, more rapidly evolved by allowing a small volume of a thoroughly boiled-out solution of phosphoric acid to enter the blood receptacle near the close of the operation. How the Gaseous Exchanges are Effected in Respiration. For the purposes of description, and in reference exclusively to its respiratory function, the blood may be looked upon as a watery solution of certain substances having respectively a slight chemical attraction for oxygen and for carbon dioxide. The chief of these substances is haemoglobin, which is concerned solely with the oxygen ; others, less perfectly known, are concerned with the carbon dioxide, and include certain saline constituents of the plasma (NaHCOg) 1 and perhaps also certain constituents of the corpuscles. The affinity of these substances for oxygen and carbon dioxide respectively is of so slight a nature that the mere exposure of the substances to a vacuum at a certain moderate temperature is sufficient to overcome the affinity, and dissociate the captive gases. Granted such a solution in the blood-vessels of the lungs, separated from the air of the air cells by the thin moist membranes of the alveolar walls, how are the gaseous exchanges of respiration brought about ? In the first place we must premise certain physical relationships which exist between gases and fluids in contact with one another. If a definite quantity of pure water be exposed to the air it will at a given temperature dissolve a definite volume of air. If the air be compressed to one-half (or one-quarter) of its original bulk the water will still absorb the same volume of it, albeit that volume now contains twice (or four times) the former quantity of gas. If the pressure be diminished again the air will escape from the water exactly in proportion to the diminished pressure. This is a con- crete example of Dalton's law that the quantity of a gas dissolved by a liquid varies directly as the pressure of the gas at the surface of the liquid. What is true of the air is true of its constituents. In the above example the quantity of the constituent oxygen absorbed after the compression of the air is twice (or four times) the quantity absorbed before compression. But the quantity which is absorbed of any constituent of a gaseous mixture depends, not upon the total pressure exerted on the liquid by all the gases of the mixture in contact with its surface, but by the partial pressure, or fraction of the total pressure, exerted by the constituent in question. To take an ex- ample, let us suppose that a solution of oxygen in water made at a certain definite pressure is exposed to an atmo- sphere of pure nitrogen exerting as great a pressure, or even a greater, on the surface of the liquid ; notwithstand- ing this pressure the oxygen would escape from solution as readily as if the solution had been placed in a perfectly vacuous space. If water be exposed in a certain space of air (which is, roughly, a mixture of nitrogen and oxygen), the oxygen which it is capable of absorbing would be In a vacuum 2HNaC0 8 splits up into + C0 2 + II 2 O.
