Differences between arterial and venous blood.
— A knowledge of the chemical and physical differences between arterial and venous blood, or, in other words, between the blood imme diately before and immediately after it has passed through the lungs and been subjected to the action of the atmospheric air, consti tutes part of the data requisite for discussing the Theory of Respiration. Although many able chemists and physiologists have of late years directed their attention to this subject, yet, from its inherent difficulties, much discre pancy of observation and conflicting evidence still require to be cleared up and reconciled. Most, if not all, of the comparative analyses of the venous and arterial blood hitherto pub lished are of' considerably less value for our present purpose than they may at first appear, since only those of the venous blood flowing from the right side of the heart, and the arte rial blood flowing from the left side of the heart or along the arteries, ought properly to be taken into account. The blood returning along the veins of the abdominal viscera, and entering the heart by the cava inferior, differs in composition from that entering the heart by the cava superior, for, independently of other reasons, a quantity of water and certain sub stances taken into the stomach are absorbed by the mesenteric and gastric veins. The composition of the blood in the large veins at the lower and lateral parts of the neck must also be somewhat affected by the lymph and chyle poured into that portion of the venous system. The analyses of venous and arterial blood taken at the same time from the carotid artery and the jugular vein, — the plan most generally followed in these researches, — are better fitted for throwing light upon the changes the blood undergoes in the perform ance of nutrition and secretion than of respi ration.
The most marked difference, more espe cially in warm-blooded animals, between ar terial and venous blood is that of colour, — arterial blood being of a scarlet red, and ve nous blood of a dark Modena hue. The extent of this difference of colour between the blood in the arteries and in the veins varies in the different vertebrata, and is greater in birds and in the mainmalia than in reptiles and fishes ; and it also varies in different con ditions of the body and surrounding media in the same animal. In animals exposed to
artificial high temperatures*, or living in warm climates+, when the energy of the re spiratory function is naturally diminished, the venous blood may be of a brighter colour than usual, while the arterial may be less so, and it may then be difficult to distinguish the one kind from the other. In certain cases of high febrile excitement of the circulation, as in acute rheumatism when the blood passes rapidly and abundantly through the lungs, the blood in the veins may be of a scarlet colour : on the other hand, where the a6-ation of the blood is imperfect, as during the state of hy bernation, in certain diseases, or from some mechanical impediment to the free passage of the air into the lungs, the blood flowing along the arteries approaches more or less the dark colour of venous blood.
The temperature of the arterial blood in the left side of the heart, aorta, and large vessels springing frotn it, is higher than the venous blood by from 1° to 2° Fahr., according to Dr. John Davy t, and 1°'01 C (1°. 818 Fahr.) on an average, according to Becquerel and Breschet.§ According to Dr. Davy, the ca pacity of venous blood for caloric is 852, that of arterial blood 839.
The specific gravity of venous is somewhat greater than that of arterial blood. Dr. Davy gives the specific gravity of arterial blood as 1050, that of venous as 1053.11 Some of those who have published analyses of both kinds of blood, procured more solid materials and less water from venous than from arterial blood; others again have obtained the oppo site result; while Denis, in his analysis of the blood of a dog, observed no difference in this respect. The number of instances, — taking the rnore trust-worthy analyses only into ac count, where the quantity of water was greater in the arterial than in the venous blood decidedly preponderates. In all pro bability the relative quantity of water in the two kinds of blood is determined by the rela tive extent of the loss of that fluid by the arterial blood at the kidneys, lungs, skin, &c., and of the supply entering the veins from without, but chiefly through the mesenteric veins.