CONDITION OF THE GASES IN THE BLOOD. 161 



readily set free by pneumic acid. This gas is a product of excretion and is not engaged 

 in any of the vital functions ; while oxygen, which has an all-important function to per- 

 form, unites immediately with the blood-corpuscles and is not easily disengaged except 

 when it undergoes transformation in the process of nutrition. In addition to this 

 excrementitious carbonic acid, there is another portion which is a permanent constituent 

 of the blood, in the carbonates, and cannot be set free without the use of reagents. 



Nitrogen exists in the blood in the same condition of solution in the plasma as 

 carbonic acid. 



Mechanism of the Interchange of Gases between the Blood and the Air in the Lungs. 

 The gases from the air pass into the blood, and the gases of the blood are exhaled 

 through the delicate membrane which separates these two fluids, in accordance with 

 laws which are now well understood. The first to point out the power of gases thus 

 to penetrate and pass through membranes was the late Dr. J. K. Mitchell, of Philadel- 

 phia. His attention was first directed to this subject by noticing the escape of gas from 

 gum-elastic balloons filled with hydrogen. Observations on the lungs of the snapping 

 turtle filled with air and placed in an atmosphere of carbonic acid or nitrous oxide, 

 showed a very rapid passage of gas from the exterior to the interior. Dr. Mitchell 

 recognized the passage of gases through membranes into liquids and the exhalation of 

 gases which were in solution in these liquids. He noted this action in the absorption of 

 oxygen and the exhalation of carbonic acid in the lungs, although he fell into the error of 

 supposing that there was no carbonic acid in solution in the blood and that it was ex- 

 haled as soon as formed. A few years later, Dr. Eogers, of Philadelphia, enclosed a 

 fresh pig's bladder, filled with venous blood, in a bell-glass of oxygen. In two hours a 

 quantity of oxygen had been consumed and a large quantity of carbonic acid had made 

 its appearance. 



We have already seen that the blood is exposed to the air in the lungs, separated 

 from it only by a very delicate membrane, over an immense surface. The membrane, 

 far from interfering with the interchange of gases, actually favors it ; and thus, in 

 obedience to the laws which regulate endosmosis between gases and liquids, the oxygen 

 is continually passing into the blood and the free carbonic acid is exhaled. 



General Differences in the Composition of Arterial and Venous Blood. All observers 

 agree that there are certain marked differences in the composition of arterial and venous 

 blood, aside from their free gases. The arterial blood contains less water and is richer 

 in organic and most inorganic constituents than the venous blood. It also contains a 

 larger proportion of corpuscles. It is more coagulable and offers a larger and firmer 

 clot than venous blood. The only principles which are constantly more abundant in 

 venous blood are water and the alkaline carbonates. According to Longet, 10,000 parts 

 of venous blood contained 12'3 parts of carbonic acid combined, and the same quantity 

 of arterial blood contained but 8'3 parts. The deficiency of water in the blood which 

 comes from the lungs is readily explained by the escape of watery vapor in the expired air. 



An important distinction between arterial and venous blood is one to which we have 

 already incidentally alluded; viz., that the former has a uniform composition in all parts 

 of the arterial system, while the composition of the latter varies very much in the blood 

 coming from different organs. Arterial blood is capable of carrying on the processes of 

 nutrition, while venous blood is not and cannot even circulate freely in the systemic 

 capillaries. 



Relations of Respiration to Nutrition, etc. It has been demonstrated that all tissues, 

 so long as they retain their absolute integrity of composition, have the property of appro- 

 priating oxygen and exhaling carbonic acid, independently of the presence of blood; and 

 that the arterial blood carries oxygen from the lungs to the tissues, there gives it up, and 

 receives carbonic acid, which is carried by the venous blood to the lungs, to be exhaled. 

 11 



