RESPIRATORY PROCESSES BEFORE BIRTH 135 



The tension of carbon dioxide in the tissues = 5 to 9 ; in the venous 

 blood it = 3.8 to 5.4 ; in the air-cells it = 2.8 ; in the external air it = 

 0.03. For the same reason that oxygen diffuses from the external air, 

 where the pressure is 20.96, to the tissues, where the tension is zero, 

 carbon dioxide passes from the tissues, where the tension is 5 to 9, to 

 the external air, where it is 0.03 (Fredericq). 



The carbon dioxide of the blood is contained chiefly in the plasma, 

 but a small quantity exists in the red corpuscles, in which this gas is 

 slightly more soluble than in pure water. It is somewhat difficult to 

 determine the exact condition of carbon dioxide in the plasma. It is 

 estimated that about five per cent exists in simple solution, seventy-five 

 to eighty-five per cent in a condition of loose combination and ten to 

 twenty per cent in firmer combination. These conditions render it 

 difficult to determine the exact tension of the gas in the plasma ; and, 

 indeed, the estimates of the tension of carbon dioxide are much less 

 exact than of oxygen. 



Respiration in the Tissues. Nearly all tissues contain coloring 

 matters known as histohematins. The most important of these is found 

 in the muscles and is called myohematin. It is thought that the oxygen 

 of the blood combines with these histohematins, the process involving 

 true tissue-respiration. At all events, the interchange of gases between 

 the tissues and the blood are exactly opposite to the changes that take 

 place in the lungs. In the lungs, the blood loses carbon dioxide and 

 gains oxygen ; in the tissues, the blood loses oxygen and gains carbon 

 dioxide. The real seat of respiration is in the tissues. 



RESPIRATORY PROCESSES BEFORE BIRTH 



It is commonly admitted that one of the most important uses of the 

 placenta the one, indeed, most immediately connected with the life 

 of the foetus is a respiratory interchange of gases, analogous to that 

 which takes place in the gills of aquatic animals. The placental villi 

 are bathed in the blood of the uterine sinuses, and this is the only way 

 in which the foetal blood can receive oxygen. Legallois observed a 

 bright red color in the blood of the umbilical vein ; and on alternately 

 compressing and releasing the vessel, he saw the blood change in color 

 successively from red to dark and from dark to red. Zweifel has 

 demonstrated the presence of oxyhemoglobin in the blood of the um- 

 bilical vessels by means of the spectroscope, showing that it contains 

 oxygen. As oxygen is thus adequately supplied to the system, the 

 foetus is in a condition similar to that of animals in which artificial 

 respiration is maintained. The want of oxygen is fully met, and there- 



