352 CHANGES IN THE TISSUES. [BOOK n. 



will continue to live, to move, to produce carbonic acid for a while 

 without any fresh oxygen, as we know of old it will without any 

 fresh food; it will continue to do so till the explosive compounds 

 which the oxygen built up are exhausted ; it will go on till the 

 vital clock has run down. 



To sum up, then, the results of respiration in its chemical 

 aspects. As the blood passes through the lungs, the low oxygen 

 tension of the venous blood permits the entrance of oxygen from 

 the air of the pulmonary alveolus, through the thin alveolar wall, 

 through the thin capillary sheath, through the thin layer of blood- 

 plasma, to the red corpuscle, and the reduced haemoglobin of the 

 venous blood becomes wholly, or all but wholly, oxyhsemoglobin. 

 Hurried to the tissues, the oxygen, at a comparatively high tension 

 in the arterial blood, passes largely into them. In the tissues, the 

 oxygen tension is always kept at an exceedingly low pitch, by the 

 fact that they, in some way at present unknown to us, pack away 

 at every moment into some stable combination each molecule of 

 oxygen which they receive from the blood. With much but not 

 all of its oxyhsemoglobin reduced, the blood passes on as venous 

 blood. How much haemoglobin is reduced will depend on the 

 activity of the tissue itself. The quantity of haemoglobin in the 

 blood is the measure of limit of the oxidizing power of the body 

 at large ; but within that limit the amount of oxidation is deter- 

 mined by the tissue, and by the tissue alone. 



We cannot trace the oxygen through its sojourn in the tissue. 

 We only know that sooner or later it comes back combined in 

 carbonic acid (and other matters not now under consideration). 

 Owing to the continual production of carbonic acid, the tension 

 of that gas in the extravascular elements of the tissue is always 

 higher than that of the blood ; the gas accordingly passes from 

 the tissue into the blood, and the venous blood passes on not only 

 with its haemoglobin reduced, i.e. with its oxygen tension decreased, 

 bat also with its carbonic acid tension increased. Arrived at the 

 lungs, the blood finds the pulmonary air at a lower carbonic acid 

 tension than itself. The gas accordingly streams through the thin 

 vascular and alveolar walls, till the tension without the blood-vessel 

 is equal to the tension within. At the same time the blood finds 

 in the air of the pulmonary alveoli a supply of oxygen, more than 

 adequate to convert the greater part of the reduced haemoglobin 

 back again to oxyhaemoglobin. Thus the air of the pulmonary 

 alveoli, having given up oxygen to the blood and taken up carbonic 

 acid from the blood, having a higher carbonic acid tension and a 

 lower oxygen tension than the tidal air in the bronchial passages, 

 mixes rapidly with this by diffusion. The mixture is further assisted 

 by ascending and descending currents; and the tidal air issues from 

 the chest at the breathing out poorer in oxygen and richer in 

 carbonic acid than the tidal air which entered at the breathing in. 



