CHAP, ii.] RESPIRATION. 471 



reduced haemoglobin of the venous blood becomes wholly, or all 

 but wholly, oxyhsemoglobin. Hurried to the tissues, the oxy- 

 gen, at comparatively high pressure in the arterial blood, passes 

 largely into them. In the tissues, the oxygen-pressure 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 its oxyhaemoglobin 

 largely but not wholly reduced, the blood passes on as venous 

 blood. To what extent the 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 determined 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 com- 

 bined in carbonic acid (and other matters not now under con- 

 sideration). Owing to the continual production of carbonic 

 acid, the pressure of that gas in the extravascular elements of 

 the tissue is always higher than that in the blood; the gas 

 accordingly passes from the tissue into the blood, and the venous 

 blood passes on not only with its haemoglobin more or less 

 reduced, i.e. with its oxygen-pressure decreased, but also with 

 its carbonic acid pressure increased. Arrived at the lungs, the 

 blood finds the pulmomary air at a lower carbonic acid pressure 

 than itself. The gas accordingly streams through the thin 

 vascular and alveolar walls until the pressure without the blood 

 vessel is equal to the pressure within. At the same time the 

 blood finds in the air of the pulmonary alveoli a supply of oxy- 

 gen, more than adequate to convert, not entirely but nearly so, 

 the reduced haemoglobin back again to oxy haemoglobin. Thus 

 the air of the pulmonary alveoli, having given up oxygen to the 

 blood and taken up carbonic acid from the blood, having in 

 consequence a higher carbonic acid pressure and a lower oxygen- 

 pressure 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 breath- 

 ing in. 



