CHAP. IL] RESPIRATION. 583 



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 oxy haemoglobin largely but not wholly 

 reduced, the blood passes on as venous blood. To what extent 

 the hsemoglobin is reduced will depend on the activity of the 

 tissue itself. The quantity of hsemoglobin 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 combined in 

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

 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 in- 

 creased. Arrived at the lungs, the blood finds the pulmonary 

 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 oxygen, more than adequate 

 to convert, not entirely but nearly so, the reduced hsemoglobin 

 back again to oxyhaBmoglobin. 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 breathing in. 



