392 RELATION OF RESPIRATION TO OTHER PROCESSES [oil. XXVII. 



pressure taken with a salt solution manometer from the jugular vein. 

 It will be noticed that the fall of arterial pressure is accompanied 

 with a great rise of venous pressure due to the venous congestion just 

 described. 



The Relation of Respiration to Nutrition. 



The gaseous interchanges in the lungs constitute what is fre- 

 quently termed external respiration. Oxygen obtains an entrance 

 into the blood, and is carried to the tissues in the loose compound 

 known as oxyhaemoglobin. In the tissues, this compound is dis- 

 sociated, and the respiratory oxygen is utilised by the tissue elements 

 for the combustion processes which occur consequent on their 

 activity. Of the ultimate products, carbonic acid and a portion of 

 the water find an outlet by the lungs, to which they are transported 

 by the venous blood. The gaseous interchanges in the tissues con- 

 stitute what is known as internal or tissue respiration. 



Inspired and Expired Air. We may compare the composition of 

 the inspired or atmospheric air with that of the expired air in the 

 following table: 



The chief change is in the proportion of oxygen and carbonic acid. 

 The loss of oxygen is about 5, the gain in carbonic acid about 4*5. If 

 the inspired and expired airs are carefully measured at the same 

 temperature and barometric pressure, the volume of expired air is thus 

 found to be rather less than that of the inspired.* The conversion of 

 oxygen into carbonic acid would not cause any change in the volume 

 of the gas ; for a molecule of oxygen (0 2 ) would give rise to a molecule 

 of carbonic acid (C0 2 ) which would occupy the same volume (Avo- 

 gadro's law). It must, however, be remembered that carbon is not 

 the only element which is oxidised. Fat and protein contain a 

 number of atoms of hydrogen, which, during metabolism, are oxidised 

 to form water ; a small amount of oxygen is also used in the formation 

 of urea. Carbohydrates contain sufficient oxygen in their own mole- 

 cules to oxidise their hydrogen ; hence the apparent loss of oxygen is 

 least when a vegetable diet (that is, one consisting largely of starch 



* This diminution of volume will cause a slight rise in the proportionate volume 

 of nitrogen per cent. 



