60 HUMAN PHYSIOLOGY 



arterial blood is found in the arteries (except pulmonary 

 artery), the left heart, and the pulmonary veins. The 

 change from venous to arterial blood is brought about by the 

 taking up of oxygen and the giving off of carbon dioxide in 

 the lungs pulmonary respiration. The change from arterial 

 to venous blood is brought about by the giving off of oxygen 

 and the taking up of carbon dioxide in the tissue tissue 

 respiration. 



2. PULMONARY RESPIRATION 



The exchange of gases between the blood and the air of 

 the lungs depends upon the diffusion of gases through the 

 walls of the alveoli and capillaries. This diffusion takes 

 place from places of higher to places of lower gas pressure. 



The inhaled or exhaled air contains the following gases: 



Inspired Air. Expired Air. 



Nitrogen 79.00 vol. % 80.0 vol. % 



Oxygen... 20.96 " 16.0 " 



Carbon dioxide 0.04 " 4.0 " 



The partial pressure at 760 mm Hg is: 



Inspired Air. Expired Air. 



Oxygen 152 mm Hg 122 mm Hg 



Carbon dioxide 0.3 " 30 '' 



The pressure or tension of the gases of the blood is stated 

 in terms of the partial pressure of these gases in a vessel 

 containing the blood, necessary to keep the quantity of the 

 gases in the blood constant. The tension is : 



Arterial Blood. Venous Blood. 



Oxygen 29.6 mm Hg 21.0 mm Hg 



Carbon dioxide 22.0 4< 41.0 



The partial pressure of oxygen in inspired air is larger 

 than its tension in the venous blood ; that of carbon dioxide 

 is less. Therefore an exchange of gases between the blood 

 and the air in the lungs takes place by diffusion through the 

 walls of the alveoli and of the capillaries. 



