PHYSIOLOGY FOR BEGINNERS 



The tissues arc constantly forming carbonic acid by their 

 life, so that there is much free or loosely-combined carbonic 

 acid in them. The blood in the capillaries, on the other hand, 

 though it contains carbonic acid, partly in solution in the 

 plasma and partly in loose combination with the salts and other 

 substances in the blood, contains much less than do the tissues. 

 Some of the carbonic acid of the tissues passes into the blood, 

 and if the blood stays long enough it will go on passing into 

 the blood until the blood is just as much loaded with carbonic 

 acid as are the tissues. The oxygen passes from the blood 

 and the carbonic acid passes to the blood by the process of 

 diffusion which tends to make the pressure of each gas the 

 same in the blood and in the tissues. 



The Change of Venous Blood to Arterial Blood. 

 The change of venous blood to arterial blood takes place as 

 the blood passes through the capillaries of the lungs. The 

 venous blood is driven by the heart along the pulmonary artery 

 through the capillaries of the lungs, and having there lost 

 carbonic acid and gained oxygen, passes as arterial blood 

 along the pulmonary vein to the heart again. 



The air in the lungs, as we shall see, is separated from 

 the capillaries of the lungs by a very thin membrane only, 

 through which gases can readily pass. The venous blood 

 brought to the lungs contains more carbonic acid than the air 

 in the lungs, and some of it therefore diffuses through into the 

 air. If the same air stayed long enough in the lungs all the 

 venous blood which was sent through them would give up 

 some of its carbonic acid to it, until the air in the lungs was 

 as much loaded with carbonic acid as the blood in the capillaries 

 of the lungs, and then the process would stop. The air in the 

 lungs is, however, to a great extent renewed at each breath, 

 so that the air does not leave the lungs so heavily laden with 

 carbonic acid as is the venous blood, while, on the other hand, 

 the frequent renewal of the air in the lungs makes the giving 

 up of carbonic acid by the venous blood quicker, because the 

 greater the difference between the amounts they each contain 

 the faster is the diffusion from one to the other. 



The venous blood brought to the lungs contains much 

 haemoglobin which is not combined with oxygen, while the 

 air in the lungs contains much free oxygen. Some of this 



