174 ELEMENTARY PHYSIOLOGY 



intestines into the blood ; and so far the loss to the blood 

 by the liver is temporary only. 



Of ail the gains to the blood perhaps the most con- 

 stant is that of oxygen, and of all the losses perhaps the 

 most constant is that of carbonic acid ; but even these 

 vary a good deal at different times or under different cir- 

 cumstances. 



Broadly speaking then the blood gains oxygen from the 

 lungs, complex organic food materials from the alimentary 

 canal, and various substances which we may speak of as 

 waste substances from the several tissues ; and it loses on 

 the one hand material which we may speak of as con- 

 structive material to the several tissues ; and on the other 

 hand material which passes away by the skin, lungs, 

 and kidney, as water, carbonic acid, urea, and saline bodies. 



And while it is continually receiving heat from the 

 several tissues, it is also continually losing heat by the 

 skin, lungs, and other free surfaces of the body. 



The sources of loss and gain to the blood may be con- 

 veniently arranged in the following tabular form : — 



Sources of Loss and Gain to the Blood. ^ 

 A. Sources of Loss : — 

 I. Loss of Matter. 



1. The lungs : carbonic acid and water (fairly 



constant). 



2. The kidneys : urea, water, salines (fairly 



constant). 



3. The skin : water, salines (fairly constant). 



4. The tissues : constructive material (variable 



especially in the case of those tissues 

 whose activity is intermittent, such as the 

 muscles, many secreting glands, &c.), 

 water, &c., to form lymj)!!. 



1 The learner must be careful not to confound tho Ifisses and gains of 

 the blood with the lo.sses and gains of the bod;/ as a wliole. The two 

 differ in much the same way as the internal commerce of a country 

 differs from its export ajid import trade. 



