ARTEEIAL AND TEXOUS BLOOD. 33 



In 1000 parts of corpuscles — 



Water ... ... 



Solids 



In 1000 parts of plasma — 



Water 908-i 



Solids 91-6 



\ Insoluble salts 1'5 



Scherer and Otte give the following as the composition of human 

 venous blood : — 



The serum of the same blood yielded in 100 parts — 



Water 90-66 



Albvimin ....... 7-76 



Extractives 0.51 



Soluble salts 094 



Difference between Arterial and Venous Blood. — By arterial blood is meant 

 that which is contained in the aorta and its branches (sj-stemic arteries), in the 

 pulmonary veins and in the left cavities of the heart ; the venous blood is that of 

 the veins generally, the pulmonary arteries, and right cavities of the heart. Their 

 differences, apart from their functional effects in the living body, come under 

 the heads of coloui* and composition. 



1. Colour. Ai-terial blood, as already stated, is scarlet, venous blood dai-k, or 

 purple. Venous blood assumes the scarlet colour on exposure to an*, i. c. to 

 oxygen. This change is gi-eatly promoted by the saline matter of the serum, 

 and may be accelerated by adding salts or sugar to the blood, especially by car- 

 bonate of potash, or of soda, and by nitre. Salts added to dark blood, "svithout 

 exposm-e to oxygen or air, cause it to assume a red colom-, but not equal in 

 brightness to tliat of arterial blood. On the other hand, the addition of a 

 little water darkens the blood. According to Stokes, the coiijuscles in the 

 former case " lose water by exosmosis, and become thereby highly refractive, in 

 consequence of which a more copious reflexion takes place at the common sur- 

 face of the coiimscles and siirrounding fluid. In the latter case they gain water by 

 endosmosis, which makes their refractive power more nearly equal to that of the 

 fluid in which they are contained, and the reflexion is consequently diminished."'* 

 But the ]presence of sei-um or of saline matter is not indispensable to the bright- 

 ening, for although the clot when washed free from serum scarcely if at all 

 reddens on exposure to oxygen, yet it is found that the red matter when squeezed 

 out of the clot and dissolved in water, still becomes brighter and clearer on 

 exposure to oxygen, whilst the colom- is darkened (and the solution becomes 

 turbid from deposition of paraglobulin), on being shaken with carbonic acid. As 

 in this case the colouring matter is extracted from the corpuscles and is red- 

 dened by oxygen without the presence of salts, it is plain that the difference of 

 colour of arterial and venous blood essentially depends, not on a difference in the 

 figure or density of the corpuscles, bat on the alteration produced in the colour- 



* Proc. Royal Soc. , vol. xiii. p. 362. 



