60 ARTERIAL AND VENOUS BLOOD. 



contain less of all these, but in addition it holds the used-up or effete 

 substances derived from the tissues, and the products of their retro- 

 gressive metabolism being more numerous, there is in venous blood a 

 larger amount of C0 2 . It is evident also that the blood of certain 

 veins must have special characters, e.g., that of the portal and 

 hepatic veins. 



The following are the most important points of difference between 

 arterial and venous blood : 



Arterial Blood contains 



more 0, 

 less C0 2 , 

 more water, 

 more fibrin, 

 more extractives, 



more sugar, 

 fewer blood-corpuscles, 

 less urea. 



It is bright red and not 

 dichroic. 



more salts. As a rule it is 1C. warmer. 



The bright red colour of arterial blood depends on the presence of 

 oxyhaemoglobin, whilst the dark colour of venous blood is due to its 

 smaller proportion of oxyhsemoglobin, and the quantity of reduced 

 haemoglobin which it contains. The dark change of colour is not to 

 be attributed to the larger quantity of C0 2 in venous blood (Marchand); 

 for if equal qualities of be added to two portions of blood, and if 

 C0 2 be added to one of them, the colour is not changed (Pfliiger). 



40. Quantity of Blood. 



In the adult the quantity of blood is equal to T V part of the body- 

 weight (Bischoff), in newly-born children T V (Welcker). 



According to Schiicking, the amount of blood in a newly-born child depends to 

 some extent upon the time at which the umbilical cord is ligatured. The amount 

 .T5- f the body- weight when the cord is tied at once, while if it is tied some- 

 what later it may be -J-. Immediate ligature of the cord may, therefore, deprive 

 a newly-born child of 100 grammes of blood. Further, the number of corpuscles 

 is less in a child after immediate ligature of the umbilical cord, than when it is 

 tied somewhat later (Helot). 



Various methods are adopted to ascertain the amount of blood, but 

 perhaps that of Welcker is the best. 



The methods of Valentin (1838), and Ed. Weber (1850), are not now used, as 

 the results obtained are not sufficiently accurate. 



Method Of Welcker (1854). Begin by taking the weight of the animal to be 

 experimented on ; place a cannula in the carotid, and allow the blood to run 

 into a flask previously weighed, and in which small pebbles (or Hg) have been 

 placed in order to defibrinate the blood by shaking. Take a part of this detibrin- 

 ated blood, and make it cherry-red in colour by passing through it a stream of CO 



