Physiology of the Blood. 



[The blood is aptly described by Claude Bernard as an internal medium which 

 acts as a " go-between " or medium of exchange for the outer world and the tissues. 

 Into it are poured those substances which have been subjected to the action of the 

 digestive fluids, and in the lungs or other respiratory organs it receives oxygen. 

 It thus contains new substances, but in its passage through the tissues it gives up 

 some of these new substances, and receives in exchange certain waste products 

 which have to be got rid of. Its composition is thus highly complex. Besides 

 carrying the neiv nutrient fluids to the tissues, it is also the great oxygen-carrier, 

 as well as the medium by which some of the waste products, e.g., CO.,, urea, are 

 removed from the tissues, and brought to the organs, e.g., the lungs, kidneys, skin, 

 which eliminate them from the body. It is at once a great pabulum-supplying 

 medium and a channel for getting rid of useless materials. As the composition of 

 the organs through which the blood flows varies, it is evident that its composition 

 must vary in different parts of the circulatory system ; and it also varies in the 

 same individual under different conditions. Still, with slight variations, there are 

 -certain general physical, histological, and chemical properties which characterise 

 blood as a vjkole.] 



1. PHYSICAL PROPERTIES. (1) Colour. The colour of blood varies from a 

 bright scarlet-red in the arteries to a deep, dark, bluish-red in the veins. Oxygen 

 (and, therefore, the air) makes the blood bright red ; want of oxygen makes it 

 dark. Blood free from oxygen (and also venous blood) is dichroic i.e., by reflected 

 light it appears dark red, while by transmitted light it is green. [Arterial blood is 

 monochroic] 



In thin layers blood is opaque, as is easily shown by shaking blood so as to form 

 bubbles, or by allowing blood to fall upon a plate with a pattern on it, and pouring- 

 it off again. [Printed matter cannot be read through a thin layer of blood spread on 

 a glass slide.] Blood behaves, therefore, like an " opaque colour," as its colouring- 

 matter is suspended in the form of fine particles the blood-corpuscles. 



Hence, it is possible to separate the colouring-matter from the fluid part of the blood by 

 filtration. This is accomplished by mixing the blood with fluids which render the blood- 

 corpuscles sticky or rough. If mammalian blood be treated with one-seventh of its volume of 

 solution of sodic sulphate, or if frog's blood be mixed with a 2 per cent, solution of sugar (Joh. 

 Midler) and filtered, the shrivelled corpuscles, now robbed of part of their water, remain upon 

 the filter. 



(2) Reaction. The reaction is alkaline, owing to the presence of disodic 

 phosphate, Na 2 HP0 4 , and bicarbonate of soda. After blood is shed, its alkalinity 

 rapidly diminishes, and this occurs more rapidly the greater the alkalinity of the 

 blood. This is due to the formation of an acid, in which, perhaps, the coloured 

 corpuscles take part, owing to the decomposition of their colouring-matter. A high 



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