314 THE BLOOD 



against 7-l~8-3/>t in dried films and 6-8-7-5 in dried and stained 

 specimens. Their greatest thickness is very nearly 0-3 of the 

 diameter, i.e. in man, from 'i-i/x in a dried specimen to 2-7/x for a 

 natural cell. It will be seen, therefore, that if the shape is the same 

 in both cases, the cell as it occurs in the blood stream is larger 

 in every way than when dried in a film. The thickness across the 

 narrowest part is about 2ja. 



The volume and surface area of the cell can be calculated from 

 the diameter and thickness. This gives the human red cell a 

 volume of between 70 and 80/x^ and an area of 120/x2. The value 

 thus obtained for the volume agrees reasonably with that found 

 from dividing the percentage volume of cells found by the 

 haematocrite by the number of cells actually counted per unit 

 volume. 



The size of the red cell is markedly altered by the j^H of the 

 plasma in which it is suspended. Increase of COg, by decreasing 

 the alkalinity of the blood, produces swelling (cf. swelling of 

 colloids, Chap. VIII.). Therefore venous blood will always 

 have larger erythrocytes than arterial blood. We shall discuss the 

 significance of this in the next chapter. 



Shape. In contact with plasma the human red cell is a circular 

 biconcave discoid. A peculiar shape like this gives one the 

 impression of the application of a constant distorting force. If 

 the corpuscle, as is the almost general opinion to-day, is an elastic 

 bag filled with fluid containing, it is said, colloidal matter dispersed 

 through it, one would expect it to be spherical. Several views 

 have been advanced to explain the central cavities. 



1. Those who consider that the cell is a sponge-hke body easily 

 find an explanation of the persistence of the shape, i.e. it is imposed 

 on the cell by the stroma within it. The flattening they attribute 

 to the loss of the nucleus and the consequent loss of some water. 

 Nucleated red cells have about 70-90 per cent, water, while non- 

 nucleated cells have only between 60 and 70 per cent, by volume. 

 Unfortunately the change from the practically spherical erythro- 

 blast to the typical disc-like erythrocyte is not synchronous with 

 the disappearance of the nucleus from the former. Further, the 

 nucleus is not thrown out of the cell. It undergoes disintegration 

 in situ, and its fragments are gradually absorbed by tlie cell. 

 Flattening occurs later. 



2. The shape may l3c due to surface forces. Since the membrane 

 of the cell is mainly lipide, the cell, as a whole, may be regarded as 

 a lipide drop. Shape is imposed on any such drop by surface 

 forces. If these forces are uniformly applied, a spherical form 

 results ; if unevenly, an egg-shaped body, and so on. It is difficult 



