CHAPTER 3 



BODY FLUIDS AND CIRCULATION 



But the circulation which I discovered teaches clearly that there is a 

 necessary outward and backward flow of the blood, and this at different 

 times and places, and through other and yet other channels and pas- 

 sages ; that this flow is determined also, and for the sake of a certain 

 end, and is accomplished in virtue of parts contrived for the purpose 

 with consummate forecast and most admirable art. 



William Harvey: Letter to P. M. Slegel, 1651 



GENERAL FUNCTIONS OF BODY FLUIDS 



In the preceding chapter the composition of body fluids has been described, 

 and it has been shown that many animals are capable of maintaining an 

 internal medium of composition different from the environment. In primi- 

 tive animals the dissolved constituents of the body fluids consist pre- 

 dominantly of mineral substances, but in higher forms the blood and body 

 fluids are found to contain large amounts of organic substances, sometimes 

 in colloidal form, including respiratory pigments (Tables 2.12, 2.13, 

 4.9,4.10). 



Experimentally it has been demonstrated that isolated tissues of many 

 simpler marine invertebrates are capable of functioning in sea water, e.g. 

 polychaetes, holothurians and sipunculoids. In these animals the body 

 fluids differ only slightly from sea water in mineral composition (Table 

 2.9). Organs from more complex animals, such as decapod crustaceans and 

 vertebrates, require balanced media approximating their natural blood or 

 haemolymph in composition in order to maintain optimal activity (Appen- 

 dix, p. 675). This is a reflexion of the increasing degree of homoeostasis 

 attained by these forms. As animals have become more complex they have 

 achieved a greater degree of control over the internal milieu, the composi- 

 tion of which is maintained within rather narrow limits, favourable to 

 efficient functioning of the body cells. 



The Protozoa are of such small size that functional exchange of material 

 between the protoplasm and environment is carried out efficiently by 

 diffusion across the plasma membrane. The simplest metazoans — sponges 

 and coelenterates — lack body cavities, and the spaces between the tissue 

 layers are filled with a solid gelatinous matrix. Internal and external mem- 

 branes are exposed to sea water and even when the jelly layer becomes very 

 thick, as in the mesogloea of large pelagic medusae, it is noteworthy that 

 the cellular content is very sparse. Moreover, metabolic activity of these 

 lowly animals proceeds at a low rate, which can be satisfied by processes 

 of diffusion. 



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