GESTATION 



998 



previous reports, it is evident that the 

 placenta, due to its similarity to an arterio- 

 venous aneuryism, is partially responsible 

 for the hypervolemia. The marked increase 

 in the uterine size and vascularity during 

 pregnancy (Barcroft and Rothschild, 1932) 

 will also account for a considerable amount 

 of the increase in the blood volume. In ad- 

 dition, the tendency for dilation of the 

 peripheral blood vessels may account for 

 another portion of the increase in the blood 

 volume. Thus, in general, the hypervolemia 

 of pregnancy can be attributed primarily 

 to structural changes in the circulatory sys- 

 tem. 



It cannot be denied, however, that the 

 cardiovascular system is influenced by the 

 changes in the endocrine balance during 

 gestation. It is well known that the placenta 

 elaborates large amounts of sex steroids 

 and corticoids. It has also been suggested 

 that there is an increased production of 

 thyroxine and ADH at this time. In ad- 

 dition, water-soluble extracts of the preg- 

 nant sow's ovaries have been shown to 

 cause water retention and anemia in the 

 rabbit (Zarrow and Zarrow, 1953). The 

 resultant hormonal balance becomes some- 

 what precarious as the additional secretions 

 of the glands tend to build up the blood 

 titers of the sex steroids and other sub- 

 stances which influence water metabolism. 

 It is possible that when a proper balance of 

 these factors is maintained, the pregnancy 

 is normal and the various requirements of 

 the fetus are provided without disrupting 

 the distribution of the body fluids outside 

 of the vascular system. However, if the 

 balance is not maintained, the animal tends 

 to accumulate fluid, and edema and other 

 pathologic complications result. 



VIII. Plasma Proteins 



It has long been known that the plasma 

 proteins play a significant role in the fluid 

 balance of the organism and as such are 

 also involved in water balance during preg- 

 nancy (]\Iack, 1955). In addition, the 

 plasma proteins are of importance in many 

 other functions, such as heat and energy 

 source and replacement of tissue in which 

 function they act as a protein source when- 

 ever needed and form the metabolic pool. 

 These proteins are synthesized in general. 



in the liver and reticuloendothelial system 

 and may be classified as albumins or globu- 

 lins although many different entities of 

 these two classifications are known to 

 exist. 



The maintenance and stabilization of 

 blood volume and the equilibrium of fluid 

 exchange between the extravascular and 

 intravascular compartments is a function 

 of the albumin fraction of special signifi- 

 cance in i)regnancy, in addition to its other 

 functions of acting as carrier for other 

 substances and sui^jilying of nutrients. The 

 regulation of blood volume by albumin 

 depends on its osmotic action and is of 

 much greater significance than the globu- 

 lins. Approximately 4.6 gm. albumin and 

 3.17 gm. globulin per 100 ml. of plasma 

 are found in the normal, nonpregnant 

 woman. At least four types of glol)ulins are 

 present in the plasma among which are 

 found the lipoproteins, prothrombin, fi- 

 brinogen, antibodies, and several hormones. 



Although both the plasma proteins and 

 albumin drop during pregnancy, this does 

 not necessarily indicate a drop in the total 

 available albumin protein. An increase in 

 the plasma volume compartment of 25 per 

 cent as seen in pregnancy could easily 

 result in an increase in the total amount 

 of circulating protein. However, as the 

 total circulating blood volume increases in 

 pregnancy, the albumin fraction and y- 

 globulin seem to be diluted whereas the 

 other globulins become more concentrated. 

 Nevertheless, the globulins cannot compen- 

 sate for the albumin loss and the total 

 protein decreases. Mack (1955) has listed 

 several possible explanations for the above 

 paradox : ( 1 ) the small albumin molecule 

 may diffuse more freely into tissues and 

 across placental membrane, and (2) al- 

 bumin synthesis cannot keep pace with util- 

 ization. 



Innumerable studies on the plasma i^-o- 

 teins of women during pregnancy have I'e- 

 vealed markedly consistent changes in the 

 albumin-globulin ratio of the plasma. The 

 concentration of total protein and albumin 

 decreases while the total globulin increases. 

 The trend is apparent by the first trimester 

 and continues throughout gestation. A re- 

 turn to the nonpregnant pattern is seen 

 shortly after parturition. The total protein 



