BLOOD AND CIRCULATION 



(Readings: Weisz, pp. 433-457. S.P.T., pp. 141-153; 163-168. Villee, Chapters 

 16 and 17. Also B. W. Zweifach, "The Microcirculation of the Blood," Sci. 

 Am. 200, No. 1, 54-60, Jan. 1959, Reprint No. 64. C. J. Wiggers, "The Heart," 

 Sci. Am. 196, No. 5, May 1957, Reprint No. 62. W. B. Wood, Jr., "White 

 Blood Cells vs. Bacteria," Sci. Am. 184, No. 2, 48-52, Feb. 1951, Reprint 

 No. 51. M. B. Zucker, "Blood Platelets," Sci. Am. 204, No. 2, 58-64, Feb. 

 1961. 



One of the principal problems facing a cell as 

 part of a multicellular organism is that it no 

 longer has free access to the external environ- 

 ment. To obtain water, salts, and organic 

 nutrients, to get rid of wastes, and for gas ex- 

 change, it must depend on some sort of circula- 

 tory system. The importance of the circulation 

 in maintaining an animal can hardly be over- 

 estimated. By far the largest cause of death in 

 man is failure of the circulation. 



Beyond its nutritive and excretory roles, the 

 circulatory system in vertebrates performs an 

 essential function in defending the animal from 

 invasions of foreign organisms and foreign 

 molecules. A failure of these defense mechan- 

 isms can lead to death as surely as the failure 

 in the nutritive and excretory functions of the 

 blood. 



The blood of higher animals is a complex 

 tissue. It may be separated by centrifugation 

 into a fraction composed of cells, and a cell-free 

 liquid fraction called the plasma. The plasma is 

 a complex solution of proteins, sugars, salts, and 

 other substances. One of the plasma proteins, 

 fibrinogen, is the precursor of the insoluble _/zin>j 



of the blood clot. The remainder of the plasma 

 after the clot has been removed is called serum. 

 For both the nutritive and defensive roles of the 

 circulatory system, both cells and plasma are 

 needed. 



Let us first consider the nutritive function of 

 the blood. Many substances are carried in water 

 solution in the plasma and are transported to the 

 cells in this fashion. Other substances are ad- 

 sorbed on proteins in the blood and are carried 

 in this way. Gas exchange presents further 

 problems. A little oxygen and somewhat more 

 carbon dioxide can be dissolved in the plasma; 

 but the major transport of both these gases in 

 vertebrates depends upon the red pigment, 

 hemoglobin, an iron-porphyrin-protein. The 

 hemoglobin is carried in specialized cells, the red 

 blood cells or erythrocytes. About as much 

 hemoglobin is packed into these cells as they 

 can possibly hold. Some 30% of the red blood 

 cell or 95% of its dry weight is hemoglobin. 

 The red blood cells are nonmotile, and do little 

 more than carry hemoglobin. In mammals these 

 cells lose their nuclei before maturing; and as you 

 would expect, from that point on they run down 



80 



