Chap. 12 CIRCULATION AND TRANSPORTATION BODY FLUIDS 203 



blood that creates the blue-gray tinge of clam broth. Hematin, the most im- 

 portant pigment in the higher animals, is a near relative of the pigment of 

 chlorophyll, the substance in plants which can utilize energy from the sun. 



In the lungs and in the- gills of aquatic animals where oxygen pressure is 

 high, hemoglobin combines with oxygen and forms oxyhemoglobin, an un- 

 stable combination which colors the blood bright red (Chap. 13). In the 



Fig. 12.6. Photograph of blood vessels in ear of living rabbit. The picture 

 is taken through glass pressed against the skin with the camera focused into the 

 vessels. The red cells are moving through the capillaries in rolls (rouleaux), the 

 number of these probably increased by the pressure of the glass. The extreme thin- 

 ness of the capillary wall is an evidence of the ease with which certain cells pass 

 through it. (Courtesy, Bremer and Weatherford: Textbook of Histology, ed. 6. 

 Philadelphia, The Blakiston Co., 1944.) 



tissues of the body the oxygen pressure is low and the oxygen separates from 

 the oxyhemoglobin leaving reduced hemoglobin which darkens the blood, 

 usually in the veins. Hemoglobin also takes part in transporting very small 

 amounts of carbon dioxide from the tissues to the lungs or gills. 



With the aid of isotopes the life span of human red cells has been shown 

 to be about 120 days. They wear out and fragments of them are eaten by 

 macrophages (phagocytic cells) in many parts of the body, especially in the 

 spleen, bone marrow, and liver. In a healthy human adult about one million 

 red cells are thus destroyed per second and a comparable number of new 

 ones are added per day as the blood passes through the red marrow of the 

 bones. 



Blood Counts. By diluting a small, measured quantity of blood and 

 spreading it upon a special ruled slide, the different kinds of blood cells can 



