CHAPTER XXIII 

 THE BLOOD AND CIRCULATION 



BLOOD 



We have seen that digestion prepares the food for the 

 cells and that by means of absorption this food enters the 

 blood. The blood is the common carrier between the ab- 

 sorptive surface of the small intestine and all the tissues of 

 the body. It circulates within a closed system of tubes or 

 blood-vessels, and substances that pass into or out of the 

 blood must pass through the walls of the blood-vessels. 



Composition of the blood. — If the finger is pricked with 

 a clean needle and a drop of blood placed on a slide and 

 looked at under the microscope, the blood will be seen as 

 yellowish liquid, containing a great many tiny round disks 

 and a few particles of irregular shape. The yellowish liquid 

 is the plasma, the disks and other floating cells are corpuscles 

 (red and white). 



Structure and function of the red corpuscles. — ^The 

 red corpuscles are biconcave disks without nuclei and con- 

 tain a red pigment, hcemoglobin. It is the function of 

 haemoglobin to carry oxygen to the tissues and carbon dioxide 

 from the tissues. 



Structure and function of white blood corpuscles. — 

 The white blood corpuscles are of many sizes and without 

 definite form. They are colorless and nucleated. Many 

 of them move about in the blood plasma like amcebae. 

 Some of the white blood corpuscles (phagocytes) take up 

 from the blood foreign organisms such as disease produc- 



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