A STUDY OF THE CIRCULATION OF BLOOD 147 



terns of capillaries before it can begin a second round of the 

 body. 



Changes in the Composition of the Blood. The composition 

 of the blood is continually changing in its passage through 

 the various tissues of the body. We may, perhaps, make 

 clearer these various changes by expressing them in tabular 

 form as follows : 



In muscular, nerve, 

 and other tissues, 



In lining of mouth, 

 stomach, intestines, 



In lungs, 



In kidneys and skin, 



BLOOD LOSES 



Materials needed for 

 growth, repair, and 

 production of en- 

 ergy. 



Materials needed for 

 the manufacture of 

 digestive juices (and 

 for growth and re- 

 pair). 



Carbon dioxid and 

 water. 



"Water and urea. 



BLOOD GAINS 

 Wastes formed by me- 

 tabolism (carbon di- 

 oxid, water, urea;. 



Digested nutrients 

 (proteids, fat, 

 starch, sugar, min- 

 eral matters, and 

 water) and wastes. 



Oxygen. 



Carbon dioxid. 



Inappropriateness of the Terms "Arterial" and "Venous." 

 Thus far we have avoided the terms ar-te'ri-at and ve'nous as 

 applied to blood. These terms are commonly used in text- 

 books, but they often give rise to considerable confusion. 

 By arterial blood is meant the bright scarlet blood that 

 comes back from the lungs to the left heart, whence it is 

 distributed through the aorta to all parts of the body. The 

 word arterial, however, suggests arteries, and orie easily 

 jumps at the conclusion that arterial blood is always car- 

 ried in arteries. This is not true ; the pulmonary artery 

 carries the dark colored or so-called venous blood. In the 

 same way venous blood is commonly supposed to flow only in 

 veins ; but we must bear in mind that the pulmonary veins 

 carry arterial blood. The difference in the color of the 

 blood is due almost entirely to the amount of oxygen that 

 is present. Hence, we may avoid all possible confusion by 

 calling the bright scarlet blood ox'y-gen-a-ted blood, or blood 



