336 



THE HUMAN BODY 



ventricle, rv, and thence through the pulmonary artery, pa, to 

 the lung capillaries, pc, from which the pulmonary veins, pv, 

 carry it to the left auricle, which drives it into the left ventricle, Iv, 

 and this again into the aorta. 



Arterial and Venous Blood. The blood when flowing in the pul- 

 monary capillaries gives up carbon dioxid to the air and receives 

 oxygen from it; and since its coloring mat- 

 ter (hemoglobin) forms a scarlet compound 

 with oxygen, it flows to the left auricle 

 through the pulmonary veins of a bright 

 red color. This color it maintains until it 

 reaches the systemic capillaries, but in 

 these it loses much oxygen to the sur- 

 rounding tissues and gains much carbon 

 dioxid from them. But the blood coloring- 

 matter which has lost its oxygen has a 

 dark purple color, and since this unoxidized 

 or " reduced" hemoglobin is now in excess, 

 the blood returns to the heart by the venae 

 cavae of a dark purple-red color. This 

 hue it keeps until it reaches the lungs, 

 when the reduced hemoglobin becomes 

 +v, Fl iS' 3 7 '~ D i iagram , of again oxidized. The bright red blood, rich 



the blood vascular system, 



showing that it forms a in oxygen and poor in carbon dioxid, is 



single closed circuit with , i i i i i , i 11 



two pumps in it, consisting known as "arterial blood" and the dark 



of the right and left halves ,, or i a<s "-rr^ Klr^r!"- anrl it miicf Ka 

 of the heart, which are rep- recl as VenOUS t 



resented separate in the borne in mind that the terms have this 



diagram, ra and rv, right 



auricle and ventricle ; la and peculiar technical meaning, and that the 



cle; ao, aorta ? c? systemic pulmonary veins contain arterial blood, 



capillaries ;^vc, ven cavse; an( j ^ e pulmonary arteries, venous blood; 



pulmonary capillaries; pv', the change from arterial to venous taking 



pulmonary veins. , . ,, . . , - 



place in the systemic capillaries, and from 



venous to arterial in the pulmonary capillaries. The chambers 

 of the heart and the great vessels containing arterial blood are 

 shaded red in Figs. 102 and 103. 



The Structure of the Arteries. A large artery can by careful 

 dissection be separated into three coats : an internal, a middle, and 

 an outer. The internal coat tears readily across the long axis of the 

 artery and consists of an inner lining of flattened nucleated cells, 



