CIRCULATION OF THE BLOOD 



1388 



CIRCUS 



special purpose. One system carries the blood 

 through the body, forming the body, or sys- 

 temic, circulation; the second carries the blood 

 through the lungs, forming the lung, or pul- 

 monary, circulation; and the third carries the 

 blood through the liver, forming the liver, or 

 portal, circulation. The last is properly a part 

 of the systemic circulation. 



Each system is double, that is, it consists of 

 two sets of pipe lines the arteries, which carry 

 the blood from the heart, and the veins, which 

 carry the blood back to the heart. In the body 

 and liver circulation the arteries carry red 

 blood, but in the lung circulation they carry 

 dark blood. In all the systems the capillaries 

 are the fine, hairlike pipes which connect the 

 arteries and veins. 



Wonders of the Circulation. It requires 

 about thirty seconds for a drop of blood to 

 make the rounds from the left ventricle of the 

 heart and back to it, following the route out- 

 lined below, and it is estimated that all the 

 blood in the body passes through the heart 

 every two or three minutes. The best author- 

 ities estimate that 900 pounds, or 112 gallons, 

 or nearly three barrels of blood pass through 

 each ventricle of the heart in one hour. Since 

 the same amount passes through each, this, 

 would make six barrels of blood pumped by 

 the ventricles every hour. The force expended 

 by the heart in one hour would lift 11,250 

 pounds one foot, or it would raise a man 

 weighing 160 pounds seventy feet. The total 

 work performed by the heart in a day is esti- 

 mated to be equal to about 7.5 horse power. 



Circulation Through the Body. The blood is 

 forced by the contraction of the heart from the 

 left ventricle into the aorta, or great artery, 

 from which main branches lead to the different 

 parts of the body the trunk, head, arms and 

 legs. Each of these branches divides and sub- 

 divides until the arteries become capillaries, 

 which form a complete network all through 

 the body, and through all these branches and 

 capillaries the blood flows, carrying nourish- 

 ment to the tissues and taking up worn-out 

 and poisonous matter. 



As the capillaries are formed by the branch- 

 ing of the arteries at one end, so at the other 

 they unite to form minute veins, which them- 

 selves unite into larger veins, until we find 

 large veins side by side with the larger arteries ; 

 through these the blood is returned to the 

 heart, where it enters the right auricle through 

 the vena cava. See HEART, for illustration of 

 its parts; also color plate herewith. 



Circulation Through the Lungs. The blood 

 returned to the heart through the vena cava 

 has given up much of its nutritive material 

 and has taken the worn-out matter and carbon 

 dioxide from the tissues. Moreover, the waste 

 products absorbed by the lymphatics enter the 

 blood in the vena cava so that the stream 

 returned to the heart is full of impurities, 

 which account for its darker color. The cir- 

 culation through the lungs, which has for its 

 purpose the purification of the blood, begins 

 at the right auricle, from which the blood 

 passes to the right ventricle. By the contrac- 

 tion of the heart, the blood is forced from the 

 right ventricle through the pulmonary arteries 

 to the lungs, where it is distributed by the 

 capillaries to- the air cells. Here it gives up 

 carbon dioxide and other impurities and ab- 

 sorbs oxygen. It is then returned through 

 the pulmonary veins to the left auricle, and 

 again begins its journey through the body. 



Circulation Through the Liver. The blood 

 from the stomach, spleen, pancreas and intes- 

 tines is gathered up by a vein called the portal 

 vein and carried to the liver. It passes through 

 the capillaries of the liver and is then returned 

 through the hepatic veins to the large veins 

 leading to the heart. During its passage 

 through the liver the bile is secreted from the 

 blood, and other important changes occur, 

 among them the removal of certain poisons 

 that enter the blood from the intestines. W.A.E. 



Related Subjects. The reader interested in 

 this subject will find the articles in these vol- 

 umes on the following topics helpful : 

 Aorta Heart 



Arteries Liver 



Bile Lungs 



Blood Lymphatics 



Capillaries Pulse 



Harvey, William Veins 



CIRCUS, sir'kus, a traveling show of musi- 

 cians, clowns, acrobats and animals, a form 

 of amusement which has been especially pop- 

 ular since exploited by Phineas T. Barnum 

 (which see). "Circus day" is a never-to-be- 

 forgotten event, especially to the boy or girl 

 in the country. First, the billboards with their 

 amazing pictures advertise the coming won- 

 ders; and then, when at last the day comes, 

 early in the morning, before sunrise even, the 

 howling of the animals tells the boy the "circus 

 has come to town." So off he hurries to watch 

 the absorbing task of unloading. Then later 

 in the day he hears the bands playing and sees 

 the parade of gilded wagons, smiling ladies, 

 the clowns and the animals. And later he 



