172 



THE BLOOD AND ITS CIRCULATION 



smaller and smaller, ever-branching arteries, a large part of the 

 blood passes directly to the muscles ; some of it goes to the nerv- 

 ous system, kidneys, skin, and other organs of the body. It gives 

 up food and oxygen in these tissues, receives the waste products 

 of oxidation while passing through the capillaries, and returns to 

 the right auricle through two large vessels known as the vence cavce. 

 It requires only from twenty to thirty seconds for the blood to 

 make the complete circulation from the ventricle back again to 

 the starting point. This means that the entire volume of blood 

 in the human body passes through the various organs of the body 

 three or four thousand times a day.^ 



Portal Circulation. — Some of the blood, on its way back to the heart, 

 passes to the walls of the food tube and to its glands. From there it is sent 

 with its load of absorbed food to the liver. Here the vein which carries the 

 blood (called the portal vein) breaks up into capillaries around the cells of the 

 liver, where it gives up sugar to be stored as glycogen. From the liver, blood 

 passes directly to the right auricle. The portal circulation connects the stomach 



and the small intestine with the liver. 

 It is the only part of the circulation 

 where the blood passes through two sets 

 of capillaries on its way from auricle to 

 auricle. 



Circulation in the Web of a Frog's 

 Foot. — If the web of the foot of a live 

 frog or the tail of a tadpole is examined 

 under the compound microscope, a net- 

 work of blood vessels will be seen. In 

 some of the larger vessels the corpuscles 

 are moving rapidly and in spurts ; these 

 are arteries. The arteries lead into 

 smaller vessels hardly greater in diameter 

 than the width of a single corpuscle. 

 These are capillaries, which may be fol- 

 lowed into larger veins, in which the blood moves regularly. This illustrates 

 the condition in any tissue of man where the arteries break up into capillaries, 

 which unite to form veins. 



Structure of the Arteries, Veins, and Capillaries. — A distinct difference in 

 structure exists between the arteries and the veins in the human body. The 

 arteries, because of the greater strain received from the blood which is pumped 

 from the heart, have thicker muscular walls, and in addition are very elastic. 

 (See figure on page 169.) Veins are much thinner-walled than arteries and 



1 See Hough and Sedgwick, The Human Mechanism, page 136. 



Capillary circulation in the web of 

 a frog's foot, as seen under the com- 

 pound microscope. 



