THE FLOW OF BLOOD IN THE BODY 177 



thin walls the oxygen of the air readily penetrates to the 

 red blood cells; and the carbonic acid gas just as readily 

 passes from the blood to the air. As a result of this 

 change, the blood becomes of a bright red color, and is 

 called arterial blood. From the capillaries of the lung the 

 arterial blood is collected into the pulmonary veins and 

 carried to the left auricle, and then to the left ventricle, 

 where it is ready to make another circuit of the body. 



307. Summary of the circulation of the blood. In making 

 a complete circuit of the body the blood passes through the left auricle, 

 and through the mitral valve to the left ventricle ; then past the left 

 sem f lunar valve to the aorta, and then through the arteries to all parts 

 of the body ; then through the capillaries into the veins, and back to 

 the heart ; next through the right auricle, then the right ventricle, 

 then through the pulmonary artery to the capillaries of the lung', then 

 through the pulmonary veins to the left auricle once more. Thus in 

 making the complete circuit of the body, a drop of blood passes through 

 the heart twice, and through two different sets of capillaries. The 

 circuit of the body in general is called the systemic circulation, and that 

 through the lungs is the pulmonary circulation. 



308. The portal system of circulation. The blood from 

 the capillaries of the stomach and intestine is collected 

 into a single vein, called the portal vein, which goes to the 

 liver and there divides into capillaries. The liver capil- 

 laries can be considered as millions of small tubes which 

 are substituted for a few inches of the portal vein. Just 

 outside of the liver they empty into three veins which 

 open into the ascending vena cava. The circulation 

 through the liver is sometimes called the portal circulation. 



309. Time required in the complete circulation. It re- 

 quires about twenty seconds for a drop of blood to go the round of the 

 circulation from the left ventricle back to its starting point. All the 

 blood passes through the heart about once every two or three minutes. 

 All the arteries, except the pulmonary artery, carry bright red arterial 



ov. PHYSIOL. 12 



