The right heart pumps blood received from all over the body to the 

 capillaries of the lungs. 



The "Double Circulation" The blood-stream courses from any point 

 and back to die start only by passing through both sides of the heart — that 

 is, through both the pulmonary, or lung, circuit and the systemic, or body, 

 circuit (see illustration, p. 191). 



This "double circulation" of all warm-blooded animals makes possible a 

 rapid exchange of carbon dioxide for oxygen. In the human body all the 

 blood passes through the heart (and therefore through the capillaries of 

 the lungs) once in from twenty-three to thirty seconds. The exchange of 

 gases between the air sacs of the lungs and the capillaries is by osmosis (see 

 page 208). 



Changes in Circulation In the frog and some of the reptiles there is 

 only one ventricle, so that the heart pumps a mixture of oxygenated blood 

 from the lungs and deoxygenated blood just returned from the other organs. 

 There is a suggestion of this condition in the unborn baby. 



In the unborn human baby the blood from the pulmonary artery is short- 

 circuited directly into the aorta, and from the right auricle into the left 

 auricle, without passing through the lungs — which have of course not yet 

 started to operate. At birth the opening between the pulmonary artery and 

 the aorta ordinarily closes at once; the opening between the two auricles, 

 widiin a few days. Occasionally, however, these passages do not close nor- 

 mally. The baby is bluish, for some of the blood is not aerated in the lungs. 

 A "blue baby" often survives, but only if these "short-circuits" close. 



Changes in the Blood While in the capillaries of the various tissues of 

 th-e body the blood absorbs from the surrounding lymph carbon dioxide, 

 urea, and other substances that are present in relatively large proportions. 

 By osmosis it also loses food materials, salts, oxygen and enzymes that are 

 relatively more abundant in the blood than in the surrounding liquids. In 

 certain parts of the body additional changes take place in the composition of 

 the blood. In the kidneys much of the urea, salts, and other waste sub- 

 stances is removed from the blood. 



In addition to furnishing the cells of the body with a uniform supply of 

 materials, the blood in its circulation tends to equalize the temperature of 

 the body tissues, much as the circulating water in a car's radiator cools the 

 engine. Among all living things, birds and mammals have the most deli- 

 cately balanced internal fluid media. 



Lymph taken from a healthy body is an excellent medium for the 

 growth of living cells of many kinds. Inside the body of a mammal or bird, 

 with its "warm" interior, the conditions would seem to be ideal for the 

 growth and activities of protoplasm. But those ideal conditions cannot re- 

 main ideal very long. As the blood and lymph move rapidly through the 



192 



