THE PULMONARY CIRCULATION. 183 



the lungs as are found in the systemic circulation. The right 

 ventricle is far less powerful than the left, so that the pressure 

 of the blood in the lung vessels is less than that in the systemic 

 vessels. The respiratory movements also cause the size of the 

 blood vessels in the lungs to vary in a marked degree. These 

 changes in the capacity of the pulmonary blood vessels affect the 

 systemic blood pressure. Thus, at the height of inspiration, the 

 lungs may contain one-twelfth of the blood of the body, while 

 during expiration this amount may be lessened to one-fifteenth 

 to one-eighteenth of the total. This condition makes it possible 

 for the heart to be filled more rapidly during the later part of 

 inspiration and the beginning of expiration, than at other times, 

 and accounts for the rise of blood pressure observed at this time. 



