1674 



HANDBOOK (II- PHYSIOLOGY 



CIRCULATION II 



PULMONARY 

 VEIN 



220 



176 



132 



88 



44 22 



DIAMETER 



220 176 



OF VESSELS 



fig. 7. Relationship between vascular calibers and medial thickness in different species. Both the 

 small pulmonary arteries and the small pulmonary veins are well developed in the guinea pig, cat, 

 calf, and pig. On the other hand, in man and in the rabbit, only the arterial muscle, and in the rat 

 only the venous muscle, is well developed. [Redrawn after Takino (392).] 



and between the two ventricles on the other — subjects 

 it to a variety of mechanical influences. Consequently, 

 the following appraisal of the functional anatomy of 

 the pulmonary circulation will take into account not 

 only those features of the vascular tree which deter- 

 mine pulmonary vascular distensibility and resistance 

 to perfusion but also the extravascular structures 

 which may, under appropriate conditions, modify or 

 obscure the natural properties of the pulmonary 

 vessels (288). 



Unless expressly indicated, the anatomical descrip- 

 tions which follow derive largely from the examination 

 of the lungs of normal man at sea level. It is likely 

 that, in most respects, the generalizations about struc- 

 ture, and particularly about the relationships between 

 structure and function, apply almost as well to the 

 cat and to the dog. However, much more has to be 

 learned before the generalizations from normal man 

 at sea level can be applied directly either to other 

 test animals, such as the rabbit and the cow, or to 

 normal native residents at high altitudes, or to sea 

 level residents with abnormal pulmonary vessels or 

 parenchyma (99, 392). 



Blood Vessels 



OVERLAP OF DISTENSIBILITY AND RESISTANCE CHARAC- 

 TERISTICS. In the systemic circulation, the term 



"arteriole" is synonomous with "resistance" vessel. 

 Characteristically, the systemic arteriole has a heavy 

 coat of circular smooth muscle and a high ratio of 

 wall thickness to lumen diameter. With respect to 

 size, ''systemic arteriole" generally refers to vessels of 

 300 to 400 fi or less, depending on the organ in which 

 they are found (50). On the other hand, in the low- 

 pressure pulmonary circulation, the anatomical 

 counterpart of the systemic arteriole does not exist. 

 This lack of sphincteric precapillary vessels has several 

 implications: a) that other small vessels may con- 

 tribute appreciably to the pressure drop between the 

 pulmonary artery and veins; b) that the small pul- 

 monary vessels may also serve as storage vessels, 

 changing caliber passively with the pulmonary blood 

 volume; and c) that under appropriate conditions, 

 each of the small vascular segments may constitute 

 the dominant pulmonary vascular resistance to 

 blood flow. 



large pulmonary vessels. The pulmonary artery 

 rapidly subdivides into terminal branches which 

 have thinner walls and wider bores than the cor- 

 responding branches of the systemic arterial tree. The 

 media of the main pulmonary artery is about half 

 as thick as that of the aorta; the elastic fibers are 

 short and far less orderly than in the aorta (198). 

 The smooth muscle appears to insert on the elastic 



