222 THE BLOOD-VESSELS OF THE LUNG. 



The Blood-vessels of the lung belong to two different systems: (A) PUL- 

 MONARY VESSELS (lesser circulation). The branches of the pulmonary artery 

 accompany the bronchi and are closely applied to them. [As they proceed they 

 branch, but the branches do not anastomose, and ultimately they terminate in 

 small arterioles which supply several adjacent alveoli, each arteriole splitting up into 

 capillaries for several air-cells (Fig. 99, v, c). An efferent vein usually arises at the 

 opposite side of the air-cells and carries away the purified blood from the capillaries. 

 In their course these veins unite to form the pulmonary veins which are joined in 

 their course by a few small bronchial veins (Zuckerkandl). The veins usually 

 anastomose in the earlier part of their course, whilst the corresponding arteries do 

 not.] Although the capillary plexus is very fine and dense, its sectional area 

 is less than the sectional area of the systemic capillaries, so that the blood-stream 

 in the pulmonary capillaries must be more rapid than that in the capillaries of the 

 body generally. The pulmonary veins, unlike veins generally, are collectively 



Fig. 99. 



Semi-schematic representation of the air vesicles of the lung v, v, blood-vessels 

 at the margins of an alveolus ; c, c. its blood capillaries ; E, relation of the 

 squamous epithelium of an alveolus to the capillaries in its wall ; f, alveolar 

 epithelium shown alone ; e, e, elastic tissue of the lung. 



narrower than the pulmonary artery (water is given off in the lung), and they have 

 no valves. [The pulmonary artery contains venous blood, and the pulmonary veins 

 pure or arterial blood]. 



(B) The BRONCHIAL VESSELS represent the nutrient system of the lungs. They 

 (1-3) arise from the aorta (or intercostal arteries) and accompany the bronchi 

 without anastomosing with the branches of the pulmonary artery. Iii their 

 course they give branches to the lymphatic glands at the hilum of the lung, 



