196 A MANUAL OF PHYSIOLOGY 



From the larynx to the bronchioles the mucous membrane is 

 ciliated on its free surface, the cilia lashing upwards so as to move 

 the secretion towards the larynx and mouth. In the infundibula the 

 ciliated epithelium begins to disappear, and is absent from the alveoli. 

 Part of the nasal cavity and the upper part of the pharynx are also 

 lined with ciliated epithelium. Mucous glands are present in 

 abundance in the upper portions of the respiratory passages, but 

 disappear in the smaller bronchi. 



Blood-supply of the Lungs. The quantity of blood traversing the 

 lungs bears no proportion to the amount required for their actual 

 nourishment. Small, however, as this latter quantity is, it cannot 

 apparently be derived from the vitiated blood of the right ventricle, 

 but is obtained directly from the aortic system by the bronchial 

 arteries. These are distributed with the bronchi, which they supply 

 as well as the connective-tissue of the interlobular septa running 

 through the substance of the lung, the pleura lining it and the walls 

 of the large bloodvessels. Most of the blood from the bronchial 

 arteries is returned by the bronchial veins into the systemic venous 

 system, but some of it finds its way by anastomoses into the pul- 

 monary veins. 



The branches of the pulmonary artery are also distributed with 

 the bronchi, and break up into a dense capillary network around the 

 alveoli. From the capillaries veins arise which, gradually uniting, 

 form the large pulmonary veins that pour their blood into the left 

 auricle. 



The same quantity of blood must, on the whole, pass per unit of 

 time through the lesser as through the greater circulation, otherwise 

 equilibrium could not exist, and blood would accumulate either in 

 the lungs or in the systemic vessels. But it does not follow that at 

 each heart-beat the output of the two ventricles is exactly equal. If, 

 indeed, the capacity of the lesser circulation were constant, the 

 quantity driven out at one systole by the right ventricle would be 

 the same as that ejected at the next by the left ventricle. But it is 

 known that the capacity of the pulmonary vessels is altered by the 

 movements of respiration and probably in other ways, so that it is 

 only on the average of a number of beats that the output of the two 

 ventricles can be supposed equal. 



The time required by a given small portion of blood, e.g., by a 

 single corpuscle, to complete the round of the lesser circulation, is, 

 as we have seen (p. 124), much less than the average time needed to 

 complete the systemic circulation. In the rabbit the ratio is probably 

 about 1:5. Since all the blood in a vascular tract must pass out of it 

 in a period equal to the circulation time, the average quantity of 

 blood in the lungs and right heart of a rabbit must be about one- 

 fifth of that in the systemic vessels. On the assumption that the 

 same proportion holds for a man, not less than 900 grm. out of the 

 5^ kilos of blood in a seventy kilo man must be contained in the lesser 

 circulation, and rather more than 4^ kilos in the greater. This 

 corresponds sufficiently well with calculations from other data. 



For example, the average weight of the lungs in three persons, 



