CHAPTER IV. RESPIRATION. 



1. Importance of Lungs, The double circulation, as 

 we have seen, results in every blood-corpuscle passing 

 through the lungs once in the course of a complete circula- 

 tion. No other organs in the body are so favoured, for 

 although the rapidity of circulation is so great, that in the 

 course of a very short time there is enough opportunity 

 for a given corpuscle to try every possible route through 

 the body in turn, yet there is no compulsion on it to do so. 

 A corpuscle may do the whole circulation fifty times or 

 five hundred times and never pass through the kidney, for 

 example, but it cannot do it a single time without passing 

 through the lung-capillaries. Evidently the lung must be 

 an organ through which it is very needful for the blood to 

 flow. Why this is so is obvious, when we consider the 

 importance of a proper supply of oxygen. It is not un- 

 natural, therefore, to take the lungs, rather than the heart, 

 as the goal of the blood, and to apply separate terms to the 

 blood that is travelling from the lung-capillaries to any 

 other set of capillaries, and that travelling in the reverse 

 direction. The former is often called arterial blood, the 

 latter venous. These names are objectionable because they 

 imply a wrong idea of relationship between two kinds of 

 blood and two kinds of vessels. We have seen that the 

 essential difference between arteries and veins is one of 

 lAood-pressure, not blood-composition. Thus the jRilnionary 

 ai tery contains " venous " blood, the pulmonary veins 

 contain " arterial." To avoid these objections the terms 

 "pure" and "impure" blood are often used, instead of 

 arterial and venous, and the blood is said to be " purified " 

 in the lungs. But these terms suggest too great a difference 

 in the nature of the blood, for although " pure " blood does 

 contain more oxygen and less carbon dioxide than " impure 

 blood," it must not be supposed that all the carbon dioxide 

 of blood is removed in the lungs : as a matter of fact only 



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