237 



circulation. It has been estimated that there is about one-twelfth 

 of the whole blood quantum in the lungs during inspiration, and one- 

 fifteenth during expiration. The great degree of distensibility of 

 the pulmonary vessels allows of frequent adjustments being made, so 

 that within wide limits as much blood in a given time will pass through 

 the pulmonary as through the systemic system. The limits of their 

 adjustment may, however, be exceeded during violent muscular 

 exertion. The compressive action of the skeletal muscles returns the 

 blood to the venous cistern, and if more arrives than can be trans- 

 mitted through the lungs and oxygenated in a given time, the right 

 heart becomes engorged, breathlessness occurs, and signs of venous 

 congestion appear in the flushed face and turgid veins. The weaker 

 the musculature of the heart, the more likely is this to occur, hence 

 the breathlessness on exertion which characterizes cardiac affections. 

 Any oedema of the lung resulting from its congestion also impedes the 

 passage in of oxygen. Hence the benefit of oxygen inhalation in 

 strenuous exercise. The training of an athlete consists largely in 

 developing and adjusting his heart to meet this strain. Similarly, 

 the weak heart may be trained and improved by- carefully adjusted 

 exercise. 



Rhythmic compression of the thorax is the method of resuscita- 

 tion from suffocation, for this not only aerates the lungs, but produces 

 a circulation of blood. By compressing the abdomen to fill the heart, 

 and then compressing the thorax to empty it, the valves meanwhile 

 directing the flow, a pressure of blood can be maintained in the aorta 

 even when the heart has ceased to beat, and this if patiently continued 

 may lead to renewal of the heart-beat. 



As regards the effect of breathing upon the arterial blood-pressure, 

 the results are complex. It is generally stated that inspiration at 

 first causes a fall and then a rise of blood-pressure, and that expiration 

 causes first a rise and then a fall. The rate of the heart-beat is also 

 affected during these times, being, when the vagi are intact, slower 

 in expiration and quickened by inspiration. 



In animals under deep anaesthesia the inspiratory rise is due to 

 lessened pressure in the pericardium, and the consequent increased 

 filling of the heart. It is abolished by allowing free access of air to 

 the pericardial sac. 



In man sphygmographic tracings, taken by the suspension method 

 (Fig. 98), show that the effect on the arterial pressure varies with, 

 the type of breathing (see p. 191). 



A deep breath generally produces a fall, often accompanied by the 

 so-called pulsus paradoxus, an alteration in rhythm often considered 

 to have a pathological import, but normal in sleeping dogs, and of 

 occasional occurrence in boys at the time of adolescence. 



The pulmonary circuit may be shut off to a large extent in animals \. 

 under artificial respiration, with little or no effect upon the arterial 

 blood-pressure. Ligation of the vessels of the left lung produced 

 in eighteen cases no noticeable effect on the output of the heart per 

 second. Of the other thirteen, in eleven the pressure was decreased 



