2Q2 



RESPIRA TION 



resistance. When the vessels at the base of the heart are ligatured 

 either at the height of inspiration or the end of expiration, so as 

 to obtain the whole of the blood in the lungs, it is found that they 

 invariably contain more blood in inspiration than in expiration. 

 During inspiration, as we have seen, the right ventricle is sending 

 an increased supply of blood into the pulmonary artery; but before 

 any increase in the outflow through the pulmonary veins can take 

 place, the vessels of the lung must be filled to their new capacity. 

 The first effect, then, of the lessened vascular resistance of the lungs 

 in inspiration is a temporary falling off in the outflow through the 

 aorta, and therefore a fall of arterial pressure. As soon as a more 

 copious stream begins to flow through the lungs, this is succeeded 

 by a rise. In like manner the first effect of expiration, which in- 

 creases the resistance and diminishes the capacity of the pulmonary 

 vessels, is to force out of the lungs into the left auricle the blood 

 for which there is no room. This causes a rise of arterial blood- 

 pressure, succeeded by a fall as soon as the lessened blood- flow 

 through the lungs is established. 



The changes in the diastolic capacity of the chambers of the heart 

 itself, with the changes of pericardial pressure, must also act in the 



Fig. 129. Effect on Blood-Pressure of Inflation of the Lungs: Rabbit. Artificial 

 respiration stopped in inflation at i. Interval between 2 and 3 (not reproduced) 

 51 seconds, during which the curve was almost a straight line. Time tracing 

 shows seconds. 



same direction. It is obvious, then, how greatly the rate and depth 

 of respiration in relation to the size of the animal and the other cir- 

 cumstances already mentioned may influence the time relations of 

 the respiratory oscillations in the arterial pressure curve, so that we 

 ought not to expect them to be absolutely constant. 



In artificial respiration oscillations of blood-pressure, synchronous 

 with the movements of the lungs, are also seen. During inflation 

 (inspiration) the arterial pressure rises; during deflation (expiration) it 

 falls. When artificial respiration is stopped at the height of inflation 

 and the lungs kept inflated (Fig. 129), the arterial blood-pressure falls 

 rapidly, and continues low until the rise of asphyxia begins. In the 



