286 RESPIRATION 



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) 

 5 1 ^econds, 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 

 fall of pressure the increased intrathoracic pressure due to the inflation 

 is an important factor. When the respiration is stopped in collapse, 

 instead of a fall a steady rise of pressure occurs (as in Fig. 84, p. 186). 

 This ultimately merges in the elevation due to asphyxia, which shows 

 itself sooner than in inflation. When the tracheal cannula is closed in 

 natural respiration, no initial fall of pressure takes place (Fig. 130). 



