368 EESPIRATION [CH. XXIV. 



steeper than that in the large arteries. All these conditions are 

 reversed when, with the expiratory act, the thorax returns to its 

 former size, and the arterial blood-pressure falls in consequence. 



The effect of inspiration on arterial blood-pressure is at first 

 assisted by the pressure of the diaphragm as it descends on the 

 abdominal veins, and blood is thus sent upwards into the chest by 

 the vena cava inferior. On the other hand, this is to some extent 

 counterbalanced by the obstruction in the passage of the blood 

 downwards in the abdominal aorta, and upwards from the veins of 

 the lower extremities, but again the veins are the vessels more easily 

 influenced by moderate changes in external pressure. 



We now come to the cause of the delay we have noted in the 

 blood-pressure tracing in following the respiratory movements. One 

 effect of the diminished intra-thoracic pressure which occurs during 

 inspiration is an increase in the capacity of the pulmonary capillaries, 

 and thus, though more blood is sent into the pulmonary circulation, 

 the resulting increase in outflow is for a time delayed because the 

 capacity of the pulmonary vessels has simultaneously become greater. 

 As soon as this increase in capacity is satisfied, the accelerated flow 

 from the right heart makes itself felt on the left side with the results 

 already explained. In some animals, such as the rabbit, the rise of 

 blood-pressure occurs during expiration, and the fall accompanies 

 inspiration. This is simply because the rabbit is an animal which 

 breathes very quickly ; we have seen there is a delay in the inspiratory 

 rise of pressure ; if the animal breathes quickly enough, inspiration 

 is over and expiration has begun before the rise of pressure occurs. 

 By making the rabbit breathe slowly (Fredericq accomplished this by 

 cooling the medulla oblongata), the tracing obtained is similar to that 

 which is got from an animal like a dog, which normally breathes 

 slowly. 



When the chest of an animal is freely opened, and artificial 

 respiration performed in order to keep it alive, respiratory undulations 

 on the arterial pressure curve are still seen, but they are in the 

 reverse direction. These obviously cannot be produced in the 

 mechanical way just described. The forcible inflation with air at 

 first squeezes more blood out of the alveolar capillaries, that is, the 

 capacity of these vessels is diminished, and this temporarily increases 

 the quantity of blood thrown into the left ventricle, and so causes a 

 rise of arterial pressure. But the increased intra-alveolar pressure 

 has also been shown to lead to an increased resistance to the pulmonary 

 circulation, and the rate of flow into the left side consequently falls ; 

 the aortic pressure therefore falls ; while the pressure in the pulmonary 

 artery rises. If the high positive intra-pulmonary air-pressure 

 persisted, a condition would soon be reached, in which the increased 

 blood-pressure in the pulmonary artery would lead to a greater flow, 



