THE PULMONARY CIRCULATION 1055 



whereas the thin-walled distensible veins will be largely influenced 

 by the same factor. The total result then of the negative pressure in 

 the pleural cavities is to increase the flow of blood from the veins into 

 the heart without affecting to any appreciable degree the outflow of 

 blood from the heart into the arteries. The more pronounced the 

 negative pressure in the thorax, the greater will be the amount of 

 blood sucked into the heart from the veins. During inspiration there- 

 fore the heart will be better supplied with blood than during expira- 

 tion, and this factor in itself will tend to raise the arterial blood pressure. 

 The inspiratory descent of the diaphragm will moreover tend to 

 increase the inflow into the heart by raising the positive pressure in 

 the abdomen, so that blood is pressed out of the abdominal veins and 

 sucked into the heart and the thoracic veins. 



Still more important is the influence of the respiratory movements 

 on the circulation through the lungs. In trying to understand this 

 influence it must be remembered that the pulmonary capillaries lie 

 in a certain amount of elastic and connective tissue and are separated, 

 on the one side by the alveolar epithelium from air at the ordinary 

 atmospheric pressure, and on the other by the pleural endothelium 

 from the pleural cavity, where the pressure varies from 6 to 30 mm. 

 Hg. below the atmospheric pressure. We may therefore consider the 

 pulmonary capillaries as lying between, and attached to, two concen- 

 tric elastic bags. Under normal conditions, since these bags are always 

 tending to collapse, the inner one must be pulling away from the outer 

 one, and the outer one from the chest wall. Hence there must be a 

 negative pressure in the tissues between these two bags a negative 

 pressure which in the expiratory condition will be something between 

 and - - 6 mm. Hg., and in the inspiratory condition between and 

 - 30 mm. Hg. If we regard the average pressure within the pulmonary 

 capillaries as constant, these capillaries must be more dilated in the 

 inspiratory than in the expiratory condition. This dilatation of the 

 pulmonary capillaries will have two effects. Their capacity will 

 be increased and the resistance they present to the flow of blood 

 will be diminished. 



Let us now consider what effect these changes will have on the 

 general arterial blood pressure. We will assume that during expiration 

 the pulmonary vessels have a capacity of 25 c.c. and that the beat of 

 the right heart is forcing through them 10 c.c. of blood per second. So 

 long as the chest remains in the expiratory condition 10 c.c. of blood 

 will be flowing into the left heart and into the aorta, so that the systemic 

 blood pressure will remain constant. Now let us suppose that an 

 inspiratory enlargement of the thorax takes place, the negative pressure 

 in the pleura is increased, the two walls of the lungs are pulled farther 

 away from one another, and there is a general enlargement of the 



