THE PULMONARY CIRCULATION 95 



muscular contraction whereby blood is pumped through 

 the capillaries into the venules. The second is the movement 

 of the diaphragm — this muscle in descending tends to 

 decrease the already negative pressure in the thorax and to 

 increase the pressure in the abdomen. Most of the veins 

 being provided with valves, muscular contraction in general, 

 and contraction of the diaphragm in particular, are effective 

 only in one direction — towards the heart. On account 

 of the factors above described, the pressure in the great 

 veins may be negative. Blood may be sucked rather than 

 pushed into the heart. This is especially liable to occur 

 during deep inspiration, for under these circumstances, 

 to the negative pressure within the heart is added the 

 negative pressure within the thorax, which tends to draw 

 open the intrathoracic veins. 



The nature of the jugular pulse has already been 

 discussed. 



THE PULMONARY CIRCULATION 



The pulmonary circulation differs from the systemic 

 in two important respects. First, the peripheral resistance 

 is considerably smaller in the lungs than in the rest of the 

 body. For this reason a smaller pressure is required to 

 drive the blood through the capillaries. It is on this 

 account that the right side of the heart is much less muscular 

 than the left. In the second place, the capacity of the 

 pulmonary circulation is continually undergoing rhythmic 

 alteration, due to the alternate expansion and retraction 

 of the lung tissue which occur in respiration. This influ- 

 ences the systemic circulation in two ways. First, each 

 inspiratory movement of the chest aids the flow of blood 

 along the extra-thoracic veins, in the manner above 

 described. Secondly, in ansesthetised animals there is an 

 effect upon the arterial pressure. During inspiration there 

 is a quickening of the heart-beat due, as we have already 

 noted, to diminution of vagus control. The effect upon 



