RESPIRATORY VARIATIONS OF BLOOD PRESSURE 227 



general relaxation of the same chief trunks. The position in which the 

 venous system is in general stretched most strongly corresponds well with 

 the attitude which one takes involuntarily when, after working at a desk for 

 a long time, he stands up and stretches himself (Fig. 93). It is, therefore, 

 to be assumed that such a stretching of the trunk and of the extremities acts 

 favorably upon the venous circulation, which has been disturbed by sitting 

 too long (Fig. 94), and this quite independently of the direct effect of 

 muscles and fascia (Braune). 



It may happen at times that a greater quantity of blood flows to the right 

 heart than can be disposed of. This is possible, for example, when a powerful 

 vasoconstriction occurs throughout a large vascular region. A large quantity 

 of blood is then forced from the arteries into the veins, and from these to 

 the right heart, while at the same time the discharge of blood from the left 

 ventricle becomes more difficult on account of the high resistance in the con- 

 tracted arteries. 



We do not know how often or to what extent this may happen. We only 

 know that after extirpation of the liver, the portal vein having been previously 

 connected with the inferior vena cava, the heart is found dilated to its utmost, 

 and the great veins are filled swelling full of blood (Stolnikow). The liver 

 takes up a considerable quantity of blood like a sponge, and protects the right 

 heart from an oversupply, just as it aids in the relief of the heart when over- 

 distention occurs as a result of transfusion (cf. page 207). 



8. THE LESSER CIRCULATION AND THE RESPIRATORY 

 VARIATIONS OF BLOOD PRESSURE 



A. THE PULMONARY CIRCULATION 



In general the same laws which we have learned in our study of the blood 

 movements in the greater circulation hold good for the lesser circulation. 

 The pressure is dependent upon the quantity of blood discharged from the 

 right heart and upon the resistance in the pulmonary vessels. The quantity 

 of blood which the right ventricle forces into the pulmonary arteries depends 

 upon the quantity of blood which flows from the greater circulation through 

 the venae cavag into the right heart. This quantity is determined partly by 

 events in the aortic system, and partly by the pressure changes in the thoracic 

 cavity which accompany the different phases of respiration. 



Thus every hindrance of any moment to the flow of blood into the venae cavae 

 reduces the pressure in the pulmonary arteries. An increased supply of blood 

 to the right heart, such for example as is brought about by powerful contraction 

 of the abdominal vessels, increases the pulmonary pressure. 



We have already emphasized the fact that with each act of inspiration the 

 thoracic cavity exerts suction on the blood vessels which enter it. The right 

 ventricle, therefore, receives more blood during inspiration than during expira- 

 tion; nevertheless the pressure in the ventricle itself (Talma) and in the 

 pulmonary arteries (Knoll) declines only to rise again at the next expiration. 



These variations of blood pressure are caused partly by the effect of varia- 

 tions in the intrathoracic pressure on the thin-walled right ventricle, and partly 



