284 



ESSENTIALS OF PHYSIOLOGY. 



pressure, and the flow of blood to the heart is thus farther assisted, 

 the thick-walled ventricles and arteries being practically unaffected. 

 As the result of the additional blood reaching the heart during 

 inspiration, the amount of blood expelled from the heart at each beat 

 becomes larger, and the arterial pressure rises. Conversely, during 

 expiration the negative pressure diminishes again, less blood is sucked 

 into the heart, its output is smaller, and the arterial pressure falls. 



A second factor which conduces to the rise of arterial pressure 

 is the descent of the diaphragm during inspiration ; this, by diminish- 

 ing the size of the abdominal cavity, raises the intra-abdominal 

 pressure and squeezes blood out of the abdomen along the inferior 

 vena cava into the heart. 



According to some observers, a third factor also takes a part in 

 producing the rise of blood pressure during inspiration, namely, 



changes in the size of the 

 vessels forming the capil- 

 lary network in the lungs. 

 The lung is exposed on 

 its inner surface to atmo- 

 spheric pressure, and on its 

 outer surface to a pressure 

 which is less than atmo- 



Arierial 

 Bloodpm 



Inspiration 



Respiratory traciryr. 



FIG. 1] 4. Effect of respiratory movements on 

 arterial blood pressure. (Starling's Principles 

 of Physiology. ) 



spheric pressure. An in- 

 crease in this difference of 

 pressure, such as occurs 

 during inspiration, will dilate the capillaries in the wall of the lungs, 

 blood will flow through them more rapidly, and a larger quantity 

 will reach the left ventricle in a given time ; the additional blood 

 thus reaching and expelled from the left ventricle will raise the 

 arterial pressure. Conversely, during expiration, when the difference 

 of pressure on the two sides of the lung wall diminishes, the capillaries 

 will shrink, blood will flow less rapidly through them into the left 

 ventricle, and the blood pressure will fall. Other writers, however, 

 consider that changes in the calibre of the capillaries play no part 

 in the variations of blood pressure which accompany the respiratory 

 movements; they regard these variations as being due entirely to 

 alterations in the amount of blood which is drawn into the heart by 

 the negative pressure on the walls of the great veins, and is driven 

 out of the abdominal veins by the contraction of the diaphragm. 



The negative pressure during inspiration varies with the depth of 

 the inspiration, and, when the breathing is very forcible, the rise and 

 fall of blood pressure during inspiration and expiration may become very 



