NORMAL RESPIRATION. Ill 



accompanied by stimulation of the vasomotor centre, which produces variations of 

 the blood-pressure. 



1. Normal Respiration. Fig. 92 shows what sometimes, but by no means 

 always, happens. During inspiration, owing to the dilatation of the thorax, more 

 arterial blood is retained within the chest, while at the same time venous blood is 



-Fig. 92. 

 Influence of the respiration upon the pulse. J, inspiration ; E, expiration. 



sucked into the right auricle by the aspiration of the thorax ; as a consequence of 

 this, the tension in the arteries during inspiration must be less. The diminution 

 of the chest during expiration favours the flow in the arteries, while it retards. the 

 flow of the venous blood in the venae cavse, two factors which raise the tension in 

 the arterial system. The difference of pressure explains the difference in the form 

 of the pulse-curve obtained during inspiration and expiration, as in fig. 92 and fig. 

 3, I, III, IV, in which J indicates the part of the curve which occurred during 

 inspiration, and E the expiratory portion. The following are the points of 

 difference : (1) The greater distension of the arteries during expiration causes all 

 the parts of the curve occurring during this phase to be higher ; (2) the line of 

 ascent is lengthened during expiration, because the expiratory thoracic movement 

 helps to increase the force of the expiratory wave ; (3) owing to the increase of 

 the pressure, the dicrotic wave must be less during expiration ; (4) for the same 

 reason the elastic elevations are more distinct and occur higher in the curve near 

 its apex. The frequency of the pulse is slightly greater during expiration than 

 during inspiration. 



2. This purely mechanical effect of the respiratory movements is modified by the 

 simultaneous stimulation of the vasomotor centre which accompanies these move- 

 ments. At the beginning of inspiration the blood-pressure in the arteries is lowest, 

 but it begins to rise during inspiration, and increases until the end of the inspiratory 

 act, reaching its maximum at the beginning of expiration ; during the remainder 

 of the expiration the blood-pressure falls until it reaches its lowest level again at 

 the beginning of inspiration (compare 85,/); the pulse-curves are similarly 

 modified, and exhibit the signs of greater or less tension of the arteries correspond- 

 ing to the phases of the respiratory movements. [There is, as it were, a displace- 

 ment of the blood-pressure curve relative to the respiratory curve.] 



Forced Respiration. With regard to the effect produced on the pulse-curve 

 by a powerful expiration and a forced inspiration, observers are by no means 

 agreed. 



Valsalva's Experiment. Strong expiratory pressure is best produced by closing 

 the mouth and nose, and then making a great expiratory effort ( 60) ; at first 

 there is increase of the blood-pressure, while the form of the pulse-waves resembles 

 that which occurs in ordinary expiration, the dicrotic wave being less developed ; 

 but, when the forced pressure is long continued, the pulse-curves have all the signs 

 of diminished tension. This effect is due to the action of the vasomotor centre, 

 which is affected reflexly from the pulmonary nerves. We must assume that forced 

 expiration, such as occurs in Valsalva's experiment, acts by depressing the activity 

 of the vasomotor centre ( 371, II.). Coughing, singing, and declaiming act like 

 Valsalva's experiment, while the frequency of the pulse is increased at the same 

 time. After the cessation of Valsalva's experiment, the blood-pressure rises above 

 the normal state (Sommerbrodt), almost as much as it fell below it ; the normal 

 -condition being restored within a few minutes {Lenzmann). 



