THE BLOOD-PRESSURE CURVE 



173 



If a record of the heart beat be taken simultaneously with a blood- 

 pressure tracing, it is found that the record of rise of pressure 

 with each beat follows with a considerable latency after the record 

 of the heart itself. The respiratory waves are generally regarded as 

 caused by alterations in the volume of blood admitted to the auricle. 

 During inspiration the greater negative pressure in the thorax draws 

 more blood from the great veins into the thorax, leading to an 

 increased flow through the heart and a consequent rise in blood 

 pressure. This increased flow does not commence at the instant 



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Fig. 136. — Blood Pressure and Respiratory Tracing of a Curarised Cat 

 under Morphia. Artificial Respiration. Time Tracing Seconds. 



inspiration commences, but at a short interval after. The rise in 

 blood pressure, therefore, is not instantaneous with the com- 

 mencement of inspiration, but follows it after an interval of about 

 two to three heart beats. In expiration there is a diminished blood 

 flow to the ventricle, and therefore a fall in blood pressure, and there 

 is an analogous latency, blood pressure only beginning to fall at an 

 appreciable interval after expiration has commenced. The effect of 

 the respiratory movements in varying the blood pressure is well 

 illustrated by studying the changes during artificial respiration. 



