RECORD OF BLOOD PRESSURE. 295 



If the different parts of the circulation be represented on the 

 base line H. A. c. v., these letters corresponding to heart, arteries, 

 capillaries, and veins respectively, and if the height of the blood 

 pressure be represented on the vertical line in mm. Hg, the curve 

 h, a, c, v, would give about the relative pressure in the various 

 parts of the circulation. This shows that in the receiving chamber 

 of the heart the pressure is below zero, while the ventricular pump 

 drives it to the height of the arterial pressure 160 mm. Hg. In 

 the arteries the pressure though gradually falling is everywhere 

 high, while just before the blood reaches the capillaries, a sudden 

 fall occurs. The variation after this is merely a gentle descent 

 until the large venous trunks are reached, where the pressure is 

 below zero. From a purely physical point of view then, the ven- 

 tricle may be regarded as pumping the blood up to an elevated 

 high-pressure reservoir of small capacity (the arteries), from which 

 it rapidly falls by numerous outlets into an expansive low-lying 

 irrigation basin (the wide capillaries), whilst it slowly trickles 

 back to the well (the auricle) under the pump, which lies below 

 the surface pressure. 



From this diagram the following points can be gathered : 



1. The great difference between the pressure on the arterial 



and venous sides of the circulation. 



2. The comparatively slight difference in pressure in the dif- 



ferent parts of the arterial or of the venous systems re- 

 spectively. 



3. The suddenness of the fall in the pressure between the small 



arteries and the capillaries, where the great resistance to 

 the outflow is met with. 



4. In the large veins the pressure of the blood is habitually 



below that of the atmosphere, only becoming positive 

 during forced expirations. 



VARIATIONS IN THE BLOOD PRESSURE. 



If the blood pressure be recorded with Ludwig's Kymograph, a 

 tracing will be obtained which shows that the pressure undergoes 

 periodic elevations and depressions of two different kinds. The 



