192 



THE CIRCULATION OF THE BLOOD 



that the pressure of blood in the carotid of a rabbit is capable of supporting 

 a column of 90 to 120 mm. of mercury; in the dog 100 to 175 mm.; in the 

 horse 152 to 200 mm.; and in man the pressure is estimated to be about the 

 same as in the horse. To measure the absolute amount of this pressure 

 in any artery multiply the area of its transverse section by the height 

 of the column of mercury which is already known to be* supported 

 by the blood pressure in any part of the arterial system. The weight of a 

 column of mercury thus found will represent the absolute pressure of the 

 blood. Calculated in this way, the blood pressure in the human aorta is 

 equal to 1.93 kilogrammeters; that in the aorta of the horse being 5.2 kilo- 

 grammeters; and that in the radial artery at the human wrist only 0.08 

 kilogrammeter. Supposing the muscular power of the right ventricle to be 

 only one-fourth that of the left, the blood pressure in the pulmonary artery 

 will be only 0.5 kilogrammeter. The amounts above stated represent the 

 arterial tension at the time of the ventricular contraction. 



The arterial pressure is greatest at the beginning of the aorta, and de- 

 creases toward the capillaries. It is greatest in the arteries at the period of 

 the ventricular systole, and least during the diastole. The blood pressure 

 gradually lessens as we proceed from the arteries near the heart to those more 

 remote, and again from these to the capillaries, as it does, also, from the 

 capillaries along the veins to the right auricle. 



Arterial Blood Pressure Measurements in Man. A number of 

 instruments have been devised for estimating blood pressure in man for 



FIG. 1 88. Riva-Rocci Apparatus (schematic) for Determining Blood Pressure in Man. 



