208 CIRCULATION OF THE BLOOD 



the quantity transfused becomes so great that sooner or later the demands upon 

 the heart are excessive, and the arterial blood pressure falls in spite of the 

 abnormally large quantity of fluid in the vessels. One sometimes meets with 

 cases where the heart works powerfully enough during the transfusion to over- 

 come the increased quantity of blood, but later in the course of the experiment, 

 after transfusion has ceased, symptoms of acute fatigue suddenly appear. In 

 such cases the heart may be relieved and death averted by withdrawing a suffi- 

 cient quantity of blood from the vessels. 



We have in the circumstances mentioned above the explanation of heart 

 weakness which sometimes follows ingestion of very large quantities of fluid by 

 way of the stomach. 



The reverse processes take place in bloodletting. The heart empties itself as 

 completely as possible and drives the greatest possible quantity of blood into 

 the vessels; the vessels contract and thereby present a high resistance to the 

 blood stream; the kidneys, salivary glands and probably all the other glands 

 diminish their secretions, and there occurs an increased passage of fluid from 

 the lymph to the blood vessels. 



By the cooperation of these factors the blood pressure under normal cir- 

 cumstances varies in general within rather narrow limits, notwithstanding 

 the many influences which tend to change it in both directions. And yet, 

 in order to produce significant variations in the pressure artificially it is 

 often necessary to use only very weak stimulation. In fact, the stimulus 

 may be so slight that at times one is wholly at a loss to make out the cause 

 of the sudden increase or decrease in pressure which results. More, on this 

 subject will be found under the discussion of vasomotor nerves. 



By means of an instrument constructed on the principle of the Basch 

 Sphygmomanometer and applied to the radial artery of man, the blood pressure 

 in the sitting or reclining position was found by Hill and Barnard to be 100- 

 108 mm. Hg. It rose to 120, 130 or 140 mm. under various influences such as 

 bodily movements, but sank again very rapidly toward the value for rest when 

 the movements ceased. The blood pressure is raised also by a cold bath, but is 

 depressed by a warm one (Edgecombe and Bain). 



Blood pressure in the large arteries is not much higher than in those of 

 smaller caliber, and decreases only slightly therefore with the distance from 

 the heart. Especially is this true of diastolic pressure, whereas differences 

 of systolic * pressure are greater. The cause of this slow fall in pressure in 

 the arterial system is purely hydrodynamic in nature. The resistance against 

 which the blood pushes in the larger arteries is small in comparison with 

 that to which it is subjected in the smallest. The consequence is that the 



[ l From numerous comparative observations made on dogs, Dawson concludes that " in 

 considering variations in the systolic pressure, it is absolutely essential to distinguish 

 between end pressures obtained from the branches of the main arterial trunk and those 

 obtained from the main trunk itself. In the former case the systolic pressure shows a 

 steady and considerable falling off which becomes apparent in end pressures taken, for 

 example, in the thyroid arteries, in branches arising from the axillary and in branches 

 from the lower part of the aortico-femoral trunk. When, however, the systolic end 

 pressure is taken in the main arterial trunk, it is found that this pressure either remains 

 high (axillary and brachial) or may even greatly exceed the corresponding lateral pressures 

 in the aorta (iliac and femoral)." ED.] 



