THE CIRCULATION OF THE BLOOD 



349 



the heart it is zero or even negative; that is, less than atmospheric pressure 

 to the extent of from i to 10 mm. of mercury. 



The amount and relation of the different pressures in the three divisions 

 of the systemic vascular apparatus are approximately shown in Fig. 156. 



160 



1ZO 



Line of 

 DIASTOLIC-- 

 PRESSURE 



Line of 

 SYSTOLIC PRESSURE 



Line of 

 MEAN PRESSURE 



PULSE PRESSURE 



The differe?ice between 



D/ASTOL/C 



and 

 SYSTOLIC PRESSURE 



FIG. 156. A DIAGRAM DESIGNED TO SHOW THE AMOUNT AND THE RELATION OF THE BLOOD- 

 PRESSURE IN THE THREE DIVISIONS OF THE VASCULAR APPARATUS, AS WELL AS THE RELATION OP 

 THE DlASTOLIC, THE MEAN, AND THE SYSTOLIC PRESSURES IN THE ARTERIAL SYSTEM. Based On 



experiments made on dogs. H. Heart. A. Arteries. C. Capillaries. V. Large veins. O, O, 

 being the zero line (= atmospheric pressure), the pressure is indicated by the height of the curve. 

 The numbers on the left give the pressure (approximately) in millimeters of mercury, h. Pressure 

 in heart, a. Arteriole region showing sudden fall of pressure, c. The fall of pressure in the 

 capillaries, v. The negative pressure in the large veins. 



RESUME OF THE FACTS OF THE BLOOD PRESSURE AND OF THE 

 FACTORS WHICH CAUSE IT 



From a consideration of the foregoing facts and statements the following 

 re*sum6 may be made: i. The blood during its flow exerts a pressure against 

 the sides of the blood-vessels. 2. This pressure is the resultant on the one 

 hand of the intra-ventricular pressure developed at the time of the contrac- 

 tion, and on the other hand of the resistance to the forward movement of the 

 blood. 3. The resistance is to be sought for in the cohesion and friction of the 

 molecules of the blood and its adhesion to the walls of the vessels. 4. The 

 resistance is inversely proportional to the diameter of the vessel and is there- 

 fore least in the large arteries and veins and greatest in the arterioles and 

 capillaries. 5. The pressure is highest in the aorta where it may amount 

 in man to 150 mm. of mercury above that of the atmosphere, and lowest at 

 the ends of the venae cavae where it may be no greater than that of the atmos- 

 phere or may be even 10 mm. Hg. below it. 6. The pressure falls from the 

 beginning to the end of the vascular apparatus, though not progressively, 

 for throughout the large vessels of the arterial system it continues relatively 

 high. 7. The high pressure in the aorta is due to the total resistance of the 

 vascular apparatus and the pressure at any given point of the apparatus 

 represents the resistance yet to be overcome. 8. The high pressure in the 

 arterial system and its marked fall at the periphery is more especially the 

 result of the very great resistance at this point, known as the peripheral 



