338 



TEXT-BOOK OF PHYSIOLOGY. 



the pressure has been found to be equal to 14 mm. Hg., and in the brachial 

 vein 9 mm. of Hg. In the jugular and subclavian and other vessels near 

 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. 157. 



L ine of 

 SYSTOLIC PRESS UP 



Lm<t of 

 ME AH PRESSUPE 



Lin* of 



D/ASTOLIC 

 Pressure 



PULSE 



T^e difference between 

 SYSTOLIC 



O/ASTOL/C PPE3SVP 



FIG. 157. -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 OF 

 THE DlASTOLIC, THE MEAN, AND THE SYSTOLIC PRESSURES IN THE ARTERIAL SYSTEM, Bated OD 



experiments made on dog*. 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 THK 

 FACTORS WHICH CAUSE IT. 



I From a consideration of the foregoing facts and statements the folio 

 r&ume* may be made: i. The blood during its flow exerts a pres 

 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 f riaion of the 

 molecules of the blood, 4. The resistance is inversely proportional to the 

 diameter of the vessel and is therefore least in the large arteries ami vein ami 

 greatest in the arterioles and capillaries, 5, The pressure i highest in the 

 aorta where it may amount in man to 150 mm. of m< hove that of the 



atmosphere, and lowest at the ends of the venae cavae where it may be no 

 greater than that of the atmosphere or may be even 10 mm. \\\' i . below it. 

 6. The pressure falls from the befnnnin^ to the eml of the vascular apparat 

 though not progressively, for throughout the large vessels of the Arterial 

 System it Continues relatively hiVh. 7. The hi;/h pressure in the aorta is Hue 

 to the total resistance of the vaseul-ir ;ipp;ir;itu nno 1 the pressure al ;my 



