496 CIRCULATION OF BLOOD AND LYMPH. 



At the heart, therefore, the pressure of the blood upon the walls 

 of the veins is very small, and, indeed, owing to the circumstance 

 that the large veins lie in the thoracic cavity, in which the pres- 

 sure is below that of the atmosphere, the pressure of the blood in 

 them may also be below atmospheric pressure, although doubt- 

 less at this point (vena cava) the pressure within the vein is 

 greater than the pressure on its exterior (intrathoracic pressure). 

 Taking into account the negative intrathoracic pressure (p. 652) 

 it may be estimated that the difference in pressure between the 

 blood in the veins of the neck and that in the superior vena cava 

 is equal to 4 or 5 mms. Hg., and this difference is sufficient to drive 

 the blood into the heart and to fill and distend it rapidly during 

 diastole* To complete the general conception of the pressure 

 relations in the vascular system it is necessary to know the pressure 

 of the blood in the smallest arteries and veins and in the capillaries. 



Fig. 200. — Schematic representation of the general relations of blood-pressure (side 

 pressure) in different parts of the vascular system: a, The arteries; c, the capillaries; v, 

 the veins. The mean and diastolic pressures remain nearly constant in the arterial system, 

 as far as they can be measured accurately. The pressures in tlie veins are represented as 

 uniform at any one point. In the large veins near tlie heart there are variations of pressure 

 with each respiration and with each heart beat (Venous Pulse, p. 525). 



It is not possible — in the cases of the capillaries, for instance— to 

 connect a manometer directly with the vessels, and recourse has 

 been had to less direct methods. Older methods used for this 

 purpose which depended upon blanching the skin have not 

 proved to be satisfactory, and in more recent work efforts have 

 been made to determine the pressure necessary to block the 

 capillary flow when this flow is being observed under the micro- 

 scope. (See p. 506.) Data obtained by this method are more 

 trustworthy. They indicate a pressure in the capillaries of the skin 

 in man of 20 to 25 mms. of mercury, f 



The general relations of the pressures in arteries, veins, and 

 capillaries may be expressed in a curve such as is shown in Fig. 

 200. 



* Henderson and Barringer, "Amer. Journal of Physiology," 31, 352, 1913. 

 t Hooker, "Physiological Reviews," 1, 112, 1921. 



