3I2 TEXT-BOOK OF PHYSIOLOGY. 



arterioles, the heart's contractions remaining the same, the existing 

 pressure soon diminishes. The outflow of blood at once lessens in 

 rapidity and so continues until it counterbalances the inflow. The 

 equilibrium is again restored, but the pressure is at a lower level. 



Variations in Capillary Pressure. The pressure in the capil- 

 laries, though for the most part possessing a permanent value, is 

 subject to variations in accordance with variations in the pressure in 

 either the arterial or venous systems or both. The marked difference 

 in the pressure in the large arteries and the capillaries is partly due to 

 the resistance offered by the narrow arterioles. If the latter dilate 

 in any given area, the capillary pressure increases because of the 

 propagation into them of the arterial pressure. The reverse condition 

 would decrease the pressure. On the other hand, any interference 

 with the outflow from any given area, due to venous compression, 

 would likewise increase the pressure; any factor which would, on the 

 contrary, favor the outflow would decrease the pressure. Indepen- 

 dent of any change in the arteriole resistance, it is evident that a rise 

 in arterial pressure alone would increase the capillary pressure. If 

 both arterial and venous pressures rise, the capillary pressure increases ; 

 if both fall, it decreases. 



Variations in Venous Pressure. Independent of any change 

 in the venous pressure in a given area from local or temporarily 

 acting causes, e. g., aspiration of the thorax or heart, muscle con- 

 tractions, change of position, etc., the general venous pressure will 

 be increased by a decrease in the value of those factors which produce 

 the difference of pressure between the arteries and veins. An in- 

 crease in the value of these factors would necessarily decrease the 

 pressure. 



THE VELOCITY OF THE BLOOD. 



From the number of heart-beats per minute,^ 2, and the amount 

 of blood discharged from the left ventricle at each beat, 180 c.c., it is 

 evident that the blood must be flowing through the vascular appa- 

 ratus with a certain velocity, for during the minute the entire 

 volume of blood, 5769 grams, must have passed twice through the 

 heart. Direct observation of the escape of blood from the central 

 end of a divided artery, and from the peripheral end of a divided 

 vein, as well as of the flow through the capillaries as seen with the 

 microscope, shows that the velocity of the flow varies in different 

 parts of the vascular apparatus. In the arteries, moreover, the flow 

 is not quite uniform, but experiences alternate acceleration and 

 retardation with each heart-beat. In the capillaries and veins the 

 flow is continuous and uniform, as the conditions of the arterial walls 

 are such as to completely overcome the intermittency. 



