340 TEXT-BOOK OF PHYSIOLOGY. 



large arteries into the continuous equable stream characteristic of the capil- 

 laries. It also permits of wide variations in the amount of blood the arteries 

 can contain between their minimum and maximum distention. 



VARIATIONS IN THE BLOOD -PRESSURE. 



A. In the Arterial Pressure. It is evident from the preceding state- 

 ments that the arterial blood-pressure as a whole may be increased above the 

 normal, by: 



1. An increase in the rate or force of the heart's contraction. 



2. An increase in the peripheral resistance. 



3. An increase in both the force of the heart and the peripheral resistance 

 and that it may be brought back to the normal by a decrease in either one 

 or both of these factors. 



It is also evident that the arterial blood-pressure may be decreased below 

 the normal by: 



1. A decrease in the rate and force of the heart's contraction. 



2. A decrease in the peripheral resistance. 



3. A decrease in both the force of the heart and the peripheral resistance 

 and that it may be raised to the normal by an increase in either one or both 

 of these factors. 



If when the arterial pressure is in a condition of equilibrium the heart 

 ejects into the arteries in a given period of time an increased quantity of 

 blood as a result of an increased rate of contraction, there will be an accumu- 

 lation of blood temporarily in the arteries and a rise of pressure (the peripheral 

 resistance remaining the same), for the reason that the pressure is only 

 sufficient to force into the capillaries a given volume, in the same period of 

 time. As the pressure rises the velocity and the outflow will be increased 

 until equilibrium is restored though at a somewhat higher level. A rise 

 of pressure from an increase in the rate of the beat alone has been questioned, 

 for it has apparently been demonstrated that there is a definite relation be- 

 tween the normal rate and the volume discharged from the ventricle, and 

 that when the rate is increased, the volume discharged diminishes and hence 

 the pressure remains normal or even falls below the normal. 



An increase in the pressure is readily brought about by an increase in the 

 force or power of the contraction, the frequency remaining the same. An 

 increase in the volume of blood ejected at each contraction will necessarily 

 lead to an accumulation. With the accumulation there goes an increased 

 distention of the artery and a corresponding increase of pressure. In a 

 short time, therefore, the increased pressure will force out of the arteries 

 at a higher rate of speed this excess of blood until the outflow again equals the 

 inflow. This restores the equilibrium but establishes the mean pressure at a 

 higher level. 



If the peripheral resistance is increased by a contraction of the muscle 

 walls of the arterioles, the frequency and force of the heart remaining the 

 same, there will also be an accumulation of blood in the arteries, an increased 

 distention and consequent rise of pressure (Fig. 158). The outflow of 

 blood will at the same time be diminished. A rise of pressure from this cause 

 much beyond the normal is to a large extent prevented by a simultaneous 



