THE VELOCITY OF THE BLOOD 177 



these factors influence the blood pressure may be seen if we again 

 compare the system to the city water supply. 



Factors Which Maintain Blood Pressure. When the most 

 water is being pumped into the mains, then the water has the 

 greatest velocity and pressure. Likewise, when the heart is 

 pumping most blood into the aorta, the. velocity and the pressure 

 of blood in the vessels are the greatest. If the amount of water 

 remains constant, a uniform outflow through all the outlet tubes 

 will be maintained, but if the number of outlet tubes be dimin- 

 ished, then more water will have to flow, per minute of time, 

 through the remaining tubes; hence the velocity and the pres- 

 sure must be increased. 



The same conditions are present in the body. A narrowing of 

 the arterioles throughout the body or in some extensive vascular 

 area, causes the pressure and the velocity of the blood to be in- 

 creased in the remaining vessels, provided, of course, the heart 

 beat is unchanged. A dilation of the arterioles, on the other 

 hand, results in a fall of pressure and a decrease in the velocity 

 of the blood. In the same way also an increase or decrease in the 

 action of the heart will result in an increase or decrease in the 

 pressure and velocity of the blood. 



The dependence of these two factors, i. e., the heart and the 

 vascular system, on the maintenance of the normal blood pres- 

 sure, is seen in the fact that, with a fast heart and dilated blood 

 vessels, the blood pressure may be exactly the same as when the 

 heart is beating very slowly but the arterioles are all constricted. 

 It is apparent, therefore, that the velocity of the blood in the 

 vessels is dependent on the pressure of the blood and the extent 

 of the vascular area at the time in question. 



The Velocity of the Blood. By the velocity of the flow of 

 blood we mean the actual time it takes for a particle of blood to 

 pass between two points. If the rate were uniform throughout 

 the vascular area, we could compute the time which a particle of 

 blood would take to pass through the circulatory system. This 

 is not the case, however, for the flow of blood is much swifter in 

 the aorta than in the smaller vessels, and here again our analogy 

 between the circulatory system and the city water system applies. 





