21 8 THE CIRCULATION OF THE BLOOD 



to paralysis of the nervous mechanism and relative loss of control, 

 second, to the great direct stimulation of the cardiac muscle. The 

 constant overuse of tobacco, therefore, very sharply weakens the effi- 

 ciency of the heart. Caffeine does not lead to so great disturbance of 



the heart's nutrition as does nicotine. 



t 



THE CIRCULATION THROUGH THE BLOOD VESSELS. 



Blood-Pressure. The subject of blood-pressure has been already 

 incidentally mentioned more than once in the preceding pages; the time has 

 now arrived for it to receive more detailed consideration. 



That the blood exercises pressure upon the walls of the vessels containing 

 it is due to the following facts: 



The heart at each contraction forcibly injects a considerable amount of 

 blood, 80 to 100 c.c., suddenly and quickly into the arteries. 



The arteries are highly distensible and stretch to accommodate the extra 

 amount of blood forced into them. The arteries are already full of blood 

 at the commencement of the ventricular systole, since there is not sufficient 

 time between the heart-beats for the blood to pass into the veins. 



There is a distinct resistance interposed to the passage of the blood from 

 the arteries into the veins by the enormous number of minute vessels, small 

 arteries (arterioles) and capillaries, into which the main artery has been 

 ultimately broken up. The sectional area of the capillaries is several hun- 

 dred times that of the aorta, and the friction generated by the passage of 

 the blood through these minute channels opposes a considerable hindrance 

 or resistance in its course. The resistance thus set up is called peripheral 

 resistance. The friction is greater in the arterioles, where the current is 

 comparatively rapid, than in the capillaries, where it is slow. 



The interaction of these factors heart-beat, elastic vessels, and periph- 

 eral resistance is sufficient to maintain a flow of blood through the entire 

 circulatory system. It is the interrelation of these factors which main- 

 tains an even and steady flow through the capillaries and past the tissues, 

 where it is desirable that the conditions of blood flow should be most con- 

 stant if the purposes of nutrition are to be best accomplished. In fact, 

 we shall even find that it is the interaction of these same factors, together 

 with the possibility of variations through regulation by their nerve-motor 

 mechanisms, that we have the great variations and adjustments of blood- 

 pressure, speed of flow, volume of flow, and the regulation of volume in 

 particular parts of the body or local control. 



Arterial Blood -Pressure. That the blood exerts considerable pres- 

 sure upon the arterial walls in keeping them in a stretched or distended 

 condition may be readily shown by puncturing any artery. The blood is 

 instantly projected with great force through the opening, and the jet rises 

 to a considerable height, the exact level of which varies with the size of the 



