The fundamental problem that must be clearly understood in studying the dynamics of the circulation is how the high pressure in the arteries is kept up, or, in other words, how the arteries can exert so much pressure on the blood when the capillary outflow is so wide and free.
From the description already given of the action of the heart, it appears that each beat of the ventricle pumps some six ounces of blood into the aorta. Though coming to the left ventricle from the pulmonary circulation, the blood may, on account of the exact cooperation of the two sides of the heart, be regarded as being pumped out of the systemic veins. Thus, as far as the general consideration of the physical forces is concerned, the pulmonary circulation may be left out of the question. This pump-• ing occurs some seventy times a minute, so that a great quantity of blood is removed from the veins and forced into the arteries. The ventricles in filling the arteries have to work against considerable pressure, and may be said to pump up the blood from the low-pressure veins into the high-pressure arteries, and the result of this work is the different pressure in the two sets of vessels. During the contraction of the heart the ventricular pressure exceeds that of the aorta, while during the diastole it falls to that of the auricle or even of the great veins. The heart then is the most essential agent in keeping the arteries stretched and overfilled, and in emptying the veins.
The second important factor in enabling the high arterial blood pressure to be kept up, is the resiliency of the middle coat of the arteries. It is only on account of the great elasticity of their arterial walls, that these vessels are capable of being so overfilled, and because of the perfect resiliency of the elastic coat, that they are able to exert such powerful pressure on the blood for such an unlimited time. If the arteries were rigid tubes, to distend them with a fluid, itself inelastic, would of course be out of the question; the outflow from the distal extremity would only take place when the additional charge of blood was injected by the heart.
With each contraction the ventricle overcomes arterial pressure, and further stretches the elastic artery. The act of injecting the blood into the aorta only occupies about one-quarter of each heart beat. The semilunar valves bear the pressure of the blood in the aorta for the rest of the time. The whole force of the ventricle is therefore used up in causing arterial distention. During the greater part of the heart's cycle, the arteries are closed at their cardiac end by the aortic valves, and open at their distal end to the capillaries.
As the result of this, the blood flows constantly out of the distended arteries, through the capillaries, into the veins, and tends to equalize the pressure in the veins and arteries.
But why does not this constant outflow allow the pressure in the arteries to fall to the level of that in the veins? Or, in other words, what is the impediment offered to the escape of the blood that thus keep the arteries distended? If the arteries and veins were a set of continuous wide tubes of similar construction and capacity throughout, it would be impossible for the heart to empty the veins, overfill the arteries, and establish the great pressure difference that normally exists. Therefore some resistance equal to the pressure must be offered to the flow of the blood from the arteries into the veins.
This resistance is made up of several items, of which one alone, namely, the vital contraction of the arterioles, is sufficient to keep up the arterial pressure. No doubt the great increase of surface over which the blood has to move in the capillaries, and the pressure exercised upon them by the surrounding elastic tissues, have influence in impeding the emptying of the arteries. But the contractility of the arterioles is the most important item, as may be seen from the following consideration. The resistance offered by the capillaries is insignificant when compared with the arterial blood pressure, for the increase.of friction accompanying their greater extent of surface is counterbalanced by the decrease of friction dependent upon the great total capacity of the capillaries in comparison with that of the small arteries. The capillary resistance alone is therefore not sufficient to restrain the blood from rushing into the veins. This is seen when the arterioles are paralyzed by the destruction of the nervous mechanism controlling them; the blood then flows readily through the capillary network, the veins become engorged, the arterial blood pressure falls, and the circulation comes to a standstill, in spite of the heart's more rapid beats. We know that beyond the arterioles the pressure falls suddenly, and in the capillary network it is always very low.
Fig. 130. Tracing, showing the effect of weak Stimulation of Vagus Nerve. Stimulus applied between vertical lines. (Recording surface moved from left to right).
The four great factors in keeping up the arterial blood pressure may be thus enumerated: 1, the heart beat; 2, perfect aortic valves; 3, the elastic resiliency of the large arteries; 4, the resistance offered by the contraction of the muscular arterioles.