2. Peripheral Resistance.-Peripheral resistance is that factor, ever present in the circulatory system, which tends to retard and prevent the forward movement of the circulating blood. In the human body this is a complicated factor, being dependent in part upon (a) the diminishing diameters of the conducting tubes, particularly in the arterioles and capillaries, (b) internal friction, (c) the length of the vessels and (d) the innumerable branching of the arterial tree.

It is obvious that any obstruction, even if partial, at the outlet of any distensible tube will increase the lateral pressure exerted by the fluid in that tube. A familiar example of this is the effect of the adjustable nozzle used on the garden hose. The same physical law holds good for the arterial system, except that the intermittency of flow is a complicating factor, which includes the interrelation of two variables-the systolic and the diastolic pressures. The result of these factors is to produce a condition in which increased peripheral resistance (increased vasomotor tone) will increase systolic pressure while a diminished peripheral resistance will lower both systolic and diastolic pressure. This change invariably occurs unless some compensating factor modifies the force of the heart (energy index, page 140).

The other factors, including length of tube and internal friction, being practically constant may be safely omitted in clinical considerations, without introducing any serious element of error (for further discussion of peripheral resistance and vasomotor tone, see Chapter VII, page 138). Elasticity and Contractility of the Vessel Wall.-The elasticity and contractility of the blood-vessel walls is

dependent upon the circular muscular coat and the elastic lamina. Were it not for this quality of the arteries, the heart would be called upon to do a great deal of unnecessary work which would absorb a vast amount of valuable energy, while the flow of blood throughout the arterial system would be intermittent, as the heart at each beat would be required, in order to complete its systole, to drive the whole volume of blood forward and through the capillaries-a condition obviously incompatible with normal physiology in the body.

Thus during each systole the heart muscle performs work in maintaining arterial flow against peripheral resistance. A large part of this work, which is sudden and explosive (at systole), is compensated for by the expansion of the arterial walls.

A part of the manifest energy of the heart thus becomes for a time potential in the stretched fibers of the arterial wall. The moment that a systole is at an end, the stretched fibers recoil and continue the work of the heart in maintaining the arterial flow against peripheral resistance.

As this potential energy becomes expended the systolic pressure gradually falls and would eventually reach zero were it not for peripheral resistance plus the rhythmically recurring cardiac systole which causes the pressure to rise again.

The laminal elasticity of the vessels is very perfect and is capable of standing a pressure greater than by any chance could possibly be developed during life. According to Janeway,' quoting Gréhant and Quinguard, the carotid artery of a dog is capable of withstanding a pressure, 1 Loc. cit., p. 24.

twenty times greater than the normal blood-pressure without tearing. For the human carotid the lowest pressure at which rupture occurs is 1.29 meters of mercury, at least eight times the ordinary carotid pressure of the normal circulating blood.

4. Total Volume of the Blood.-Compared with the capacity of the arteries, capillaries and veins combined, the total volume of blood is surprisingly small, being variously estimated to be from one-twelfth to one-fifteenth of the body weight. In the normal individual the cubic capacity of the vascular system is so reduced by vasomotor control (see page 30) that the blood at all times is maintained under a considerable pressure.

While a certain amount of blood, probably about threefourths that of the normal volume, is necessary to support the circulation, still it has been found that a large amount of blood can be withdrawn without interrupting cardiac action (see Venesection, page 460) and further that the pressure rapidly returns to a point at or near normal soon after the cessation of the hemorrhage. On the other hand, Worm Müller has shown that an amount of fluid greater than the total blood volume of the body can be transfused into the vessels, without increasing the blood-pressure above a point frequently reached under normal conditions. Therefore, it would seem that, except for great changes, the volume of the circulating blood has only a slight and temporary influence on normal blood-pressure.

5. The Viscosity of the Blood.-This factor has until recently been almost entirely ignored in observations and discussions of the normal and pathologic variations in

blood-pressure. It is, however, evidently a most important factor in determining peripheral resistance. This has been brought out by Allbutt1 who states "that it is evident that friction in the arterial tree must multiply with every increase in viscosity as it has been demonstrated that nearly 200 times more of the heart's energy is expended in overcoming friction than is required in maintaining the velocity of the blood-stream." Variation in this factor, even when slight, must affect enormously the resistance offered to the passage of blood through the arterial system and therefore must profoundly affect the bloodpressure, so that we should follow most carefully any variation however small, in the viscosity of the circulating fluid. excess corp.

Determann has found in plethoric persons, with high blood-pressure an increase in viscosity, while A. Martinet has shown that in normal cardiovascular conditions the blood-pressure parallels the viscosity. It will probably be found, as experimentation is carried further, that the viscosity of the blood is a most important factor in affecting blood-pressure, and that the development of methods for its modification or control will mark an epoch in the study and treatment of diseases involving blood-pressure changes.

When the viscosity is normal or below, as in renal arteriosclerosis for example, the blood-pressure will be high. On the other hand, if the viscosity is high, while the blood-pressure is normal or below, the pulse will be weak. Martinet has endeavored to build up a clinical picture upon this basis, in which such symptoms as cold hands and

1 Clifford Allbutt, Quarterly Jour. Med., 1910, p. 242.

feet, congestion of the liver, varices and various cardiovascular disturbances predominate.

The Pulse. From our knowledge of the action of the heart, we know that blood is forced into the aorta at regular intervals, and that each charge of blood entering the aorta is felt throughout the arterial system in the form of a wave which is styled the pulse and which may be felt as a rhythmically recurring impulse (due to transitory increase in size of the vessel) in all palpable arteries.

The propagation of this wave throughout the arterial system implies a change in diameter of the vessel with a resulting stretching of the vessel wall (see Elasticity, page 45) caused by the increased increment of blood entering it. This further stretching of an already stretched vessel wall can occur only through an increase in pressure within the vessel sufficient to cause the increased diameter of the vessel which is left under the finger. It is a self-evident fact, then that there occurs alternately, in regular rhythmic cycle, a rise and fall in blood-pressure throughout the arterial system. Corresponding to the ventricular systole and diastole, the highest and lowest point of this change in pressure are termed respectively, systolic blood-pressure and diastolic blood-pressure (See Fig. 2).