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when dry fixes the trace, which thereafter will not be spoiled by handling. In Figure 16 the uppermost line shows a trace which fairly represents the successive fluctuations of the aortic pressure of the dog. The longer and ampler fluctuations are respiratory, the briefer and slighter are cardiac. In each respiratory curve the lowest point and the succeeding ascent coincide with inspiration; the highest point and the succeeding descent with expiration. The horizontal middle line is the base line, representing the pressure of the atmosphere. The base-line has been shifted upward in the figure simply in order to save room on the page. In the lowermost line the successive spaces from left to right of the reader represent successive intervals of time of two seconds each, written by an electro-magnetic chronograph. The pressure-trace taken from a vein may in certain regions near the chest show respiratory fluctuations, but nowhere cardiac ones, as the pulse is not transmitted to the veins. The venous pressure is so small, that for the practical study of it a recording manometer must be used in which some lighter fluid replaces the mercury, which would give a column of insufficient height for working purposes. The values obtained are then reduced by calculation to millimeters of mercury, for comparison with the arterial pressure. The intravascular pressure at a given moment can be measured by measuring a vertical line or "ordinate" drawn from the curve written by the manometer to the horizontal base-line. The latter represents the height of the manometric column when just disconnected from the blood-vessel; that is, when acted upon only by the weight of the atmosphere and of the solution of sodium carbonate. To ascertain the bloodpressure, the length of the line thus measured must be doubled; because the mercury in the proximal limb of the manometer sinks under the blood-pressure exactly as much as the float rises in the distal limb. A small correction must also be made for the weight of the solution of sodium carbonate.

The Mean Pressure.-The " mean pressure" is the average pressure during whatever length of time the observer chooses. The mean pressure for the given time is ascertained from the manometric trace by measurements too complicated to be explained here. As the weight and consequent inertia of the mercury cause it to fluctuate according to circumstances more or less than the pressure, the mean pressure is much more accurately obtained from the mercurial manometer than is the true height of each fluctuation, which is very commonly written too small. Therefore, it is especially the mean pressure that is studied by means of the mercurial manometer. The true extent and finer characters of the single fluctuations caused by the heart's beat are better studied with other instruments, as we shall see in dealing with the pulse.

It has been seen that the blood flows continuously through the capillaries because the pressure is continually high in the arteries and low in the veins. The reader is now in position to understand statements of the blood-pressure expressed in millimeters of mercury. The mean aortic pressure in the dog is far from being always the same even in the same animal. We have found it, in the case referred to on page 85, to be equivalent to about 121 millimeters of mercury. It will very commonly be found higher than this, and may range

up to, or above, 200 millimeters. In man it is probably higher than in the dog. The pressure in the other arteries derived from the aorta which have. been studied manometrically is not very greatly lower than in that vessel. In the pulmonary arteries the pressure is probably much lower than in the aortic system. The pressure in the small veins of the head of the dog, the cannula being in the distal stump of the external jugular vein, we have found already in one case to equal about 14 millimeters of mercury. In such a case the presence of valves in the veins and other elements of difficulty make the mean pressure hard to obtain as opposed to the maximum pressure during the period of observation.

If a cannula be so inserted as to transmit the pressure obtaining within the great veins of the neck just at the entrance of the chest, without interfering with the movement of the blood through them, and if a manometer be connected with this cannula, the fluid will fall below the zero-point in the distal limb, indicating a slight suction from within the vein, and thus a slightly "negative" pressure. This negative pressure may sometimes become more pronounced during inspiration and regain its former value during expiration. Sometimes, again, the pressure during expiration may become positive. The continuous flow from the great arteries through the capillaries to the veins, and through these to the auricle, is therefore shown by careful quantitative methods, no less than by the tube of Hales, to be simply a case of movement of a fluid from seats of high to seats of lower pressure.

The Symptoms of Bleeding in Relation to Blood-pressure. The dif ferences of pressure revealed scientifically by the manometer exhibit themselves in a very important practical way when blood-vessels are wounded and bleeding occurs. If an artery be cleanly cut, the high pressure within drives out the blood in a long jet, the length of which varies rhythmically with the cardiac pulse, but varies only to a moderate degree. From wounded capillaries, or from a wounded vein, owing to the low pressure, the blood does not spring in a jet, but simply flows out over the surface and drips away without pulsation. At the root of the neck, where the venous pressure may rhythmically fall below and rise above the atmospheric pressure, the bleeding from a wounded vein may be intermittent.

D. THE CAUSES OF THE PRESSURE IN THE ARTERIES, CAPILLARIES, AND VEINS.

The causes of the continuous high pressure in the arteries must first engage our attention.

Resistance. The great ramification of the arterial system at a distance from the heart culminates in the formation of the countless arterioles on the confines of the capillary system. We have already seen direct evidence of the friction in the minute vessels which results from this enormous subdivision of the blood-path. The force resulting from this friction is propagated back

1 H. Jacobson: "Ueber die Blutbewegung in den Venen," Reichert's und du Bois-Reymond's Archiv für Anatomie, Physiologie, etc., 1867, S. 224.

ward according to the laws of fluid pressure, and constitutes a strong resistance to the onward movement of the blood out of the heart itself. Friction is everywhere present in the vessels, but is greatest in the very small ones collectively.

Power. Where the aorta springs from the heart, the rhythmic contractions of the left ventricle force open the arterial valve, and force intermittent charges of blood into the arterial system, overcoming thus the opposing force derived from friction. The wall of the arterial system is very elastic everywhere. Thus the high pressure in the arteries results from the interaction of the power derived from the heart's beat and the resistance derived from friction. That the high pressure is continuous depends upon the capacity for distention possessed by the elastic arterial wall.

Balance of the Factors of the Arterial Pressure.-In order to study the causation of the arterial pressure, let us imagine that it has for some reason sunk very low; but that, at the moment of observation, a normally beating heart is injecting a normal blood-charge into the aorta. The first injection would find the resistance of friction present, and the elastic arterial wall but little distended. For this injection some room would be made by the displacement of blood into the capillaries. But it would be easier for the arterial wall to yield than for the friction to be overcome, so the injected blood would largely be stored within the arterial system and thus raise the pressure. Succeeding injections would have similar results; it would continue. to be easier for the injected blood to distend the arteries than to escape from them; and the arterial pressure would rise rapidly toward its normal height. Presently, however, a limit would be reached; a time would come when the elastic wall, already well stretched, would have become tenser and stiffer and would yield less readily before the entering blood; and now a larger part than before of each successive charge of blood would be accommodated by the displacement of an equivalent quantity into the capillaries, and a smaller part by the yielding of the arterial wall. Normal conditions of pressure would be reached and maintained when the blood accommodated, during each systole of the ventricle, by the yielding of the arterial wall should exactly equal in amount the blood discharged from the arteries into the capillaries during each ventricular diastole; for then the quantity of blood parted with by the arteries during both the systole and the diastole of the heart would be exactly the same as that received during its systole alone.

We see that, at each cardiac systole, the cardiac muscle does work in maintaining the capillary flow against friction, and also does work upon the arterial wall in expanding it. A portion of the manifest energy of the heart's beat thus becomes potential in the stretched elastic fibres of the artery. The moment that the work of expansion ceases, the stretched elastic fibres recoil; their potential energy, just received from the heart, becomes manifest, and work is done in maintaining the capillary flow against friction during the repose of the cardiac muscle. At the beginning of this repose the arterial valves have been closed by the arterial recoil. When, at each cardiac systole,

the arterial wall expands before the entering blood, the pressure rises, for more blood is entering the arterial system than is leaving it; when, at each cardiac diastole, the arterial wall recoils, the pressure falls, for blood is leaving the arterial system, and none is entering it. But before the fall has had timeto become pronounced, while the arterial pressure is still high, the cardiac systole recurs, and the pressure rises again, as at the preceding fluctuation.

The Arterial Pulse. The increased arterial pressure and amplitude at the cardiac systole, followed by diminished pressure and amplitude at the cardiac diastole, constitute the main phenomena of the arterial pulse. They are marked in the manometric trace by those lesser rhythmic fluctuations of the mercury which correspond with the heart-beats. The causes of the arterial pulse have just been indicated in dealing with the causes of the arterial pressure. The pulse, in some of its details, will be studied further for itself in a later chapter. For the sake of simplicity, the respiratory fluctuations of the arterial pressure have not been dealt with in the discussion just concluded. The causes of these important fluctuations are very complex and are treated of under the head of Respiration.

The arterial pressure, then, results from the volume and frequency of the injections of blood made by the heart's contraction; from the friction in the vessels; and from the elasticity of the arterial wall.

The Capillary Pressure and its Causes.-When we studied the movement of the blood in the capillaries, we found the pressure in them to be low and free from rhythmic fluctuations. In both of these qualities the capillary pressure is in sharp contrast with the arterial. What is the reason of the difference? The work of driving the blood through as well as into the capillaries is done during the contraction of the heart's wall by its kinetic energy. During the repose of the heart's wall and the arterial recoil this work is continued by kinetic energy derived, as we have seen, from the preceding cardiac contraction. The work of producing the capillary flow is done in overcoming the resistance of friction. The capillary walls are elastic. The same three factors, thenthe power of the heart, the resistance of friction, the elasticity of the wallwhich produce the arterial pressure produce the capillary pressure also. Why is the capillary pressure normally low and pulseless? The answer is not difficult. The friction which must be overcome in order to propel the blood out of the capillaries into the wider venous branches is only a part of the total friction which opposes the admission of the blood to the minuter vessels. The resistance is therefore diminished which the blood has yet to encounter after it has actually entered the capillaries. The force which propels the blood through the capillaries, although amply sufficient, is greatly less than the force which propels it into and through the larger arteries. In both cases alike the force is that of the heart's beat. But, in overcoming the friction which resists the entrance of the blood into the capillaries, a large amount of the kinetic energy derived from the heart has become converted into heat. The power is therefore diminished. As, in producing the high arterial pressure, much power is met by much resistance, and the elastic wall

is, therefore, distended with accumulated blood; so, in producing the low capillary pressure, diminished power is met by diminished resistance, outflow is relatively easy, accumulation is slight, and the elasticity of the delicate wall is but little called upon.

The Extinction of the Arterial Pulse.-But why is the capillary pressure pulseless, as the microscope shows? To explain this, no new factors need discussion, but only the adjustment of the arterial elasticity to the intermittent injections from the heart and to the total friction which opposes the admission of blood to the capillaries. This adjustment is such that the reçoil of the arteries displaces blood into the capillaries during the ventricular diastole at exactly the same rate as that produced by the ventricular contraction during the ventricular systole. Thus, through the elasticity of the arteries, the cardiac pulse undergoes extinction; and this becomes complete at the confines of the capillaries. The respiratory fluctuations become extinguished also, and the movement of the blood in the capillaries exhibits no rhythmic changes. This conversion of an intermittent flow into one not merely continuous but approximately constant affords a constant blood-supply to the tissues, at the same time that the cardiac muscle can have its diastolic repose, and the ventricular cavities the necessary opportunities to receive from the veins the blood which is to be transferred to the arteries.

A simple experiment will illustrate the foregoing. Let a long india-rubber tube be taken, the wall of which is thin and very elastic. Tie into one end of the tube a short bit of glass tubing ending in a fine nozzle, the friction at which will cause great resistance to any outflow through it. Tie into the other end of the rubber tube an ordinary syringe-bulb of india-rubber, with valves. Expel the air, and inject water into the tube from the valved bulb by alternately squeezing the latter and allowing it to expand and be filled from a basin. The rubber tube will swell and pulsate, but if its elasticity have the right relation to the size of the fine glass nozzle and to the amplitude and frequency of the strokes of the syringe, a continuous and uniform jet will be delivered from the nozzle, while the injections of water will, of course, be intermittent.

The Venous Pressure and its Causes.-The pressure in the peripheral veins is less than in the capillaries and declines as the blood reaches the larger veins. Very close to the chest the pressure is below the pressure of the atmosphere, and may sometimes vary from negative to positive, following the rhythm of the breathing. These respiratory fluctuations will be considered later. The low and declining pressures under which the blood moves through the venules and the larger veins are due to the same causes as those which account for the capillary pressure. It is still the force generated by the heart's contractions, and made uniform by the elastic arteries, which drives the blood into and through the veins back to the very heart itself. As the blood moves through the veins, what resistance it encounters is still that of the friction ahead. But the friction ahead is progressively less; the conversion of kinetic energy into heat is progressively greater. The venous wall possesses elas

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