The explanation of the respiratory undulations in the tracing of the blood pressure is difficult. Though many causes have been assigned, no single one appears to explain adequately all the changes that may occur in this phenomenon. At first sight the respiratory movements and consequent pressure changes within the thorax would seem to give a simple mechanical explanation 0f the variation in pressure. But if the change occurring in the intra-thoracic pressure be examined carefully, it will be found not to correspond exactly with the so-called respiratory wave of the pressure curve in the arterial system.
Fig. 136. Fick's Spring Manometer.
A hollow C-shaped spring (A), made of extremely thin metal, is fixed at (bb), where its cavity communicates with the tube (K). The top of the C is connected at (a) with the writing lever. Any increase of pressure in the tube (K) causes the spring to expand and move the writing point (G) up and down.
The amount of pressure exercised on the pericardial contents by the lungs varies with the respiratory movements. It is slightly decreased during inspiration and increased during expiration. The differences thus produced, however, are during ordinary respiration very slight (probably 1 mm., mercury). So slight a variation as 1 mm., mercury, cannot, by direct action on the aortic arch, cause the change of several millimetres which we see in the respiratory undulation in the arterial pressure. We must, therefore, seek the explanation in the changes it causes in the great veins.
Fig. 137. Tracing of blood pressure taken with Fick's manometer.
Owing to the lungs being very elastic and constantly tending to shrink away from the costal pleura, the pressure in the pleural cavity is less than that of the atmosphere which distends the lungs, i. e., the pleural pressure is negative. All the viscera in the thoracic cavity are habitually under the influence of the negative pressure. Thus the elastic lungs exert a kind of traction on the pericardium, and tend to cause a negative pressure within the heart and great systemic vessels, both arteries and veins. The influence is more felt by the thin-walled venae cavae in which the blood pressure is low than in the thick-walled arteries where it is high.
Fig. 138. Blood pressure and Respiratory Tracings recorded synchronously - recording surface moving from right to left - showing that the variations in pressure in the arteries (continuous line) and in the thoracic cavity (dotted line) do not exactly correspond, the latter continuing to fall after the blood pressure has commenced to rise.
The flow of blood into the left auricle from the pulmonary vessels is not influenced by the negative pressure, as pressure of the atmosphere cannot reach them.
It has been suggested that by facilitating the flow into the thorax from the great veins, the amount of blood entering the right auricle during inspiration may be increased, and thus the left ventricles may be better filled and made to beat more actively, so as to cause an elevation in the arterial pressure. The sequence of events may be read as follows. During inspiration the negative pressure on the right heart is increased; the atmospheric pressure acting on the tributaries of the superior vena cava is unchanged, while the pressure in the abdominal cavity is increased, and the inferior vena cava compressed by the muscular action. The blood thus flows more readily into the right heart, and consequently the lungs receive a larger supply of blood during this period. In expiration, on the other hand, the intra-thoracic pressure becomes less negative, the compression of the abdominal viscera is relieved, and the flow into the auricle loses somewhat in force.
But this view appears to leave the pulmonary circulation out of the question in a way hardly justifiable, since the lungs must be traversed by the blood before the increased inspiratory inflow to the right auricle can affect the left ventricle or the systemic arteries.
It must be carefully borne in mind that the left side of the heart works under different conditions, for the variations of pressure affect both the pulmonary veins and the left auricle similarly, since they are both included in the thoracic cavity, and are both subjected to a slightly varying negative pressure. The aid given to the flow into the right heart by the low intra-thorafcic pressure is quite absent on the left side, as the inflow is not assisted by atmospheric pressure; so that the thoracic movements do not exert any influence on the flow of blood from the pulmonary veins to the systemic arteries. While inspiration is taking place, the lungs receive a larger supply of blood. From the relatively small amount of blood in these organs it is probable that this slight excess has little or no influence on the amount entering the left side of the heart. The left ventricle may receive an amount of blood during expiration slightly in excess of that which it receives during inspiration. This can have but little direct effect on the pressure in the great arterial trunks.
It is more than probable that excess of blood in the heart cavities does not mechanically influence the beat or the blood pressure, but rather acts as a nervous stimulus, and excites the inhibitory centre of the heart and the depressor centres which control the arterioles.
The rejection of this indirect mechanical explanation appears necessary from the following facts: -
1. The rise in pressure is not exactly synchronous with expiration or inspiration.
2. The heart beats more slowly during expiration than inspiration.
3. This difference at once disappears if the vagi be cut and the respiratory wave becomes greatly modified.
4. Variations in the pressure like the respiratory wave occur after the respiratory movements have quite ceased.
5. The respiratory wave is observed when artificial respiration is employed, in which the forcing of air into the lungs is the cause, and not the result of the thoracic movements, so that the pressure effects are reversed.
We may conclude that a sympathy in action can be recognized in the working of the respiratory, vascular and cardiac nerve mechanisms.
The undulations known as Traube's Curves occurring in cura-rized animals when no respiratory movements are performed, have been explained by referring them to a stimulation by impure blood of the vasomotor centre, which by rhythmical impulses increases the contraction of the arterioles and causes a rhythmical variation in the blood pressure. This explanation when applied to the respiratory waves seems to be rendered unsatisfactory by the fact that these undulations go on even when the arterioles are cut off from their chief nerve centres by sections of the spinal cord. So that if these undulations are to be referred to nerve mechanism we are ignorant of the course 26 taken by the nerve impulses, for any rhythmical sympathy existing between the respiratory and vasomotor nerve centres in the medulla cannot well influence the vessels when the cord is cut.
Thus we seem forced to fall back upon the muscular coats of the arteries for an explanation of the respiratory variation in the blood pressure, and to accord to this tissue automatic rhythmical contractility.
The blood pressure in the capillaries cannot be directly measured by the means above described; it is difficult to estimate, and very variable. The slightest change of pressure in the corresponding veins or arteries causes the pressure in the capillaries to rise or fall. Thus, variations in pressure are constantly occurring in the capillaries, which cause an alteration in the rate of flow, or even a retrograde stream in some parts of the network.
The regulation of the blood supply, and, therefore, of the pressure in the capillaries, is under the control of the small arterioles which supply them; a slight relaxation of the muscle of the arterioles causes great increase in the amount of blood flowing through the capillaries, as can readily be seen with the microscope.
The blood pressure in the veins must be less than that in the capillaries, and, as has been said, must diminish as the heart is approached, where in the great veins (superior cava) the pressure is said to be rather below that of the atmosphere ( - 3 to - 5 mm., mercury). During inspiration the minus pressure may become further lowered, while, on the other hand, it is only by very forced expiration that it ever becomes equal to or at all above that of the atmosphere.
. This is a most important fact, as the suction considerably helps the flow of blood from the veins, and also the current of fluid from the thoracic duct that bears the chyle from the intestines and the fluid collected from the tissue drainage back to the blood.
The pressure of the blood in the veins may be said to be generally nil, since the veins are nowhere overfilled with blood.
The pressure, on the other hand, that can be registered and measured depends upon forces communicated from without, namely: (1) gravity; (2) the elastic pressure of the surrounding tissue; and (3) the pressure exerted by the muscle during contraction. This pressure is increased by any circumstance which impedes the flow of blood through the right side of the heart, through any large vein, or through the pulmonary circulation; but when no abnormal obstacle exists in the venous blood current the pressure in those vessels can never attain any great height, for, as we have seen, the large trunks are constantly being emptied by the heart's action.
Most circumstances which tend to lower arterial pressure also tend to raise the pressure in the veins, so that when the heart's action is weak or its mechanism faulty the venous pressure rises.
In the veins of the extremities the pressure greatly depends on the position of the limb, as it varies almost directly with the effect of gravity.
In the pulmonary circulation the direct measurement of the intra-vascular pressure is rendered extremely difficult, and possibly erroneous, by the fact that to ascertain it the thorax has to be opened. It has been found in the pulmonary artery to be in a dog 29.6 mm., in a cat 17.6 mm., and in a rabbit 12 mm. of mercury.