If the different parts of the circulation be represented on the base line H. A. C. v., these letters corresponding to heart, arteries, capillaries, and veins respectively, and if the height of the blood pressure be represented on the vertical line in mm. Hg., the curve h, a, c, v, would give about the relative pressure in the various parts of the circulation. This shows that in the receiving chamber of the heart the pressure is negative, while the

regions of the vessels.

H. Heart. a. Arterioles. v. Small Veins. A. Arteries. c. Capillaries. V. Large Veins. H. V. being the zero line (atmospheric pressure), the pressure is indicated by the height of the curve. The numbers on the left give the pressure (approximately) in mm. of

mercury.

ventricular pump drives it to the height of the arterial pressure 160 mm. Hg. In the arteries the pressure is everywhere high, while just before the blood reaches the capillaries a sudden fall occurs. The variation after this is merely a gentle descent until the large venous trunks are reached, where the blood pressure is below zero, i. e., below the pressure of the atmosphere.

From a purely physical point of view the ventricle may be

regarded as pumping the blood up to an elevated high-pressure reservoir of small capacity (the arteries), from which it rapidly falls by numerous outlets into an expansive, low-lying irrigation basin (the wide capillaries), while it slowly trickles back to the well (the auricle), which lies below the surface pressure.

From this diagram the following points can be gathered :

1. The great difference between the pressure on the arterial and venous sides of the circulation.

2. The comparatively slight difference in pressure in the different parts of the arterial or of the venous systems respectively.

3. The suddenness of the fall in the pressure between the small arteries and the capillaries, where the great resistance to the outflow is met with.

4. In the large veins the pressure of the blood is habitually below that of the atmosphere, only becoming positive during forced expirations.

VARIATIONS IN THE BLOOD PRESSURE.

If the blood pressure be recorded with Ludwig's Kymograph, a tracing will be obtained which shows that the pressure undergoes periodic elevations and depressions of two different kinds. The smaller oscillations are found to correspond with the heart beat, the larger waves have the same rhythm as the respiratory movements, and the average elevation of the mercurial column is spoken of as the mean pressure. In the large arteries of the warm-blooded animals this mean pressure varies with the size of the animal from 90 mm., mercury, to more than 200 mm. In cold-blooded animals it is comparatively low, from 22 mm. in the frog (Volkmann) to 84 mm. in a large fish.

The general mean pressure in the arteries is increased by (1), increased action of the heart; (2), increased contraction of the muscular coat of the arteries; (3), sudden increase in the quantity of blood. When the change is gradual, the vessels adapt themselves to the increase.

The opposite of these conditions may be said to have a contrary effect.

The character of the change in pressure which accompanies the heart's systole is not shown exactly in the tracing obtained by the mercurial manometer, owing to the sluggishness of the movement of the mercurial column, which, as it were, rubs off the apices of the curves. But with the spring manometer of Fick, the details of these oscillations are marked. They are of course synchronous with the arterial pulse, and follow the variations of

Blood-pressure Curve, drawn by mercurial manometer. O-x-zero line, y-y-curve with large respiratory waves and small waves of heart impulse. A scale is introduced to show height of pressure.

INFLUENCE OF RESPIRATION ON BLOOD PRESSURE.

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

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.

FIG. 137.

the thorax would seem to give a simple mechanical explanation of 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

Tracing of blood pressure taken with Fick's manometer.

system.

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 I 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.

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

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.

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 venæ cavæ in which the blood pressure is low than in the thick-walled arteries where it is high.

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.