the sphygmograph reveals respiratory oscillations; the whole line
of the tracing falls during the first part of inspiration and rises subsequently.
The circulation in the capillaries may be studied by placing under the microscope a transparent membrane such as the web of the frog’s foot, tail of tadpole, wing of bat, &c. By a special illumination one may see the shadow of the blood corpuscles moving through the retinal vessels of one’s own eye, The capilliary circulation.and even calculate the velocity of flow The diameter of the smaller capillaries is such as to permit the passage of the red blood corpuscles in single file only; their length is about 150th of an inch. The endothelial cells confine the blood from direct contact with the tissue lymph and so prevent its coagulation, but allow and regulate the exchange of material between the blood and lymph. This exchange is regulated by the vital activity of the cells, and does not follow such laws as pertain to filtration and diffusion through dead membranes. There is evidence to show that the cells of the hepatic capillaries are capable of protoplasmic movement and of phagocytosis. The pressure in the capillaries stands inclosingr relationship to that in the veins than to that in the arteries; for example, a rise of pressure in the venae cavae, other things remaining the same, raises the pressure in the hepatic capillaries to a like amount, while a rise of pressure in the aorta does not, for most of the arterial pressure is spent in overcoming the peripheral resistance. The filling of the capillaries in the skin varies greatly with temperature, posture, &c. When the hand is cold the arterioles are so constricted that blood only passes through the wider and more direct capillaries. As the skin becomes warm it flushes, the arterioles dilating and all the capillary networks becoming filled with blood. Muscular movements express the blood out of the capillaries, as may be seen by the blanching of the skin which occurs on clenching the hand. Raising the hand blanches, and lowering it congests the capillaries. The pressure and velocity in the capillaries thus constantly vary, owing to alterations in hydrostatic pressure, the pressure of the body against external objects, the contraction of the muscles, and the contraction of the arterioles. It is not possible therefore to set any definite figure to the capillary pressure or velocity. In the frog's web, with the foot confined and at rest, the velocity is about 1 mm. per second. We continually make slight movements to counteract the hydrostatic effect and prevent the congestion of blood in the capillaries of lower parts of the body. It is this tendency to congestion which makes it so difficult to stand absolutely motionless for any length of time. The red corpuscles, being the heavier, occupy the axis, and the white corpuscles the peripheral layer of the capillary stream. If an irritant is placed on the membrane it will be observed that the capillaries become wider and crowded with corpuscles, the flow slackening and finally becoming arrested owing to the assing out of the plasma through the damaged capillary wall. 'lphe white corpuscles creep out between the endothelial cells into the tissues. Such are the first phenomena of inflammation. Af ter obstruction of an artery collateral pathways are in most parts rapidly formed, for the anastomatic capillaries, stimulated by the increased blood flow, develop into arterioles and arteries.
Numerous anastomoses exist between the veins, so that if the flow of blood be obstructed in one direction it readily finds a passage in another. Muscular movement, alterations of posture and in the respiratory movements particularly forward the venous circulation. The barber’s pole of the barber surgeon was The flow in the veins. grasped to increase the flow in the old blood-letting days. The valves in the veins allow the blood to be forced only towards the heart. The pressure in the veins varies according to the hydrostatic pressure of the blood column above the point of measurement. In the horizontal position, when this factor is almost eliminated, the pressure in the large veins is about equal to 5–10 mm. of mercury, and even may become negative on taking a deep inspiration. There thus arises the danger of air being sucked into a wounded jugular vein. If air does thus gain entry it may fatally obstruct the circulation.
The venous circulation is impeded by (I) a lessening of heart power, (2) valvular defects, such as incompetence or narrowin of the orifice which they guard, (3) obstruction to the filling of the heart, as in cases of pericardia effusion, (4) obstruction of the pulmonary circulation as in coughing, by pleuritic effusion, &c. The results of venous congestion are a less efficient arterial circulation, a dusky appearance of the skin, a fall of cutaneous temperature, and an effusion of fluid into the tissue spaces producing oedema and dropsy. This last effect is not due to increased capillary pressure producing increased transudation as has been supposed, for no such increase in venous and capillary pressure persists under the conditions. It is due to the altered nutrition of the capillary endothelium and the tissues, which results from the deficient circulation.
If for any reason the left ventricle fail to maintain its full systolic output, it ceases to receive the full auricular input, and in consequence the pulmonary vessels congest. This tells back on the right heart, and the right ventricle is unable to empty itself into the congested pulmonary vessels, and this in its turn leads to venous congestion. The final result of any obstruction thus is a pooling of the blood in the venous cistern. Dyspnoea re'=l11ts'from cardiac insufficiency. veins. It is excited by the increased venosity of the blood acting- on the respiratory centre. Both excess of carbon dioxide and deficiency of oxygen excite this centre. The increased respiratory movements aid the circulation., ,
The venous side of the vascular system, owing to the great size of the veins, has a large potential capacity, while many of the capillaries in each organ are empty and collapsed, except at those periods of vaso-dilatation and hyperaemia which accompany extreme activity of function. The vascular system cannot be regarded as a closed system, for the blood-plasma, whenever the capillary pressure is increased, transudes through the capillary wall into the tissue-spaces and enters the lymphatics. Thus, if fluid be transfused into the circulatory system, it not only collects in the capacious reservoirs of the veins and capillaries-especially in the lungs, liver and abdominal organs—but leaks into the tissue-spaces. Hence the pressure in the vascular system cannot be raised above the normal for any length of time by the injection of even enormous quantities of fluid. The lymphatics of tissue-spaces must be regarded as art of the vascular system. There is a constant give and take between the blood-plasma and the tissue lymph. If the fluid part of the blood be increased, then the capillary transudation becomes greater, and the excess of fluid is excreted from the kidneys and glands of the alimentary canal. If the fluid part of the blood diminish, then fluid passes from the tissue-spaces into the Haemorrhage and transfusion. blood, and the sensation of thirst arises, and more drink is taken. The circulation may be greatly aided by the transfusion of salt solution (0.8 %) or blood after severe hemorrhage, or in states of surgical shock. Only the blood of man must be used. The direct giving of blood by connecting the radial artery of a relation to the median vein of a patient has been used as a means of effecting restoration. Blood may be withdrawn from the system slowly to the extent of 4 %, rapidly to the extent of 2 % of the body weight, without lowering the arterial pressure, owing to the compensatory contraction of the arterioles and the rapid absorption of fluid from the tissues into the blood. The withdrawal of the tissue-lymph excites extreme thirst and the great need for water which occurs after severe hemorrhage. About 75 % by weight of the tissues, excluding fat and bone, consists of water. The quantity of blood in the body is about 120th of the body weight. That of tissue-lymph is unknown, but it must be considerable, probably greater than that of the blood; The lymphatics drain off the excess of fluid which transudes from the capillaries, and finally return it to the vascular system. The interchange between tissue, blood and lymph depends on the forces of the living cells, which are as yet far from complete elucidation.
We may define the velocity of the blood at any point in a vessel as the length of the column of blood flowing by that point in a second. In the case of a tube, supplied by a constant head of pressure, we can divide the tube and measure the outflow per second; knowing the volume of this,The velocity of blood flow. and the cross area of the artery, we can determine the length of the column. This kind of experiment cannot be done on the living animal, because the opening of the vessel alters the resistance to flow, and the loss of blood also changes the physiological conditions. To determine the velocity other means must be devised. Ludwig invented an instrument called the stromuhr, consisting of two bulbs mounted on a rotating platform pierced with two holes. One bulb is filled with oil—the other with blood. The bulbs are connected together by a tube at their upper end, and the lower end of the one full of oil is brought over the hole in the platform. The central end of the artery is connected to the same hole and the peripheral end to the other, over which stands the bulb full of blood. The blood being allowed to flow displaces the oil out of the one bulb into the other; directly this happens, the bulbs are rotated and the one full of oil is again brought over the central end of the artery. The number of rotations per minute is counted, and the volume of the bulb being known we obtain the volume of blood that passes through the instrument per minute. In another instrument, the haemodromograph of Chauveau, there is inserted into the artery a ┴ tube in which hangs a small pendulum; the stem of the pendulum passing through a rubber dam which closes the vertical limb of the tube. The pendulum is defected by the flow, and the eater the velocity the greater the deflection. The deflection can be recorded by connecting the free end of the pendulum to a tambour arrangement. This instrument allows us to record and measure the variations of velocity during systole and diastole, of the heart, but it can only be used in the vessels of large animals. Still other methods have been employed by Cybulski and Stewart. The general relations of the velocity of the blood in the arteries, capillaries and veins is expressed by the curve shown in fig. 26. The velocity in the large arteries may reach 500 mm. per second
Fig. 25.—Ludwig’s Stromuhr.
