The cause of the blood's motion is simply a difference in the pressure within the various parts of the vascular system, for the heart acts as the pump filling the tube represented by the large elastic arteries, which can be more or less distended, according as (1) the outflow is impeded or facilitated by the contraction or relaxation of the muscular arterioles which form the outlet, or as (2) the inflow is increased or diminished by the greater or less activity of the heart's action.
From the foregoing facts, and what has been said of the direction of the blood current, namely, that it flows from the arteries through the capillaries into the veins, it would appear that the pressure in the arteries exceeds that in the capillaries, and that in the capillaries must in turn be greater than that in the veins, the blood flowing in the direction in which the pressure becomes less.
The different manner in which blood flows from a. cut artery and a cut vein shows that a great difference exists in the pressure within the two sets of vessels.
When a small artery is cut and the orifice directed upward the blood spurts out two or three feet, in jerks. When a vein is cut, the blood only trickles gently from the orifice, the force of the flow depending much upon the position of the part. It is well to remember that bleeding from a vein in the leg or arm can be stopped by placing the limb in a position more elevated than the rest of the body, so as to prevent the force of gravity from acting on the blood.
By means of a special form of gauge (the mercurial manometer), - which will presently be described - the exact difference in the pressure exerted by the blood against the vessel walls in the different parts of the circulation can be accurately estimated, and it has been found by direct experiment that the pressure varies, just as one would be led to expect from a consideration of its physical relationships, namely, with the direction and rate of the current and the varying width of the bed in which it flows.
The fall in pressure observed in the vessels when passing from the left ventricle to the right auricle is not even. In the arterioles it falls suddenly, and a great difference always exists between the arterial and venous pressure (p. 299). It is on account of the permanent high pressure in the arteries and comparatively low pressure in the capillaries and veins, that there is a continuous and permanent flow through the capillaries from arteries to veins.