A liquid flows through a tube as the result of a difference of pressure in the different parts of the tube. The liquid moves from the part where the pressure is higher toward that where it is lower, except where sudden and great variations of calibre occur.*

The energy of the flow corresponds with the amount of difference in the pressure, and varies in proportion to it, being continuous so long as the pressure is unequal in different parts, and ceasing when it is equalized throughout the tube.

* Although in the whole course of any system of tubes the flow of liquid must take place from the part of higher to that of lower pressure, yet if a narrow tube open abruptly into one the diameter of which for a short length is much greater, the diminution of velocity in the wide tube may cause the local pressure in it to exceed that in the narrower tube immediately preceding; so that the liquid would be actually flowing, for a short distance, from a point of lower to a point of higher pressure.

If liquid be forcibly pumped into one extremity of a long tube, such as a garden hose, a pressure difference is established, the pressure becoming greater at the end into which the liquid is pumped, a current consequently takes place toward the open end. So long as the free or distal end of the tube is quite open and on the same level as the rest, no very great pressure can be brought to bear on the walls of the tube, no matter how forcibly the pumping may go on, as the liquid easily escapes, and therefore flows the more quickly as the pumping becomes more energetic. If, however, the outflow be impeded by raising the distal end of the tube to any considerable height, or by partially closing the orifice with a nozzle or rose, then the pressure within the tube can be greatly increased by energetic pumping, and the tube being elastic will be distended.

It can be further observed in this common operation that the smaller the orifice of the nozzle the greater the pressure in the tube with a given rate of working the pump; and, the orifice remaining the same, the pressure will increase in proportion as the pump is more energetically worked. Or in other words, the pressure within the tube will depend on (a) the energy used at the pump, and (b) the degree of impediment offered to the outflow.

If the tube be resilient, and the nozzle have a small orifice so that a high pressure can be established within the tube, it will be found that the liquid will flow from the nozzle in a continuous stream, and will not follow the jerks communicated by the pump. That is to say, the interrupted energy of the pump is stored up by the elastic tube and converted into a continuous pressure exerted on the fluid. But if the tube be quite rigid, or the orifice too wide to allow the pressure within the tube to be raised sufficiently high, then the fluid will flow out of the end of the tube in jets which correspond with the strokes of the pump; i. e., the outflow will follow closely the pressure difference caused by the pump at the point of inflow.

Fig. 129. Diagram of Circulation, showing right (R H) and left (L H) hearts, and the pulmonary (P) and systemic (S) sets of capillaries.

These simple facts, which can be verified experimentally with an ordinary enema bag, a yard of elastic tubing, and a short glass tube drawn to a point, form the key to the most important dynamic principles of the circulation.