The amount of blood in the circulation and the capacity of the vascular bed should not be considered as rigidly fixed for any individual. The vascular system is in a perpetual state of varia tion on account of the continuous changes of the vasomotor tone. It is obvious that unless vasoconstriction is compensated for by a simultaneous vasodilation elsewhere it will lead to a change in the resistance as well as a change of capacity. Arterial vasocon striction will chiefly affect the resistance, while even small changes in the tone of the enormous capillary bed will mainly affect capac ity. Thus on account of these changes of capacity more or less blood may be in active circulation, and more or less blood may be left behind to fill the vascular bed.
Spleen, Lungs and Other Factors.—There are however two organs which play a predominant role as reservoirs of blood, namely the spleen and the lungs, Hy difference being that the spleen seems to change its capacity mainly as the result of active contraction and relaxation, while the lungs play a purely passive role and act only because of the very great elasticity of their blood vessels. With every increase in the pulmonary blood pressure or with a certain decrease in the intrathoracic pressure, the pul monary blood vessels contain more blood, and therefore less blood is available for the systemic circulation. On the other hand, the capacity of the spleen is to a much greater extent independent of the blood pressure, the changes in its vascularity being due to active contractions of its muscular frame. These contractions seem to be specially adapted to meet the requirements of the whole organism. Whenever, for instance, more blood is needed in cir culation, as when the animal loses blood or when blood goes to fill widely dilated vascular areas of other organs, the spleen con tracts. This also happens in anoxaemia following diminution of the oxygen supply, or during diminution of the oxygen-carrying power of the blood as in the case of carbon monoxide poisoning. As we have already seen, the vascular resistance, the fitness of the heart and the vascular capacity are the three main factors which determine the blood flow in the whole animal. There are however accessory factors, the chief of which are the respiratory movements and the contractions of the skeletal muscles. Any increase in respiratory movements leads to a more negative pres sure in the thorax, and therefore to a better aspiration of blood from the periphery into the large veins and the heart. This is
probably the main cause of the so-called respiratory variations of the blood pressure ; during inspiration the inflow into the heart is increased, and after a certain lag the output is also increased. The rhythmic contractions of the skeletal muscles press the blood from the capillaries to the veins, the back-flow of blood being pre vented by the valves of the veins.
The Nervous Control of the Blood Vessels.—During in creased activity, an organ requires more oxygen. Theoretically this could be accomplished by an increased degree of de-oxygena tion of the blood without a change in the blood flow, or by an in crease in the blood supply without any change in the degree of deoxygenation. The organism uses both methods, but the first in preference. In some cases the increase in the blood supply is such as to overcompensate the need, so that, although the oxygen consumption of the organ is increased, the blood supply is so great that each cubic centimeter of blood gives to the tissues less oxygen than usual. Increased blood supply to an active organ is due either to an increase of the general circulation rate which increases the blood flow through all the organs, or to changes in the distribution of the blood between the various organs, more blood being diverted to the active and less to the resting organs.
Our knowledge of the nervous control of the blood vessels dates from Claude Bernard's discovery that nerve fibres run in the sympathetic chain which maintain the vessels in a state of tonic constriction. Section of these nerves leads to a considerable vasodilation and therefore to an increased blood flow; stimula tion of them causes vasoconstriction. A few years after this dis covery it was found that there are also vasodilator nerve-fibres which are antagonistic in function to the vasoconstrictor nerves, and that these nerve-fibres also have in many cases a constant influence on the blood vessels ; section of them slightly diminishes the blood flow, and stimulation greatly increases it. The blood vessels are therefore, like the heart, under the influence of double innervation. The tonic state of the blood vessels does not itself depend on nervous control, and it does not completely vanish after section of all the nerves going to an organ. The function of the vasomotor nerves is merely to increase or decrease this tone.