The circulation of the blood is maintained chiefly by two forces - the rhythmic contraction of the heart muscle and the elasticity of the arteries. Other factors concerned are the compression of the veins by the muscles and the inspiratory action of the chest.

As these are the chief factors, any abnormality within them will bring about more or less general disturbances of the circulation. To these may be added alteration in the quantity or quality of the blood itself. According as to whether the effect is more marked in the systemic or in the pulmonary circulations the disturbances are more or less widely distributed.

The circulatory disorders may be cardiac in origin and either the result of muscular or valvular lesions. If muscular, there may be an excessive or, what is more common, a diminished action.

The excessive form is seldom lasting, but while present causes a rise of blood-pressure, an increased amount of blood within the vessels in the part involved, and an increase in the rate of flow. If the overaction should be long continued, as a result of hard work or by constant stimulation, there would be hypertrophy of the left ventricle.

Diminished activity is more common and more important than the above. It may be brought about in many ways. It may be the sequel of a heart muscle weakened by the infectious fevers or other diseases, by poisons, by lack of nourishment caused by anemia, or by a blocking of the coronary arteries. It may be the result of nervous disturbances with no apparent lesion of the muscle, or it may be the result of some valvular disorder.

Sometimes it results from pressure from the outside - that exerted by collections of fluid in the pericardium, in the pleurae, or by tumors or adhesions.

As a result of the weakened circulation there is an accumulation of blood in the venous circulation. If the failure is of the left ventricle, there will be a damming back of the blood in the left auricle and in the pulmonary circulation. If the right heart remains capable, the engorgement will go no further, but when it fails the right auricle becomes distended and a condition of general passive congestion ensues.

In all cases there is a decrease of arterial and an increase of venous pressure.

When the heart's action has become much weakened it will be found that the blood tends to gravitate to the more dependent portions, giving rise to hypostatic congestion. It occurs in the late stages of severe fevers and when death has taken place very slowly. The dependent tissues will become livid through the accumulation of blood, edematous from the escape of fluid from the blood-vessels, and sometimes bed-sores may result. A frequent occurrence is a collection of blood within the lungs, a condition known as hypostatic pneumonia.

The changes within the arteries may be either organic or nervous (vasomotor). Their elasticity may be diminished, and their caliber increased or diminished. The alteration in caliber may be due to changes within the tissues or to disturbances of the vasomotor control.

If there is a paralysis of the controlling nerves, the vessels dilate and hyperemia results. On the other hand, stimulation will cause contraction and subsequent anemia. When sufficiently marked, there will be an increase in the blood-pressure, interference with the heart's action, and venous congestion.

The most common organic disturbance is a sclerosis of the vessel wall, a condition leading to constant interference with the arterial circulation. Generally a hypertrophy of the left heart follows. If, however, the sclerotic changes are very widely distributed, instead of hypertrophy there may be a dilatation, on account of the resistance being too great for the heart to overcome.

Changes in the quantity of the blood, either an increase or a decrease, are generally only temporary, and soon readjust themselves, either through a contraction or a dilatation of the vessels.

Alterations in the quality have a marked effect upon the circulation, probably through the direct action of the toxic substances upon the vessel walls or upon the terminal nerve filaments.