Energy sources of blood circulation and the mechanical action of the heart.

The principle that " the movement of the blood is maintained by the heart which acts as a pump" continues to be binding in modern physiology (Evans, 1952). Certain doubts, however, appear when the power of the heart muscle is regarded as the exclusive or even the main source of energy for the whole cardiovascular system. The question may be asked, Is there no other motive force in the circulatory system, besides the heart muscle, that might be significant ? It is not easy to reconcile the principle of the dominant role of the heart, in the circulation, with the fact of the existence of the venous return energy, but one example from the field of cardiovascular surgery may illustrate that role. Fig. 1 is an angiogram of a patient with an anastomosis between the superior vena cava and the right pulmonary artery. (The patient was operated on by Dr. Glenn from New Haven, Connecticut, U.S.A.) It was a case of complete transposition of the great vessels with stenosis of the pulmonary artery, and after the operation a great improvement in the patient's condition was observed. It is difficult to understand the haemodynamics of this part of the patient's circulation, where the systemic venous circulation is connected directly -by-passing the heart-with the arterial pulmonary circulation. How is it possible for the blood, pumped by the heart, impeded by the dense net of capillaries, to cross them and still retain enough energy to pass the second " filter " of the pulmonary capillaries as happened in Dr. Glenn's (1958) case ? The energy of the venous blood flow can hardly be explained by the small resistance of the capillaries. On the contrary, it seems that the strength of the resistance met by the blood during its passage through the capillaries is underesti-