Monary Circulation

Since the coronary blood flow is so readily affected by the heart beat, great difficulties are encountered in the study of the regula tion of the coronary circulation. A knowledge of all the influences affecting it is, however, extremely important because the activity of the heart, and therefore the fate of the organism, ultimately depends on the blood supply to the cardiac muscle. It is known that even temporary occlusion of the coronary blood vessels is fatal. The disease known as Angina pectoris, which usually leads to death, is most probably due to such occlusion as the result of arteridsclerosis or thrombosis of the arteries ; it is highly probable that in some cases it may also be due to disorders of the nerves which regulate the tone of the coronary blood vessels. In most cases of weakening of the heart, the coronary flow increases. Lack of oxygen produces an enormous augmentation of the coronary circulation, thus ensuring the survival of the heart so long as there is a trace of oxygen left in the blood.

The Influence of Posture on the Circulation.

The circula tion remains efficient not only in the horizontal but also in the erect position. Yet in a man standing six feet high the hydro static pressure of a column of blood reaching from the vertex to the soles of the feet is equal to 14 cm. of mercury. The blood, owing to its weight, continually presses downwards, and under the influence of gravity would sink if the veins and capillaries of the lower parts were sufficiently extensile to contain it. During the evolution of man, there have been developed special mechanisms by which the determination of the blood to the lower parts is pre vented, and the assumption of the erect posture rendered possible. The abdominal viscera are slung upwards to the spine, while below they are supported by the pelvic basin and the wall of the abdo men, the muscles of which are arranged so as to act as a natural waistband. In tame hutch rabbits, with large patulous abdomens, death may follow in from 15 to 20 minutes if the animals are sus pended and immobilized in the erect posture, for the circulation through the brain ceases, and the heart soon becomes emptied of blood. If, however, the capacious veins of the abdomen be con fined by an abdominal bandage, no such result occurs. Man is naturally provided with an efficient abdominal belt, although this is often rendered toneless by neglect of exercise or gross and indo lent living. The splanchnic arterioles are maintained in tonic con traction by the vasomotor centre, and thus the flow of blood to the abdominal viscera is confined within due limits.

The veins of the limbs are broken into short segments by valves, and these support the weight of the blood in the erect posture. Every contraction of the skeletal muscles compresses the veins of the body and limbs, for these are confined beneath the taut and elastic skin. Guided by the valves of the veins, the blood is by such means driven upwards into the venae cavae.

Circulation During Muscular Exercise.

An attempt to understand the changes in the circulation during muscular exercise can only be made if every part of the complex regulation of the cardio-vascular system is kept in mind.

In muscular exercise, every available mechanism concerned in the regulation of the blood flow is brought into operation, every thing being centred on the achievement of two objects, (a) to carry the all important oxygen to the muscles, and (b) to remove the excess of waste products as quickly as possible. In violent exercise the common effort of all the cardio-vascular mechanisms involved is often inadequate, but fortunately the muscles and other organs do not require an immediate supply of sufficient oxygen to fulfill their needs ; fortunately also the muscles can with stand a certain degree of accumulation of waste products without much alteration of the intensity of the activity. In other words the muscles can to some extent run into debt as regards the oxygen supply. But the excess oxygen requirements must at some time be made good, or the activity will have to stop. The complete adaptation of the vascular and respiratory systems, when the oxygen supply and the removal of waste products entirely meets the demand, is achieved comparatively slowly. The first period of severe muscular activity proceeds under somewhat less favourable conditions than hold after complete adjustment has been attained. This is probably the explanation of the phenomenon of "second wind." The organism is better adapted to exercise, the more rapidly this state of adjustment is reached.

Since the arterial blood is normally about 95% saturated with oxygen, not much extra oxygen can be obtained by increasing the saturation of the haemoglobin. As already explained, the muscle obtains the required oxygen in two ways : (a) by increased utiliza tion of the oxygen of the blood, i.e., by a greater desaturation of the haemoglobin, and (b) by an increase in the blood flow. The first factor alone would be inadequate even if the muscles were able to remove all the oxygen from the blood. A simple example will serve to illustrate this. If we take the total blood flow as 4.5 litres per minute, even if we make the impossible assumption that the whole of the blood goes through the muscles, it will mean that the muscles will only be supplied with about Boo cc. of oxygen per minute. Now the normal oxygen requirements at complete rest are about 30o cc. per minute, and during severe exercise they may rise to 3,00o cc. per minute or more. Moreover, the desaturation of haemoglobin is never carried to completion, and even during severe muscular exercise the venous blood still contains some oxygen. Thus the increased oxygen demand can be met to only a limited extent by increasing the coefficient of the oxygen utili zation.