Monary Circulation

The output of the heart can be measured at any time by opening the tube X, clamping tube Y, and allowing the blood to flow into a graduated cylinder. The volume changes of the heart at each beat can be measured by the so-called cardiometric method. A glass cardiometer of the shape shown in fig. 9 is fitted over the beating heart. The opened pericardium is tied around the lip of the cardiometer, which is then connected with a tambour with a slack rubber membrane. The movements of this membrane are recorded by a lever on the smoked surface of a revolving drum. The difference between the diastolic and the systolic volume thus recorded is obviously equal to the amount of blood put out by the two ventricles during a single heart beat.

This preparation enabled Starling and his co-workers to study the main features of the physiological activity of the heart. It was found that the heart itself cannot modify the blood flow and that, within wide variations of the heart rate and of the arterial re sistance against which the heart beats, the output remains con stant provided the inflow is not changed. This means that the heart will adapt the strength of its beat within very wide limits, and will perform the work required if it is within the functional capacity of the organ. If the inflow into the heart is say i,000 cc. a minute, this output will be maintained if the heart rate is r oo or 200 beats per minute, or if it has to beat against a pressure of 6o or 16o mm. of mercury. Increase in the heart rate does not modify the output, but it increases the maximum amount of blood which the heart is able to expel, since at the faster heart rates the inflow of blood can be increased without causing over-distension of the ventricles. Another important observation made by Starling was that, in the heart-lung preparation, the heart rate is independent of the pressure against which the heart works and of the output. The heart rate is here determined by the temperature of the S-A node, i.e., by the temperature of the circulating blood. It is clear that the extent of this effect will vary from heart to heart, depend ing on the natural rhythmicity of the node.

The Work of the Heart.— The energy of the contraction of the heart is expended (a) in forc ing a certain amount of blood against a certain resistance pre sented by the arterial pressure, and (b) in imparting a certain velocity to the blood. The work done by each ventricle can be calculated from the formula : Where W is the work done, M is the mass of blood expelled at each beat, r is the mean arterial pressure, v is the velocity at the root of the aorta, and g the factor of acceleration. The work of the right ventricle is approximately -} of that of the left. The work

of both ventricles in the human heart at rest is about i oo gram meters per beat, which is equivalent to about io,000 kilogram meters in 24 hours. During very strenuous muscular exercise when the output is considerably increased, the work of the heart per beat is about 400 gram.-meters, or 8o,000 kilogram-meters in 24 hours. This rate of work could probably be maintained for not more than a few minutes.

The energy required for the cardiac contraction is derived from the oxidation of the deposits of glycogen (possibly also of fats) within the heart itself. It was found by Lovatt Evans that, on in creasing the arterial pressure from 8o-14o m.m. of mercury, the oxygen consumption of a heart was increased from 228 to 404 cc. per hour; and on increasing the output from 9.3 to 92 litres per hour, it increased from 155-649 cc. per hour. The maximum efficiency of the heart is of the same order as that found for skeletal muscle, namely 20-28%.

The Nervous Regulation of the Heart Beat.

In the vertebrates, the heart is supplied with two sets of nerve fibres; those which pass from the central nervous system in the vagus nerve, and those which pass in the sympathetic nerves (fig. io). The cardiac fibres of the vagus terminate around nerve cells sit uated in the heart itself (preganglionic fibres) ; the ganglionic cells serve as relays from which new fibres (postganglionic fibres) emerge and run directly to the cardiac muscle and to the S-A node. The sympathetic fibres leave the spinal cord by the anterior roots mainly of the second and third dorsal nerves, run in the white remi communicantes to the Stellate ganglia where they end; from the,Stellate ganglia, postganglionic fibres begin, which go to the various parts of the cardiac muscle.

The Vagus Nerve.—In 1845 the brothers Weber made the im portant discovery that stimulation of the vagus nerve retards or even arrests the heart beat. The cardio-inhibitory nerves have since then been found in all classes of vertebrates and in many invertebrates. During a stimulation of the vagi, the heart beat is considerably retarded or stops altogether, and as the result of this the arterial blood pressure falls. Blood accumulates on the venous side of the heart and is not forced out in sufficient amount to maintain the blood pressure. If the stimulation of the vagus is prolonged, the heart often begins to beat again with a slow rhythm; this beat is seen to be confined to the ventricles only, the auricles still remaining at a standstill. We speak of such a beat as an "escape." The ventricle is really beating at its own idio ventricular rhythm in response to impulses originating within itself.