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The observations reported in this paper support the view that a vigorous and properly timed atrial contraction, by transporting blood into the ventricle and by approximating the atrioventricular valve prior to ventricular systole, provides a mechanism by which mean atrial pressure is maintained at a substantially lower level for any ventricular end-diastolic pressure than would be the case if these functions of the atrium were not operative. The central nervous system, by way of efferent sympathetic pathways, can regulate appropriately the vigor and timing of atrial systole as well as the duration of the diastolic filling period. The net effect of such regulation is to decrease mean atrial pressure at any given heart rate and cardiac output, and to permit rapid heart rates which could not otherwise be tolerated without elevations of atrial pressure. The fact that MLAP-LVEDP is independent of aortic pressure and stroke volume, and of heart rate over certain ranges suggests that this hemodynamic measurement is uniquely dependent on, and therefore useful in the analysis of, atrial performance.

In the analog computer for the human systemic circulatory system, the electrical equivalents of a segment of artery as designed up till now are not satisfactory, because they lack representation of the sleeve effect, which results from the interaction between viscous and inertial forces during pulsatile blood flow. First, a network which accounts for this effect was derived, starting out from entirely different equations. It was surprising to find that this does not alter the circuit completely, but only requires adding to the former circuit a corrective network which, however, may have considerable influence. Second, the effect of the anomalous viscosity of blood was taken into account. Its influence on oscillatory flow has been investigated by Taylor, who derived an expression for the relation between pressure-gradient and flow under the assumption that there exists a small boundary layer of lower viscosity at the wall, and found a similar formula for the case in which the blood is assumed to slip at the wall. The modifications of the electrical circuit, necessary to represent these conditions, are given in this paper and turn out to be remarkably simple.