The graph above combines cardiac and vascular function curves.  The cardiac function curve, labeled as cardiac output, illustrates the Frank-Starling effect.  The Frank-Starling effect states that as cardiac muscle is increasingly stretched, the cardiac output increases (up to a limit, as illustrated by the flat portion of the curve).  This is essentially the same length-tension relationship that is seen in skeletal muscle.

The venous return curve represents how venous return changes with variations in right atrial pressure.  Where the venous return curve intersects with the x-axis, it indicates a venous return of zero.  At this point, all circulatory flow ceases, and the pressure throughout the circulatory system equalizes.  This average pressure is termed the mean systemic filling pressure, which is a measure of the degree of filling of the circulatory system relative to the circulatory capacity.  Increases in blood volume (such as with excessive hydration, dashed line) shift the x-intercept to the right, while decreased blood volume (acute hemorrhage, dashed line) shifts it to the left (Choices A and B).  Total peripheral resistance (TPR) affects the slope of both the venous return and cardiac output curves.  Increases in the TPR decrease the slope of both curves, as higher vascular resistance restricts venous return and increases afterload.

The dashed lines in the question image depict decreased cardiac output and an unchanged venous return (unchanged blood volume and TPR).  An isolated decrease in cardiac output indicates decreased contractility that is not the result of decreased preload (because the venous return line is unchanged).  This indicates inhibited contraction either from the action of a negative inotropic drug or an injury to the myocardium, such as a myocardial infarction.

(Choice C)  Chronic anemia causes an increase in cardiac output in an effort to meet the metabolic demands of the tissues.  This causes an increase in the slope and height of the cardiac output graph.  Venous return also increases somewhat due to decreased blood viscosity.

(Choice E)  Anaphylaxis causes widespread venous and arteriolar dilation along with increased capillary permeability and third-spacing of fluids.  This results in a serious drop in venous return (shifted down and leftward).  Cardiac contractility also increase as the body attempts to maintain blood pressure.

Educational objective:
Myocardial infarction causes a sharp decrease in cardiac output due to loss of function of a zone of myocardium.  On a cardiac function curve, myocardial infarction would decrease both the slope and the maximal height of the line.