This is intended to be a critical analysis of commonly held concepts about a major issue in our understanding of the dynamic balance of cardiac output and venous return. It attacks the idea that central venous pressure, e.g., the pressure at the entrance to the right atrium, somehow controls venous return. That idea is part of the conceptual legacy of the data published as "venous return curves" by Guyton and co-workers. Many have interpreted these as showing that venous return was driven through the resistance of the venous system by the pressure difference between mean circulatory pressure and central venous pressure.

But mean circulatory pressure has no physical meaning as a driving pressure behind venous return; central venous pressure is not in any sense an independent variable in the control of venous return; and the apparent resistance suggested by the sloped portion of venous return curves is not the resistance of the venous system.

These conclusions are developed according to the following outline. The text is organized into a series of pages which may be accessed in sequence or via the outline below. Graphics can be accessed separately via the list below.

Page 1: Statement of the problem. The prevailing model taken as a first approximation for the physical properties of the venous system. The specific errors in interpretation of venous return curves in terms of this model.

1. Introduction
2. Lumped properties of the venous system

   Page 2: Background; how the idea of cardiac output curves was extended to venous return curves. The fundamental point of confusion about what was controlled in the experiments on venous return.

3. Background: cardiac output curves
4. Background: moving the focus from the heart to the vasculature

   Page 3: The experimental procedure in which cardiac output, not venous return, was controlled to adjust central venous pressure

5. Experimental procedure for obtaining "venous return curves".
   1. What was the signal guiding the control of pump output?
   2. What was the independent variable?
   3. Summary of experimental basis of venous return curves

   Page 4: The idealized venous return curve, what the parameters of the curve mean in terms of a simple physical model.

6. Features of venous return curves
   1. Mean circulatory pressure
   2. Linear sloped segment
   3. Venous return curves in terms of a simple physical model

   Page 5: Summary and conclusions.

7. Summary: valid and invalid interpretations of venous return curves
   1. Fallacious interpretation of the sloped segment of venous return curves
   2. Fallacious concept that venous return can be stopped by elevating central venous pressure
   3. The utility of vascular function curves in describing cardiovascular equilibrium through the negative feedback relationship between cardiac output and central venous pressure
   4. Limitations
8. Conclusion
9. Bibliography

Equations are developed for the model described in the paper as well as a predecessor in a separate document