Cardiac output and venous return as interdependent and independent variables

Area of Science:

• Cardiovascular Physiology
• Hemodynamics

Background:

• In steady states, cardiac output (CO) and venous return (VR) are equal, making their distinction seem redundant.
• However, during transient physiological or pathological conditions (non-steady states), CO and VR can temporarily diverge, necessitating a clear distinction.

Purpose of the Study:

• To elucidate the dynamic relationship between cardiac output and venous return during non-steady states.
• To identify which variable, CO or VR, acts as the independent determinant of circulatory changes in various perturbations.

Main Methods:

• The study conceptually analyzes the interplay between CO and VR based on established hemodynamic principles.
• It examines how pressure gradients and vascular resistance influence VR, and how CO provides the energy for VR.
• The analysis focuses on identifying the independent variable in transient states.

Main Results:

• Venous return (VR) is determined by the pressure difference between systemic capillaries and the right ventricle, and total venous resistance.
• In certain non-steady states (e.g., exercise, hemorrhage), VR acts as the independent variable, with CO becoming dependent.
• Conversely, in other conditions (e.g., myocardial infarction), CO may be the independent variable.

Conclusions:

• Distinguishing between CO and VR is essential for understanding circulatory dynamics during non-steady states.
• Identifying the independent variable (CO or VR) is key to explaining circulatory adjustments in diverse physiological and pathological scenarios.