Learning Objective

By the end of this section, learners should be able to:

1. Describe the cardiac and vascular function curves and their intersection as the operating point of the circulatory system.
2. Explain how changes in inotropy, venous return, and total peripheral resistance (TPR) affect cardiac output (CO) and right atrial pressure (RAP).
3. Recognize physiological and pathological examples that shift these curves.

Overview

The operating point of the heart is defined by the intersection of the cardiac function curve and the vascular function curve. At this point:

• Cardiac output (CO) = venous return (VR)
• Right atrial pressure (RAP) represents the pressure at the entry to the right heart.
• The system is closed, so any change in one parameter affects the other to maintain equilibrium.

Key Curves

1. Cardiac Function Curve

• Represents the relationship between RAP and cardiac output.
• Influenced mainly by inotropy (contractility).

Effects of Changes in Contractility:

2. Vascular Function Curve

• Represents the relationship between RAP and venous return.
• Influenced mainly by venous tone, circulating volume, and TPR.

Effects of Changes in Venous Return:

Effects of Changes in Total Peripheral Resistance (TPR):

• ↑ TPR → CO may decrease; effect on RAP unpredictable
• ↓ TPR → CO may increase; effect on RAP unpredictable
  Examples:
  • Vasopressors ↑ TPR
  • Exercise or arteriovenous shunts ↓ TPR

Interactions

• Reinforcing changes: Exercise simultaneously ↑ inotropy and ↓ TPR → maximal increase in CO
• Compensatory changes: Heart failure ↓ inotropy → fluid retention ↑ Preload → helps maintain CO

Activity