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Blood volume, the venous system, preload, and cardiac output
Insights
Cardiac output depends on heart rate, contractility, and preload. Venoconstriction mobilizes blood volume reserves, increasing cardiac output and maintaining circulation during hemorrhage.
Area of Science:
- Cardiovascular Physiology
- Hemodynamics
Background:
- Cardiac output is a key determinant of circulatory function.
- It is influenced by cardiac contractility, afterload, preload, and ventricular compliance.
- The pressure-volume loop is a valuable tool for illustrating these relationships.
Purpose of the Study:
- To explain the determinants of cardiac output.
- To describe the role of blood volume reserves and venoconstriction in maintaining circulation.
- To highlight the importance of the splanchnic bed as a blood volume reservoir.
Main Methods:
- Utilized the pressure-volume loop model to illustrate cardiac function.
- Described the interplay between stressed and unstressed blood volumes.
- Explained the mechanisms of venous return and its impact on cardiac preload.
Main Results:
- Cardiac output is regulated by heart rate, contractility (Emax), afterload, and diastolic properties.
- Peripheral circulation determines the position of the pressure-volume loop.
- Venoconstriction converts unstressed blood volume to stressed volume, supporting cardiac output.
Conclusions:
- The splanchnic bed serves as a major blood volume reserve.
- Mobilization of this reserve via venoconstriction is crucial for maintaining cardiac output, especially after hemorrhage.
- The reflex control of venoconstriction remains an area for further investigation.
Abstract:
Cardiac output is determined by heart rate, by contractility (maximum systolic elastance, Emax) and afterload, and by diastolic ventricular compliance and preload. These relationships are illustrated using the pressure-volume loop. Diastolic compliance and Emax place limits determined by the heart within which the pressure-volume loop must lie. End-diastolic and end-systolic pressures and hence the exact position of the loop within these limits are determined by the peripheral circulation. In the presence of minimal sympathetic tone, some 60% of total blood volume is hemodynamically inactive and constitutes a blood volume reserve (the unstressed volume). The remainder of the blood volume (the stressed volume) and the compliance of the venous system determine the venous pressure. This venous pressure together with venous resistance determines venous return, right atrial pressure, cardiac preload, and hence cardiac output. Venoconstriction causes conversion of unstressed volume to the stressed volume, the blood volume reserve is converted into hemodynamically active blood volume. After hemorrhage this replaces the lost stressed volume, while in other situations where total blood volume is not reduced, it allows a sustained increase in cardiac output. The major blood volume reserve is in the splanchnic bed: the liver and intestine, and in animals but not man, the spleen. A major unsolved problem is how the conversion of unstressed volume to stressed volume by venoconstriction is reflexly controlled.