Venous Return and Cardiac Function: Key Factors
Factors Affecting Venous Return and Cardiac Function
Venous return is the amount of blood flowing from the veins into the right atrium each minute. Several factors influence the increase in venous return:
- Active Muscle Pumping: This is a major factor, especially at the calf level, where it acts as a peripheral heart. Proper assessment and treatment of muscular and osteoarticular disorders are crucial in managing venous disease.
- Thoracic Aspiration: This effect is relatively minor and is partially offset by increased intra-abdominal pressure during inspiration.
- Venous Tone: Mediated by the autonomic nervous system.
- Peripheral Vascular Resistance: In inactive territories.
- Pulsation of Neighboring Arteries: This produces periodic compression of the satellite venous axis.
- Plantar Venous Compression: During walking.
- Vis a Tergo: The residual left ventricular thrust transmitted to veins through capillaries and arteriovenous anastomoses.
The valve system is another major factor, along with the muscle pump. Its functions include:
- Orienting Venous Flow: Making muscle pumping and other centripetal factors effective. Valves prevent reflux into the distal sector and guide flow from the superficial to the deep system.
- Fragmenting the Hydrostatic Column: This prevents distal hypertension.
Other factors, such as pain receptors, temperature, and emotions, also play a role.
Hydrodynamic Regulation of Cardiac Function
Increased venous return triggers the hydrodynamic regulation of cardiac function. This regulation involves adjusting cardiac output, preload, afterload, and contractility.
- Cardiac Output: This is heart rate multiplied by stroke volume. In adults, it typically ranges from 4 to 7 liters per minute. Stroke volume depends on preload, afterload, and contractility.
- Preload: This is the force that stretches the myofibrils (heart wall, primarily the atria) during diastole. Increased preload leads to a more intense and rapid recovery of the myofibrils, enhancing cardiac contraction and increasing stroke volume. However, excessive stretching can lead to laxity and decreased cardiac output. This phase is related to the Frank-Starling law (greater blood influx leads to increased contractility).
- Contractility: This refers to the intensity of myofibril contraction. Both increased and decreased contractility can reduce cardiac output.
- Afterload: This is the pressure the ventricles must overcome to open the pulmonary and aortic valves and eject blood. Increased resistance forces the ventricles to contract more strongly. Systemic vascular resistance (for the left ventricle) and pulmonary vascular resistance (for the right ventricle) are the main factors influencing afterload.
Bainbridge Reflex
The Bainbridge reflex is a hydrodynamic response characterized by increased heart rate in response to increased venous return.