** The Skeletal Muscle Pump and Cardiac Output

 During the 1950s, Arthur Guyton and coworkers conducted an extensive series of experiments that, together with what had been learned earlier by Starling, Wiggers, and many others, led to the conclusion that, in fact, the normal heart could do little more than pump what returned to it from the veins; it had no control over the rate of output but could only respond to the flow of blood into its chambers. Guyton concluded that cardiac output was controlled instead by the factors that regulated the flow of blood from the body back to the heart. While this may seem to be a straightforward hypothesis, the circulatory mechanisms involved in mediating the control are complex and interrelated with other systems, and consequently, the full understanding of cardiac output regulation requires command of many aspects of physiology. ……….Physiologists and clinicians learned many things from Arthur Guyton; among the most important may be that worthwhile understanding of physiology requires the study of more than tissues or organs, but instead analysis of complete systems, with time as a variable. Control of Cardiac Output; NCBI Bookshelf https://www.ncbi.nlm.nih.gov/books/NBK54469/toc/?report=reader

“During upright posture, skeletal muscles help maintain venous return and consequently cardiac output by compressing underlying veins in order to increase blood flow back to the heart (skeletal muscle pump). Thus, the role of lower leg muscles (calf skeletal muscles) in regulating blood pressure homeostasis while attaining upright posture is indispensable.” Skeletal Muscle Pump Drives Control of Cardiovascular and Postural Systems Ajay K. Verma,  Amanmeet Garg,  Da Xu,  Michelle Bruner,  Reza Fazel-Rezai,  Andrew P. Blaber &  Kouhyar Tavakolian 

Skeletal Muscle Pump – Peripheral veins work in concert with the muscular contraction to increase venous return to the heart. When muscles (such as the quadriceps) contract (during walking, running etc), the valves are forced open to increase the venous return.

• The Respiratory Pump: Respiration – During inspiration, venous return increases as the thoracic cavity’s pressure becomes more negative. This reduced intrathoracic pressure draws more blood into the right atrium. This results in greater venous return.
• Venous Compliance – Increased sympathetic activity will reduce venous compliance. This increases the venous pressure and venous return as when blood flow into the veins increases, it cannot dilate to accommodate the increased blood. Instead, pressure in the veins rises and blood flow through the vessels increases to empty the veins faster.
• Blood Volume – The greater the blood volume in the veins, the greater the blood flow and venous pressure. The heart can accommodate the increase in blood volume because of the Frank-Starling mechanism (the greater the stretch, the greater the contractility of the heart).
• The Heart- must be working efficiently to pump blood out of the veins and maintain CVP. [Central Venous Pressure].

This explains why respiratory rate, the respiratory pump, locomotor activity and the skeletal muscle pump are key to venous return and cardiac output and will be put to work, involuntarily, if necessary when adequate circulation of blood is in peril for any reason.