Venous return simplified with air-plethysmography, modelling and Sack Theory

The usual mantra taught by experts is to explain venous return using (i) pressure gradients, (ii) ankle joint movements and (iii) the suction effect of inspiration. This is supported with data obtained directly from venous pressure measurements and indirectly using ultrasound calculations. Whilst these veno-dynamic factors undoubtedly assist in the venous return process, the primary mechanism is missing from the standard teaching curriculum. Evidence for this is the observation that most patients with calf muscle pump (CMP) inactivity or failure of active inspiration have an excellent venous return. Examples include persons on mechanical ventilation, in a wheelchair from paralysis or amputees. Chair sleeping is another example. The first strategy of this paper is to explain venous return using calf volume changes in response to gravitational positioning. It relies on the premise that arterial supply volume equals venous drainage volume. When this system is challenged by gravitational positioning, the resulting calf volume changes demand an explanation in terms of an inequality in the inflow = outflow hypothesis. Large volume shifts illustrate the powerful ability of gravity to change venous drainage dynamics. The second strategy is to use modelling with water, beakers, bags and tubes to explain upward flow against hydrostatic columns over a metre high. Whilst this is a data free exercise, the experiments are easily repeatable and understandable. They will depict pressure using height instead of pressure transducers (which are themselves calibrated using liquid columns). Most important, it will demonstrate that pressure is not the cause of the flow but the expression of the feature of a hydrodynamic system. The final strategy is to place Sack Theory into context as the hidden environment making venous drainage possible. It relies on the fact that our bodies are made of collapsible “sacks”, liquids and tissues that compress like liquids. These are surrounded by a hierarchy of enveloping membranes with each absorbing their enclosed weight and transferring their contents into weightless tissue. Once transformed, gravitational forces are negated making upward flow energy efficient. Collapsible venous drainage tubes are recognised as one such envelope (sack). Elementary child-friendly models are illustrated, and the role of trans-membrane pressure neutralisation is highlighted. Veno-dynamic equations will not be used.