What Goes Around Comes Around—Venous Return

By its nature, circulatory physiology is also susceptible to circular reasoning because every part of an interconnected system is affected by, and affects, every other part. If we're not careful, we end up saying things like ‘venous return equals cardiac output’ when, in the steady state, that is true by definition and nothing new is gained. If we grant that right atrial pressure (PRA) is the ‘downstream’ pressure for venous return, then it follows that PRA should be inversely related to venous return (and therefore, to cardiac output). If we simply apply Ohm's law to the cardiovascular system, we forget that the mean arterial pressure not only contributes to venous return but also is sustained by venous return. If venous return fails for any other reason (unrelated to arterial pressure), so too will mean arterial pressure eventually fail.