The blueprint: contemporary venous physiology and measurement

This narrative review presents an exploration of venous blood flow regulation across integrative physiological and technical measurement themes. First, the biomechanical and tissue properties of veins in relation to arteries are discussed, emphasizing a role for venous compliance and the importance of considering volume stress. These functional characteristics are then discussed under the lens of sympathetic activation and local vaso-responsive features, highlighting the interplay of volume stress and whole body sympathetics. Subsequent discussion delves into the determinants of venous return, considering the interplay of known contributors, including the respiratory and muscle pumps, then identifying newly considered mechanisms of venous return under the common theme of graded volume stressors. Venous valves are also given special consideration as an important area to apply new imaging technology to identify the intricacies of flow through valves under volume stressed conditions. Blood flow estimation methods are discussed in detail, highlighting the difficulties of applying Doppler ultrasound to research-grade venous blood velocity and flow metrics. We introduce vector flow imaging as a promising technology to characterize complex venous blood flow behavior in vivo with key advantages over conventional imaging techniques. The review concludes by considering implications of the discussed venous principles in the context of applied physiology, including orthostatic stress, exercise, and healthy aging. Together, the review provides a holistic understanding of venous physiology, highlighting its importance in human cardiovascular physiology.