Introduction

The basis for understanding the pathophysiology of congenital heart disease is an understanding of normal hemodynamics. Several decades ago Strandness and Sumner, in a monograph entitled Hemodynamics for Surgeons, noted that “In developing an understanding of the physiology of the circulation, it is essential that the basic units and relations of the physical factors involved be understood.”[1] Important concepts include fluid pressure and energy, vascular resistance and impedance, Poiseuille’s law of fluid flow, determinants of laminar vs. turbulent flow patterns, and the Fick principle and its use in calculating blood flows, vascular resistances, and shunts. It is important to remember that the hydrodynamic equations outlined below are for idealized conditions and that the situation in the normal circulation may differ quantitatively from values generated by using these formulas. However, they are presented to provide a conceptual background that will generally provide directionally correct inferences to be drawn regarding the circulation.