Chapter 9: Physics for the Vascular Specialist

INTRODUCTION

Hemodynamic principles describe how blood flows in arteries and veins. This includes the relationship between pressure, flow, and resistance in normal, diseased, and collateral vessels. The clinical significance of arterial disease such as obstruction depends on its location, severity, and duration, as well as on the ability of the circulation to compensate, by changing cardiac output and developing collateral pathways. The aim of this chapter is to describe the principles and laws that are essential to understanding the pathophysiology and the fundamentals of the treatment of vascular disease. There are a few principles that govern flow in tubes including blood vessels; however, blood vessels are more compliant than rigid tubes making blood vessels more complex than rigid tubes. Additionally, blood vessels are affected by biological and hormonal factors that add to the complexity of the flow in blood vessels.

ARTERIAL HEMODYNAMICS

Fluid Pressure and Energy

Blood flows through the arterial system in response to differences in total fluid energy. The pressure in a fluid system is defined as force per unit area, which is expressed in units such as dynes per square centimeter (dyn/cm²) or millimeters of mercury (mmHg). Total fluid energy (E) is made up of potential energy (Ep) and kinetic energy (Ek). The potential energy (Ep) consists of intravascular pressure (P) and gravitational potential energy. Intravascular pressure (P) has three components: (1) dynamic pressure produced by cardiac contraction, (2) hydrostatic pressure, and (3) static filling pressure.¹ The specific gravity of blood and the height of the point of measurement above or below a reference level (usually the right atrium) determine the hydrostatic pressure. The hydrostatic pressure is described by the equation:

p: the specific gravity of blood (~1.056 g/cm³)

g: the acceleration due to gravity (980 cm/sec²)

h: the distance in centimeters above or below the right atrium

The static filling pressure illustrates the residual pressure that exists in the absence of flow and is determined by the volume of blood and the elastic properties of the vessel wall. In contrast to hydrostatic pressure, static filling pressure is usually low (5–10 mmHg).² Gravitational potential energy is the work a volume of blood can do because of its height above a specific reference level. The formula for gravitational potential energy is: +pgh. Since gravitational potential pressure and hydrostatic pressure usually cancel each other out, and static filling pressure is relatively low, the cardiac contraction dynamic pressure is the predominant component of potential energy (E_p). This is expressed as follows:

Kinetic energy (E_k), on the contrary, is of more biological and physiological significance and is the ability of blood to do work because of ...