Page 38 - Libro 2

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PART I — INTRODUCTION TO THE VASCULAR SYSTEM
Hydrostatic pressure increases in lower portions of the body due to the weight of the column of blood within the vessels. The farther below the reference point, the greater the hydrostatic pressure. The formula for gravi- tational potential energy is the same as hydrostatic pressure but with an opposite sign. Thus, gravitational potential energy and hydrostatic pressure tend to can- cel each other out. More information about hydrostatic pressure will be discussed in Chapter 3.
THE BERNOULLI PRINCIPLE
The Bernoulli principle states that when a fluid flows without a change in velocity from one point to an- other, the total energy content remains constant, providing no frictional losses. However, in reality, there is always some energy that is “lost.” Of course, energy cannot be lost, but it is merely transferred to a different form. In the vascular system, energy is mostly dissipated in the form of heat due to friction.
The total energy in the vascular system is a bal- ance between potential energy (pressure) and kinetic energy (velocity). If the velocity of blood goes up, there must be a pressure decrease. One can think of this as taking energy from one form (the blood pressure) in order to increase another form of energy (the blood velocity). This principle is used in cardiac imaging. By measuring the velocity at the stenotic valve, one can determine the pressure drop across a valve and thus the clinical significance (Fig. 2-1).
VISCOSITY AND INERTIA
In the vascular system, energy “losses” are the re- sult of viscosity and inertia. Viscosity is the prop- erty of a fluid that resists the force tending to cause
Bernoulli’s Principle
fluid to flow. It can be defined as the friction exist- ing between bordering layers of fluid. Imagine two open containers, one filled with water and one filled with honey. If both containers were tilted to allow the liquids to pour out, the water would flow more quickly than the honey. The honey is more viscous than the water, and more viscous fluids flow more slowly. Blood viscosity increases with increases in hematocrit (the concentration of red blood cells). Hematocrit is the most important influence on blood viscosity.
Inertia is the tendency of a body at rest to stay at rest or a body in motion to stay in motion unless acted upon by an outside force. It is one of the fun- damental principles of physics described by Sir Isaac Newton. A classic example of the force of inertia is, when riding in a car when the brakes are applied, an individual has the tendency to move forward. The seat belt is the outside force that stops this forward movement. Inertial losses in the vascular system oc- cur whenever blood is forced to change direction or velocity. In order to change direction, a force needs to be applied and some energy is “lost.” Inertial “losses” depend on the density and velocity of the blood flow. In blood vessels, energy “losses” due to viscosity effects are greater than those due to inertia.
VELOCITY AND FLOW
Often, the terms “blood velocity” and “blood flow” are used interchangeably, but they mean two differ- ent things. Velocity refers to the rate of movement (displacement) with respect to time. It has the units of distance per unit of time, such as centimeters per second or meters per second. Blood flow is also re- ferred to as volume flow. It represents the volume of something moved per unit of time. It has the units of milliliters per second, liters per second, milliliters per minute, or liters per minute.
Velocity and flow are related by the equation:
High pressure
Low pressure
High pressure
Low velocity
High velocity
Low velocity
A =
Q = 10 mL/s
v =
2 cm2
5 cm/s
V 􏰀 _Q A
10 cm2
1 cm/s
1 cm2
10 cm/s
Figure 2-1 Illustration of the Bernoulli principle. In a vessel where the area decreases and the blood velocity increases, the pressure must decrease.
Figure 2-2 This illustrates the changes in velocity as a result of changes in diameter. Note there is a steady rate of flow throughout the conduit.