Page 42 - Libro 2

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PART I — INTRODUCTION TO THE VASCULAR SYSTEM
results in changing the flow profile into a low- resistance pattern with antegrade flow through- out the cardiac cycle. The same change can occur within the superior mesenteric artery. After eating, this tissue bed also vasodilates, changing the flow profile into a low-resistance pattern. In both these circumstances, the low-resistance pattern results in an increase in blood flow to meet the increased metabolic demand.
LAMINAR AND TURBULENT FLOW
Under certain conditions, flow in a cylindrical tube (or blood vessel) will be laminar or streamlined. At the entrance to a vessel, all elements of the blood flow stream will have the same velocities (often re- ferred to as plug flow). As the flow progresses in the vessel, a thin layer in contact with the wall will adhere to the wall and become motionless. The layer of fluid next to this layer must move against this mo- tionless layer and therefore moves slowly due to fric- tion between the layers. The adjacent more central layer travels a little more rapidly. The layers at the center of the tube move the fastest and their velocity is equal to twice the mean velocity across the entire cross section of the tube. At a distance equal to sev- eral tube diameters away from the entrance, laminar flow becomes fully developed. Thus, at the begin- ning of a vessel, the flow profile is rather blunted. The longitudinal velocity profile becomes parabolic at a point several diameters away from the entrance of a vessel (Fig. 2-9).
Turbulent flow is described as irregular motions of the fluid elements. Definite laminae are no lon- ger present but rapid radial mixing occurs. A greater pressure is required to move a given flow of fluid through a tube under turbulent conditions as com- pared to those for laminar flow.
Turbulence is best defined in terms of a dimen- sionless quantity, the Reynolds number (Re). The Reynolds number is proportional to the inertial
Figure 2-9 Flow through a tube illustrating plug flow at the entrance of the tube and parabolic flow distal to the entrance.
forces and to the viscous forces acting on a fluid. In the vascular system, Re is directly proportional to the velocity of the blood, the density of the blood, and the radius of the blood vessel. It is inversely proportional to the viscosity of the blood. Because blood density and viscosity are relatively constant, turbulence develops mainly due to changes in the velocity of blood and the size of the blood vessel. For Re below 2,000, flow will be laminar. For Re above 2,000, turbulence will develop. As blood flows through a stenosis, the vessel radius is re- duced by the presence of atherosclerotic disease, and velocity increases. This results in turbulence, which is routinely documented as the flow exits the stenotic area.
THE ARTERIAL SYSTEM: A HYDRAULIC FILTER
The principal function of the arterial system is to dis- tribute blood to the capillary beds throughout the body. The arterial system consists of various-sized vessels with varying volumes and distensibility. The arterial system, composed of elastic conduits and high-resistance terminals, constitutes a hydraulic filter analogous to resistance-capacitance filters of electrical circuits. Hydraulic filtering converts the in- termittent (pulsatile) output of the heart to a steady flow through the capillaries. Steady flow in the capil- laries ensures adequate exchange or nutrients and wastes.
The entire stroke volume is discharged from the heart during systole. Part of the energy of the car- diac contraction is dissipated as the kinetic energy of the forward blood flow. The remainder is stored as potential energy by the distensible arteries. During diastole, the elastic recoil of the arterial walls con- verts the potential energy into blood flow. This pro- duces antegrade flow in late diastole (Fig. 2-10). If the arterial walls were rigid, no capillary flow would occur during diastole.
Arterial elasticity and capacitance are essen- tial properties to allow for proper blood flow. The change in volume divided by the change in pressure represents capacitance or compliance. Normal ca- pacitance of arteries is greatest over a median range of pressure variations. (Just like a balloon, which is hardest to inflate at the very beginning and again at just before it is completely full, but it is easiest to inflate at intermediate volumes.) Capacitance de- creases with age as the vessel walls become rigid. As a vessel wall becomes stiffer with age, this results in an increase in systolic pressure as well as pulse pressure. Pulse pressure is the difference between the systolic and diastolic pressures.