Page 59 - Libro vascular I

Chap-05.qxd 29~8~04 13:25 Page 50
50
PERIPHERAL VASCULAR ULTRASOUND
blood vessel wall varies considerably depending on its position within the vascular system.
Arteries and veins are composed of three layers of tissue, with veins having thinner walls than arter- ies. The outer layer is called the adventitia and is predominantly composed of connective tissue with collagen and elastin. The middle layer, the media, is the thickest layer and is composed of smooth muscle fibers and elastic tissue. The intima is the inner layer and consists of a thin layer of epithelium overlying an elastic membrane. The capillaries, by contrast, consist of a single layer of endothelium, which allows for the exchange of molecules through the capillary wall. It is possible to image the struc- ture of larger vessel walls using ultrasound and to identify the early stages of arterial disease, such as intimal thickening.
The arterial tree consists of elastic arteries, mus- cular arteries and arterioles. The aorta and subcla- vian arteries are examples of elastic or conducting arteries and contain elastic fibers and a large amount of collagen fibers to limit the degree of stretch. Elastic arteries function as a pressure reservoir, as the elastic tissue in the vessel wall is able to absorb a proportion of the large amount of energy generated by the heart during systole. This maintains the end diastolic pressure and decreases the load on the left side of the heart. Muscular or distributing arteries, such as the radial artery, contain a large proportion of smooth muscle cells in the media. These arteries are innervated by nerves and can dilate or constrict. The muscular arteries are responsible for regional distribution of blood flow. Arterioles are the smallest arteries, and their media is composed almost entirely of smooth muscle cells. Arterioles have an impor- tant role in controlling blood pressure and flow, and they can constrict or dilate after sympathetic nerve or chemical stimulation. The arterioles dis- tribute blood to specific capillary beds and can dilate or constrict selectively around the body depending on the requirements of organs or tissues.
WHY DOES BLOOD FLOW?
Energy created by the contraction of the heart forces blood around the body. Blood flow in the arteries depends on two factors: (1) the energy available to drive the blood flow, and (2) the resistance to flow presented by the vascular system.
A scientist named Daniel Bernoulli (1700–1782) showed that the total fluid energy, which gives rise to the flow, is made up of three parts:
● Pressure energy (p)—this is the pressure in the fluid, which, in the case of blood flow, varies due to the contraction of the heart and the disten- sion of the aorta.
● Kinetic energy (KE)—this is due to the fact that the fluid is a moving mass. KE is dependent on the density () and velocity (V) of the fluid
KE 12 rV 2 (5.1)
● Gravitational potential energy—this is the ability of a volume of blood to do work due to the effect of gravity (g) on the column of fluid with density () because of its height (h) above a reference point, typically the heart. Gravitational potential energy (gh) is equivalent to hydrostatic pressure but has an opposite sign (i.e.gh). For example, when a person is standing, there is a column of blood—the height of the heart above the feet— resting on the blood in the vessels in the foot (Fig. 5.1A) causing a higher pressure, due to the hydro- static pressure, than that seen when the person is lying down (Fig. 5.1B). As the heart is taken as the reference point, and the feet are below the heart, the hydrostatic pressure is positive. If the arm is raised so that it is above the heart, the hydrostatic pressure is negative, causing the veins to collapse and the pressure in the arteries in the arm to be lower than the pressure at the level of the heart.
The total fluid energy is given by:
Total fluid energy pressure energy kinetic energy
gravitational energy Etot p (rgh) 12 rV 2 (5.2)
Figure 5.2 gives a graphical display of how the total energy, kinetic energy and pressure alter with continuous flow through an idealized narrowing. Usually the kinetic energy component of the total energy is small compared with the pressure energy. When fluid flows through a tube with a narrowing, the fluid travels faster as it passes through the nar- rowed section. As the velocity of the fluid increases in a narrowed portion of the vessel, the kinetic energy increases and the potential energy (i.e., the