Page 66 - Cardiac Nursing

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CHAPTER
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S S S S Systemic and Pulmonary Circulation and
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O O O O Oxygen Delivery
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Elizabeth J. Bridges / Joseph O. Schmelz
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Th st f ri st ic of t y te mi el elastin, these large conducting arteries are able to distend to twice
The structural and functiionall hcharactteristicss of thhe systemic icir-
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cu culation determine the continuouss adjustments in flow, pressure, , th their unloaded lengthh. The ability of the capacitive arteries to dis-
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and resistance that occur in each vascular bed and that are vital de- tend is important in cuushioningg pulsatile flow, such that the bllood
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termiinants off issue functionn. Blood flow and nut irient ex hchange fl flow to thhe organs/tissue is almost a constant flow. During systole,
in various vascular beds are affected by the structural and meta- the aorta and proximal large vessels store approximately 50% to
bolic characteristics of the vascular bed, the physical factors that 60% of the stroke volume. During diastole, the distended vessels
affect flow and the exchange of materials across the blood vessel recoil and move the remaining blood to the periphery. This phe-
wall, the local factors originating from the metabolically active nomenon is referred to as a “Windkessel function,” which is the
cells and vascular endothelium that regulate flow to individual transformation of pulsatile flow in the central arteries to constant
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vascular beds, and local and systemic neuroendocrine regulation. flow in the periphery. As the arteries approach the periphery, they
The combined regulation of cardiac output, blood pressure, and become smaller in diameter, and there is a relative decrease in
systemic vascular resistance determines tissue blood flow and, ul- elastin and a relative increase in smooth muscle in the tunica me-
timately, the survival of each organ system and the body as a dia. 7,8 These peripheral arteries are referred to as muscular arteries.
whole. This chapter describes the basic anatomy and physiology The small arteries (prearteriolar vessels with a diameter less
of the systemic and pulmonary circulation; Chapter 3 describes than 500 mm) receive nervous stimulation primarily from nora-
the overall regulation of cardiac output and blood pressure. drenergic stimuli, with the nerve terminals located in the adventi-
tia. Unlike the larger arteries, in which sympathetic neural
constriction is activated by 1 and postsynaptic 2 receptors, the
STRUCTURAL CHARACTERISTICS small arteries are noradrenergically constricted mainly by the post-
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OF THE VASCULATURE AND synaptic 2 receptors. The small arteries are also sensitive to
LYMPHATICS endothelium-derived relaxing and contracting factors. Of clinical
importance, abnormal small artery wall structure is an independ-
ent predictor of cardiovascular events (e.g., stroke, myocardial in-
Blood vessels are usually classified in the following manner: aorta, farction, death). 10,11 However, it may be possible to reverse these
large arteries; main arterial branch, small arteries, arterioles; ter- changes with vasodilator therapy. 12
minal arterioles, capillaries, postcapillary venules; venules, small
veins, main venule branch, large veins, and the vena cava. 1–3 Microvascular Bed
These classifications are based on structural characteristics such as
diameter, wall thickness, and the presence of muscle. Although The term microcirculation denotes the vascular and lymphatic mi-
blood vessel diameter is often used to characterize different vessels, crocirculation. The vascular microcirculation consists of (1) large
it is not an appropriate criterion to use for classification, because and small arterioles (precapillary resistance vessels); (2) terminal ar-
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differences in vessel size reflect the state of vessel contraction as terioles, which in many tissues serve as so-called precapillary
well as differences between organ systems and species. 3,4 sphincters; and (3) other precapillary structures such as capillaries;
With the exception of the capillaries, the systemic vasculature is and (4) nonmuscular venules, known collectively as the exchange
composed of three layers: the tunica intima or internal layer, which vessels, and muscular venules (postcapillary resistance vessels).
consists of the endothelium and the basal membrane; the tunica The term lymphatic microvasculature refers specifically to the ter-
media, which consists of smooth muscle and a matrix of collagen, minallymphatic vessels.
elastin, and glycoproteins; and the tunica adventitia, which con-
sists of connective tissue (Fig. 2-1). The muscularis in the artery is Arterioles
a concentric ring, which allows for vasoconstriction. In contrast, As the vessel diameter decreases from the small arteries to the
the venous musculature is organized into small bundles at right an- arterioles, the number of smooth muscle layers decreases from
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gles. In the larger arteries and veins, the tunica adventitia also con- approximately six layers in the 300- m vessels to a single layer of
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tains blood vessels that supply the vessel wall (vasa vasorum). The irregularly dispersed smooth muscle in the 30- to 50- m vessels. 7
vascular endothelium, which is a metabolically active barrier, is a At this point, the vessels are referred to as arterioles. The smallest
primary mediator of vascular function and is discussed in detail. arteriolar branches (8 to 20 m in diameter) are called the termi-
nal arterioles. 13 In some cases, smooth muscle extends beyond the
Arteries intersection of the terminal arterioles with the nonmuscular cap-
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illaries into structures known as ll k precapillary sphincters. 14 Th
The ter-
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Arteries in which the media contains smooth muscle and elastin minal arterioles and precapillary sphincters control the distribu-
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are called elastic arteries. Because of the considerable amount of tion of blood supply to the exchange vessels. 15
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