proliferation of smooth muscle cells. The thickening of the intima is also
called an atheromatous plaque and worsens with repeated damage to the
endothelium. It is most dangerous in small vessels, particularly the coro-
nary arteries, where occlusion can result in a myocardial infarction. Ather-
osclerotic plaques also lead to thrombi and aneurysms.
157.The answer is b.(Young, pp 147, 149. Junqueira, pp 208, 211, 215.
Ross, p 371.)The blood vessel in the electron micrograph is an arteriole
(small artery) involved in intraorgan blood flow. There is only one layer of
smooth muscle, but a distinct internal elastic membrane is present.
There is no visible internal elastic membrane in a venule. A capillary lacks
smooth muscle and is composed only of a single layer of endothelial cells.
The aorta and large arteries (answer a)contain extensive elastic fibers
that permit rapid arterial wall stretch in response to the force of ventricular
contraction during systole (120 to 160 mm Hg) followed by sudden relax-
ation (60 to 90 mm Hg) during diastole. Blood is ejected from the left ven-
tricle into the large arteries only during systole; however, blood flow is
uniform because of the elasticity of the large, conducting arteries.
The muscular (distributing) arteries regulate blood flow to organs
(answer c).Muscular (medium) arteries contain more smooth muscle
than the arteriole (answer b)in the figure and distribute blood to organs.
Contraction of muscular arteries is regulated by local factors as well as
sympathetic innervation. The degree of contraction regulates blood flow
between organs. When the tunica media of the muscular artery is con-
tracted, less blood flow occurs to the organ. In a more relaxed state, there
is increased blood flow to the same organ.
The thoracic duct (answer d)returns lymphocytes from the lymphoid
compartment to the circulation. The thoracic duct shows complete disor-
ganization in the wall with no distinct media or adventitia. The large veins
(answer e),such as the vena cava, that return blood to the heart contain
smooth muscle bundles in the adventitia and are also the only vessel in
which one sees both cross sections and longitudinal sections of smooth
muscle in the same vessel.
158.The answer is c.(Junqueira, p 223. Ross, p 248.)After blood is
removed from the body, a clot forms that contains platelets, erythrocytes,
leukocytes, and a clear, yellow fluid known as serum. Hematocrit is the
volume of erythrocytes per unit volume of blood (e.g., 40 to 50% in adult
264 Anatomy, Histology, and Cell Biology