Page 406 - Textbook of Pathology, 6th Edition

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Section III SYSTEMIC PATHOLOGY

The Blood Vessels
Chapter 15
Chapter 15
and Lymphatics

The blood vessels are closed circuits for the transport of blood 3. Tunica adventitia. The outer coat of arteries is the tunica
from the left heart to the metabolising cells, and then back to adventitia. It consists of loose mesh of connective tissue and
the right heart. The blood containing oxygen, nutrients and some elastic fibres that merge with the adjacent tissues. This
metabolites is routed through arteries, arterioles, capillaries, layer is rich in lymphatics and autonomic nerve fibres.
venules and veins. These blood vessels differ from each other The layers of arterial wall receive nutrition and oxygen
in their structure and function. from 2 sources:
1. Tunica intima and inner third of the media are nourished
ARTERIES by direct diffusion from the blood present in the lumen.
2. Outer two-thirds of the media and the adventitia are
NORMAL STRUCTURE
supplied by vasa vasora (i.e. vessels of vessels), the nutrient
Depending upon the calibre and certain histologic features, vessels arising from the parent artery.
arteries are divided into 3 types: large (elastic) arteries, As the calibre of the artery decreases, the three layers
medium-sized (muscular) arteries and the smallest arterioles.
progressively diminish. Thus, there are structural variations
SECTION III
Histologically, all the arteries of the body have 3 layers in the three types of arteries:
in their walls: the tunica intima, the tunica media and the Large, elastic arteries such as the aorta, innominate,
tunica adventitia. These layers progressively decrease common carotid, major pulmonary, and common iliac
with diminution in the size of the vessels. arteries have very high content of elastic tissue in the media
1. Tunica intima. This is the inner coat of the artery. It is and thick elastic laminae and hence the name.
composed of the lining endothelium, subendothelial Medium-sized, muscular arteries are the branches of elastic
connective tissue and bounded externally by internal elastic arteries. All the three layers of arterial wall are thinner than
lamina. in the elastic arteries. The internal elastic lamina appears as
Endothelium is a layer of flattened cells adjacent to the a single wavy line while the external elastic lamina is less
flowing blood. Narrow junctions exist between the adjoining prominent. The media primarily consists of smooth muscle
Systemic Pathology
endothelial cells through which certain materials pass. The cells and some elastic fibres (Fig. 15.1).
integrity of the endothelial layer is of paramount importance Arterioles are the smallest branches with internal diameter
in maintenance of vascular functions since damage to it is 20-100 μm. Structurally, they consist of the three layers as in
the most important event in the initiation of thrombus forma- muscular arteries but are much thinner and cannot be
tion at the site. distinguished. The arterioles consist of a layer of endothelial
Subendothelial tissue consists of loose meshwork of cells in the intima, one or two smooth muscle cells in the
connective tissue that includes myointimal cells, collagen, media and small amount of collagen and elastic tissue
proteoglycans, elastin and matrix glycoproteins. comprising the adventitia. The elastic laminae are virtually
lost.
Internal elastic lamina is a layer of elastic fibres having Capillaries are about the size of an RBC (7-8 μm) and
minute fenestrations.
have a layer of endothelium but no media. Blood from
2. Tunica media. Tunica media is the middle coat of the capillaries returns to the heart via post-capillary venules and
arterial wall, bounded internally by internal elastic lamina thence into venules and then veins.
and externally by external elastic lamina. This layer is the In the following pages, diseases of arteries are discussed
thickest and consists mainly of smooth muscle cells and under 3 major headings: arteriosclerosis, arteritis (vasculitis)
elastic fibres. The external elastic lamina consisting of and aneurysms. This is followed by brief account of diseases
condensed elastic tissue is less well defined than the internal of veins and lymphatics, while the vascular tumours are
elastic lamina. described at the end of the chapter.