Page 243 - Concise Pathology for Exam Preparation ( PDFDrive )

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Blood Vessels

The blood supply of heart comprises three major vessel types: arteries (carry blood from
the heart to the systemic circulation); capillaries (responsible for exchange of water and
chemicals between the blood and tissues); and veins (carry blood from capillaries back
towards the heart).

Arteries
The arteries and veins have a similar structure with three layers, from inside to outside:
1. Tunica intima (thinnest layer): It is composed of a single layer of flattened cells
(lining endothelium) held together by a polysaccharide matrix composed of collagen,
proteoglycans and elastin. Outside this is present a thin layer of connective tissue (sub-
endothelial connective tissue) and circularly arranged elastic bands called internal
elastic lamina, which separates the intima from the media.
2. Tunica media (thickest layer): This is limited on the inside by the internal elastic
lamina and on the outside by another thick elastic band called external elastic lamina
(the latter separates the media from adventitia). Tunica media is constituted by con-
nective tissue and polysaccharide substances and is rich in vascular smooth muscle
(especially arteries), which controls the calibre of the vessel.
3. Tunica adventitia: It is made of loose connective tissue and elastic fibres and contains
nerves that supply the vessel as well as nutrient capillaries (vasa vasorum). The inner part
of the tunica media receives nourishment by direct diffusion of nutrients and oxygen from
the lumen whereas the outer part of the media is nourished by the vasa vasorum.

Veins
Veins are different from arteries in the following ways:
1. They have a thinner wall.
2. The three tunicae are less well defined.
3. The elastic tissue is scanty and not well organized into internal and external elastic
laminae.
4. The media is richer in collagen and contains less smooth muscle.

Capillaries
These are 7–8 microns in diameter and are lined by endothelial cells which form its tunica
intima (inner layer) with pericytes forming its tunica adventitia (outer layer).

Arterioles
They are the smallest branches of the arterial tree which have a diameter less than
100 microns. The intima of an arteriole is composed of endothelial cells which rest on
a basement membrane. Larger arterioles may have a fine internal elastic lamina. The
arteriolar media is composed of one or two layers of smooth muscle cells and the
adventitia is insignificant.
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