The Arteries
Arterial histology
The arteries of the systemic circulation receive blood from the heart and deliver it to the organs
and tissues. The structure of the vessels directly reflects their position in the system and their
function as capacitance or distribution vessels. The elastic arteries receive the ejected stroke
volume from the heart and absorb the increased volume. The muscular arteries control and direct
flow to the tissues.
Elastic arteries
The elastic arteries are the aorta
and its major branches. The requirement of these vessels is that
they damp the pulsatile nature of blood flow and protect the end organs
from damage that would
result from direct coupling to a peristaltic pump. The structure of these vessels is such that they
are able to dilate in response to the increased pressure which results from the pulsatile cardiac
output. Following dilation, the vessels must recoil to force blood through the circulation. In
common with all vessels, elastic
arteries have three layers - the intima, media and adventitia. The
intima of elastic arteries is formed of a simple squamous epithelium, the
endothelium. The
endothelial cells rest on a basement membrane and a thin layer of connective tissue matrix. The
media is an extremely thick layer formed of alternating sheets of elastin and smooth muscle cells.
In elastic arteries with more than 20 layers, small vessels, the vasa vasorum, provide nutrition.
The sheets of elastin are perforated and associated with loosely packed collagen fibres. The
smooth muscle cells are electrically connected to varying extents by gap junctions. The thickness
of the media is greatest close to the heart and is reduced by the termination of the aorta in the
abdomen. The adventitia is a thin connective tissue coat on the outside of the media. The
primary cells present are fibroblasts which produce the elastin and collagen of the connective
tissue matrix. The adventitia of elastic arteries contains few nerve fibres. Control of arterial
diameter is limited in these vessels since they dilate passively to increased pressure and recoil due
to their natural elasticity. The elastic ability of elastic arteries is reduced with age as the elastin
becomes less elastic, and by stiffening of the wall due to development of calcified atherosclerotic
plaques. The adventitia may also contain further branches of the vasa vasorum system.
Muscular arteries
The elastic arteries eventually lose the organized elastic layers of the media until only an
external
and internal elastic lamina remain. The media of these vessels then is composed largely of
numerous layers of smooth muscle with fragments of elastin and collagen
between the cells. The
adventitia gains innervation, which is most dense in arterioles where catecholaminergic and
peptidergic nerves play a significant part in the control of vessel diameter. The endothelium lining
the intima appears similar in both elastic and muscular arteries but there are regional differences in
the distribution of receptors and the ability of the endothelium to induce contraction or relaxation
in the overlying media.
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