= Solution
\b[Both the <aorta> and a <vena cava> have an endothelial lining and three broadly corresponding tunics; the <aorta> is dominated by an elastic muscular media, whereas a caval <vein> has a relatively thin media and a prominent collagenous adventitia.] Their appearance reflects their different mechanical conditions, not simply the <oxygen> content of their <blood>.
The <tunica intima> of each begins with a continuous <endothelium> on its <basement membrane>, supported by subendothelial <connective tissue>. A normal section must be distinguished from age-related or pathological thickening. In the <aorta>, the intima merges towards a very thick <tunica media>. Unlike a typical muscular <artery> with one conspicuous internal elastic boundary, an elastic <artery> contains so many <elastic lamellae> that selecting one line as the uniquely dominant elastic membrane is less useful.
The aortic media contains repeated, approximately concentric fenestrated <elastic lamellae>, with <smooth muscle cells>, <collagen> and other matrix components between them. The elastic elements extend during ventricular ejection and recoil between beats; <smooth muscle> participates in matrix maintenance and regulation of wall properties. The outer <tunica adventitia> contains <collagen>, <elastic fibres>, <nerves> and <vasa vasorum>. Those small vessels serve the outer wall, which is too thick to be supplied entirely by diffusion from luminal <blood>.<Blood Vessels in the Histology Guide>[Slides MH 065 and MH 066, aorta: elastic lamellae, smooth muscle and outer wall]{ref}
A <vena cava> has a wide lumen relative to its wall and is easily distorted or collapsed in a fixed empty preparation. Its intima is relatively thin. The media contains fewer circular smooth-<muscle> and elastic elements than the aortic media and is much less dominant. Its adventitia is often the thickest coat, with substantial <collagen>, elastic tissue and conspicuous longitudinal bundles of <smooth muscle>, particularly in the inferior vena cava. The relative amount varies with the sampled caval region; a thin-walled <vein> must not be described as a vessel without any <muscle>. <Vasa vasorum> and <nerves> occur in its outer wall too.<Anatomy and Physiology 2e by J. Gordon Betts and others/20.1 Structure and Function of Blood Vessels>[Shared Structures; Arteries; Veins: tunics, elastic arteries and venous wall proportions]{ref}
Ordinary peripheral venous valves are folds of intima, but a valve is not a necessary feature of every large <vein> or every caval section. The <aorta> likewise has no series of valves distributed down its wall; the aortic valve is at its cardiac origin. Thus two circular profiles alone should be compared by wall organization, not diagnosed by assuming the <vein> must display a valve.
An elastic-fibre stain makes the aortic lamellae conspicuous, while ordinary stained sections show nuclei and the relative distribution of <muscle> and <connective tissue>. A transverse section cuts longitudinal caval <muscle> bundles across, whereas a longitudinal section displays their length; recognizing orientation prevents confusing those bundles with glands or isolated cellular nodules. The low-pressure, highly compliant venous reservoir and the pressure-bearing elastic arterial conduit are consequently different uses of the same broad tissue classes.
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