How do capillaries compare to arteries and veins in terms of structure, specifically regarding the presence of endothelial cells and tunica media?

Structural Comparison of Capillaries, Arteries, and Veins

Capillaries are fundamentally different from arteries and veins in that they consist solely of a single layer of endothelial cells without any tunica media, whereas arteries and veins possess all three vessel wall layers (tunica intima, media, and adventitia) with varying proportions and compositions. 1

Capillary Structure

• Capillaries represent the simplest vascular structure, containing only endothelial cells that form the vessel wall, completely lacking smooth muscle cells, elastic fibers, and the organized tunica media found in larger vessels 1
• The absence of tunica media in capillaries is a defining structural feature that distinguishes them from all other vessel types and enables their primary function of facilitating exchange between blood and tissues 2
• Capillaries are surrounded by pericytes rather than smooth muscle cells, with pericytes characterized by longitudinal processes spanning multiple vessels and comprising approximately 85% of intraparenchymal microvasculature coverage 3
• Pericytes do not express smooth muscle actin (αSMA) and are not contractile in the same manner as arterial smooth muscle cells, distinguishing capillary mural cells from those of arteries 3

Arterial Structure

• Arteries possess all three distinct layers: tunica intima (endothelium with basement membrane), tunica media (smooth muscle cells and elastic fibers), and tunica adventitia (connective tissue) 3, 4
• The tunica media dominates arterial wall composition, particularly in intracranial arteries where the ratio of intima:media:adventitia to total wall thickness is 0.17:0.52:0.31 respectively 3
• Arterial smooth muscle cells are ring-like cells that robustly express αSMA and surround approximately 15% of brain microvessels in precapillary and arteriolar segments 3
• Arteries contain an external elastic membrane in extracranial locations, though intracranial arteries lack this structure, with the outermost muscle cells representing the boundary between media and adventitia 3
• Arterial walls are significantly thicker than capillaries, with intracranial arteries averaging 0.094 mm wall thickness compared to coronary arteries of similar size at 0.876 mm 3

Venous Structure

• Veins contain the same three layers as arteries but with markedly different proportions—the tunica media is much thinner and the tunica adventitia is proportionally thicker than in arteries 4
• Venous smooth muscle is less organized and contains fewer elastic fibers compared to arteries, reflecting the lower pressure environment in which veins function 4
• The endothelial layer in veins shares the same basic structure as arterial endothelium but exhibits different phenotypic characteristics and responses to stimuli based on venous-specific gene expression 1

Functional Implications of Structural Differences

• The single endothelial layer of capillaries optimizes them for their primary function of nutrient and gas exchange, with minimal barrier thickness facilitating diffusion 2, 1
• The thick tunica media in arteries enables them to withstand high pressures and actively regulate blood flow through vasoconstriction and vasodilation mediated by smooth muscle cell contraction 3, 4
• Only arteriolar and precapillary smooth muscle cells are contractile and able to modulate vasomotility, while capillary pericytes display calcium transients that do not correlate with vessel diameter changes 3
• The absence of smooth muscle in capillaries makes them vulnerable to different pathological processes than arteries—capillaries can undergo rarefaction (regression) in response to injury, while arteries undergo fibrotic remodeling 5

Clinical Significance

• Understanding these structural differences is critical for interpreting vascular pathology, as large and medium-sized vessels react differently to injurious stimuli compared to capillaries due to differences in matrix abundance, mural cell density, and adventitial composition 5
• Capillary vulnerability differs from arterial vulnerability: after stroke, capillaries may remain occluded due to thrombosis rather than smooth muscle spasm, as they lack contractile smooth muscle cells 3
• The blood-brain barrier function resides at the capillary level where tight junctions between endothelial cells regulate permeability, a function not present in the same manner in larger vessels 3, 6