Vessel Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 357

Tabl e 7. 6. Wall structure difference in blood macro- and microcirculation (ColF: collagen fiber;
EC: endothelial cell; ElnF: elastin fiber; FB: fibroblast; vSMC: vascular smooth muscle cell).

Macrocirculation Microcirculation

Intima Endothelium

Endothelium (glycocalyx) (monolayer of ECs,

Subendothelium (vSMC, ElnF) blood-wall interface)

IEL
Media (vSMC, ElnF) 1-2 layers of vSMC in arterioles,

pericytes in capillaries

EEL Elastic externa ( ± )

Adventitia (FB, ElnF, ColF) Absent in arterioles

Nervi vascularis (end on vSMC)
Vasa vasorum (cap plexi)

Capillaries

Capillaries 16 are small exchange vessels composed of endothelium surrounded by a
basement membrane.
A capillary bed can encompass 2 types of vessels: (1) true capillaries which
branch mainly from metarterioles 17 and yield mass exchange between cells and
blood and (2) a short vascular shunt that directly connects the arteriole and venule
at opposite ends of the capillary bed.
Three structural types exist. Continuous capillaries in muscles, skin, lung,
and central nervous system are defined by a continuous basement membrane and
tight intercellular clefts and have the lowest permeability. Fenestrated capillaries
in exocrine glands, renal glomeruli, and intestinal mucosa are characterized by
perforations in endothelium and thus by relatively high permeability. Discontinuous
capillaries in the liver, spleen, and bone marrow, are defined by large intercellular
and basement membrane gaps and, consequently, very high permeability.
Precapillary sphincters are rings of smooth myocytes at the origin of true
capillaries that regulate blood flow into tissues. Mass flux across capillaries depends
mainly on 6 variables: (1) capillary and interstitial hydrostatic pressure; (2) capillary
and interstitial oncotic pressure; and (3) filtration and reflection coefficients.

16 In the second half of the seventeenth century, M. Malpighi identified the capillaries that close the
blood circulation loop between arteries and veins, demonstrated earlier by W. Harvey. About at the
same time, C. Aselius discovered the lymphatic vessels.
17 Terminal short segment of artriole with smooth myocytes forming a precapillary sphincter that
controls the blood entry into that capillary bed.