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Compared with arteries at the same level of branching, veins have larger diameters, larger lumina, and thinner walls with less distinct layers, all adaptations to the low pressures found on the venous side of the circulation (see
Fig. 10.1 ). Thus, veins are more prone to dilation, external compression, and penetration by tumors or inflammatory processes. In veins where blood flows against gravity (e.g., in the lower extremities), backflow is prevented by valves. Collectively, the venous system has a huge capacitance and normally contains approximately two thirds of the blood.
Lymphatics are thin-walled, endothelium-lined channels that drain lymph (water, electrolytes, glucose, fat, proteins, and inflammatory cells) from the interstitium of tissues, eventually reconnecting with the blood stream via the thoracic duct. Lymphatics transport interstitial fluid and inflammatory cells from the periphery to lymph nodes, thereby facilitating antigen presentation and cell activation in the nodal tissues—and enabling continuous monitoring of peripheral tissues for infection. This can be a double-edged sword, however, as these channels can also disseminate disease by transporting microbes or tumor cells to distant sites.
Endothelium is a continuous sheet of cells lining the entire vascular tree that regulates many aspects of blood and blood vessel function (
Table 10.1 ). Resting ECs maintain a non-thrombogenic blood-tissue interface (Chapter 4), modulate inflammation (Chapter 3), and influence the growth and behavior of other cell types, particularly SMCs.
Table 10.1 Endothelial Cell Properties and Functions
ProductionofextracellularmatrixCollagen,proteoglycans
ModulationofbloodflowandVasoconstrictors: endothelin,ACEvascularreactivityVasodilators: NO,prostacyclin
RegulationofinflammationandIL-1,IL-6,chemokinesimmunityAdhesionmolecules:VCAM-1,ICAM,E-selectin,P-selectinHistocompatibilityantigens
ACE, Angiotensin-convertingenzyme;CSF, colony-stimulatingfactor;FGF, fibroblastgrowthfactor;ICAM, intercellularadhesionmolecule;IL, interleukin;LDL, low-densitylipoprotein;NO, nitricoxide;PDGF, platelet-derivedgrowthfactor;TGF-β, transforminggrowthfactor-β;VCAM, vascularcelladhesionmolecule.
Normotension Laminar flow Non-adhesive, non-Growth factors (e.g., VEGF) thrombogenic surface
Hypertension Increased expression ofCytokines procoagulants, adhesionComplement molecules, andBacterial products proinflammatory factorsLipid products Altered expression ofAdvanced glycation chemokines, cytokines,end-products and growth factorsHypoxia, acidosis
Fig. 10.2 Basalandactivatedendothelialcellstates.Normalbloodpressure,laminarflow,andstablegrowthfactorlevelspromoteabasalendothelialcellstatethatmaintainsanonthromboticsurfaceandappropriatevascularwallsmoothmuscletone.Injuryorexposuretocertainmediatorsresultsinendothelialactivation,astateinwhichendothelialcellsdevelopadhesive,procoagulantsurfacesandreleasefactorsthatleadtosmoothmusclecontractionand/orproliferationandmatrixsynthesis.