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Figure 18.1a Generalized structure of arteries, veins, and capillaries.

Artery

Vein

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Tunica intima

• Internal elastic membraneTunica media(smooth muscle and elastic fibers)• External elastic membrane

Tunica externa(collagen fibers)

Lumen

Artery

Capillarynetwork

Lumen

Vein

Endothelial cellsCapillary

Basement membrane

• Vasa vasorum

Figure 18.1b Generalized structure of arteries, veins, and capillaries.

Valve

• Endothelium• Subendothelial layer

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Figure 18.2 The relationship of blood vessels to each other and to lymphatic vessels.Venous system Arterial system

Large veins(capacitancevessels)

Largelymphaticvessels

Elasticarteries(conductingarteries)

Musculararteries(distributingarteries)

LymphnodeLymphaticsystem

Small veins(capacitancevessels) Arteriovenous

anastomosis

Lymphaticcapillaries

Sinusoid Arterioles(resistancevessels)Terminalarteriole

MetarteriolePrecapillarysphincter

Capillaries(exchangevessels)

Thoroughfarechannel

Postcapillaryvenule

Heart

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Figure 18.3a Capillary structure.

Pericyte

Red bloodcell in lumen

Intercellularcleft

Endothelialcell

Basementmembrane

Tight junctionEndothelialnucleus

Pinocytoticvesicles

Continuous capillary. Least permeable, and most common (e.g., skin, muscle).

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Figure 18.3b Capillary structure.

Pinocytoticvesicles

Red bloodcell in lumen

Fenestrations(pores)

Intercellularcleft

Endothelialcell

EndothelialnucleusBasement membrane

Tight junction

Fenestrated capillary. Large fenestrations (pores) increase permeability. Occurs in areas of active absorption or filtration (e.g., kidney, small intestine).

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Figure 18.3c Capillary structure.

Endothelialcell

Red bloodcell in lumen

Largeintercellularcleft

Nucleus ofendothelialcell

Incompletebasementmembrane

Sinusoid capillary. Most permeable. Occurs in speciallocations (e.g., liver, bone marrow, spleen).

Tight junction

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Chapter Opener 18

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Figure 18.4 Anatomy of a capillary bed.Vascular shunt

Precapillary sphinctersMetarteriole Thoroughfare

channel

Terminal arteriole

Truecapillaries

Postcapillary venule

Sphincters open—blood flows through true capillaries.

Terminal arteriole Postcapillary venule

Sphincters closed—blood flows through metarteriole – thoroughfare channel and bypasses true capillaries.

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Figure 18.5 Relative proportion of blood volume throughout the cardiovascular system.

Pulmonary bloodvessels 12%

Systemic arteriesand arterioles 15% Heart 8%

Capillaries 5%

Systemic veinsand venules 60%

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Figure 18.7 The muscular pump.

Venous valve (open)

Contracted skeletalmuscle

Venous valve(closed)

Vein

Direction of blood flow