BLOOD VESSELS

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© 2013 Pearson Education, Inc.

Blood Vessels

• Delivery system of dynamic structures that begins and ends at heart– Arteries: carry blood away from heart;

oxygenated except for pulmonary circulation and umbilical vessels of fetus

– Capillaries: contact tissue cells; directly serve cellular needs

– Veins: carry blood toward heart

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Artery

Vein

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Structure of Blood Vessel Walls

• Lumen– Central blood-containing space

• Three wall layers in arteries and veins– Tunica intima, – Tunica media, and – Tunica externa

• Capillaries– Endothelium with sparse basal lamina

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Tunica intima• Endothelium• Subendothelial layer• Internal elastic membrane

Tunica media(smooth muscle and elastic fibers)• External elastic membrane

Valve

Tunica externa(collagen fibers)

Lumen

Artery Capillary network

Lumen

Vein

Endothelial cells

Capillary

Basement membrane

• Vasa vasorum

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Tunics

• Tunica intima– Endothelium lines lumen of all vessels

• Continuous with endocardium• Slick surface reduces friction

– Subendothelial layer in vessels larger than 1 mm; connective tissue basement membrane

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Tunics

• Tunica media– Smooth muscle and sheets of elastin– Sympathetic vasomotor nerve fibers control

vasoconstriction and vasodilation of vessels

• Influence blood flow and blood pressure

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Tunics

• Tunica externa (tunica adventitia)– Collagen fibers protect and reinforce; anchor

to surrounding structures – Contains nerve fibers, lymphatic vessels

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Venous system Arterial system

Large veins(capacitancevessels)

Largelymphaticvessels

Elasticarteries(conductingarteries)

Musculararteries(distributingarteries)

Lymphnode

Lymphaticsystem

Small veins(capacitancevessels)

Arteriovenousanastomosis

Lymphaticcapillaries

Sinusoid

Arterioles(resistancevessels)

Terminalarteriole

MetarteriolePrecapillarysphincter

Capillaries(exchangevessels)

Thoroughfarechannel

Postcapillaryvenule

Heart

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Arterial System: Elastic Arteries

• Large thick-walled arteries with elastin in all three tunics

• Aorta and its major branches

• Large lumen offers low-resistance

• Act as pressure reservoirs—expand and recoil as blood ejected from heart– Smooth pressure downstream

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Arterial System: Muscular Arteries

• Distal to elastic arteries– Deliver blood to body organs

• Thick tunica media with more smooth muscle

• Active in vasoconstriction

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Arterial System: Arterioles

• Smallest arteries

• Lead to capillary beds

• Control flow into capillary beds via vasodilation and vasoconstriction

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© 2013 Pearson Education, Inc.

Capillaries

• Microscopic blood vessels

• Walls of thin tunica intima– In smallest one cell forms entire

circumference

• Pericytes help stabilize their walls and control permeability

• Diameter allows only single RBC to pass at a time

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Capillaries

• In all tissues except for cartilage, epithelia, cornea and lens of eye

• Provide direct access to almost every cell

• Functions– Exchange of gases, nutrients, wastes,

hormones, etc., between blood and interstitial fluid

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Capillaries

• Three structural types1. Continuous capillaries

2. Fenestrated capillaries

3. Sinusoid capillaries (sinusoids)

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Continuous Capillaries

• Abundant in skin and muscles– Intercellular clefts allow passage of fluids and

small solutes

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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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Fenestrated Capillaries

• Some endothelial cells contain pores (fenestrations)

• Permeable

• Function in absorption or filtrate formation (small intestines, endocrine glands, and kidneys)

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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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Sinusoid Capillaries

• Fewer tight junctions; usually fenestrated; larger intercellular clefts; large lumens

• Blood flow sluggish – allows modification– Large molecules and blood cells pass

between blood and surrounding tissues

• Found only in the liver, bone marrow, spleen, adrenal medulla

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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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Blood Flow Through Capillary Beds

• Precapillary sphincters regulate blood flow into true capillaries– Blood may go into true capillaries or to shunt

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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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Venous System: Venules

• Formed when capillary beds unite– Smallest postcapillary venules– Very porous; allow fluids and WBCs into

tissues– Consist of endothelium and a few pericytes

• Larger venules have one or two layers of smooth muscle cells

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Veins

• Formed when venules converge

• Have thinner walls, larger lumens compared with corresponding arteries

• Blood pressure lower than in arteries

• Thin tunica media; thick tunica externa of collagen fibers and elastic networks

• Called capacitance vessels (blood reservoirs); contain up to 65% of blood supply

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Pulmonary bloodvessels 12%

Systemic arteriesand arterioles 15% Heart 8%

Capillaries 5%

Systemic veinsand venules 60%

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Veins

• Adaptations ensure return of blood to heart despite low pressure– Venous valves prevent backflow of blood

• Most abundant in veins of limbs

– Venous sinuses: flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)

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© 2013 Pearson Education, Inc.

Artery

Vein

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Systemic Blood Pressure

• Pumping action of heart generates blood flow

• Pressure results when flow is opposed by resistance

• Systemic pressure– Highest in aorta– Declines throughout pathway– 0 mm Hg in right atrium

• Steepest drop occurs in arterioles

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Systolic pressure

Mean pressure

Diastolicpressure

120

100

80

60

40

20

0

Aor

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Art

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Art

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Cap

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Ven

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Vei

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Ven

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Blood pressure (mm Hg)