الرحيم الرحمن الله الرحيم بسم الرحمن الله بسم

HISTOLOGY OF THE BLOOD VESSELS

HISTOLOGY OF HISTOLOGY OF THE BLOOD VESSELSTHE BLOOD VESSELS

By the end of this lecture, the student should be able to identify and describe the microscopic structure of the wall of the blood vessels including:

a. Elastic arteries.a. Elastic arteries.

b. Muscular (medium-sized) arteries.b. Muscular (medium-sized) arteries.

c. Medium-sized veins.c. Medium-sized veins.

d. Blood capillaries.d. Blood capillaries.

Blood vessels:Blood vessels:– Arteries:Arteries:

• Elastic artery.Elastic artery.

• Muscular (distributing) (medium-sized) artery.Muscular (distributing) (medium-sized) artery.

• Arterioles.Arterioles.

– Blood capillaries.Blood capillaries.

– Veins:Veins:

• Venules.Venules.

• Small veins.Small veins.

• Medium-sized veins.Medium-sized veins.

• Large veins.Large veins.

General Structure of Blood VesselsGeneral Structure of Blood Vessels

The wall of blood The wall of blood vessel is formed of vessel is formed of three concentric three concentric layers:layers: Tunica intima Tunica intima

(interna)(interna) Tunica mediaTunica media Tunica adventitia Tunica adventitia

(externa)(externa)

Tunica IntimaTunica Intima Is the innermost layerIs the innermost layer Composed of:Composed of:

Single layer of flattened Single layer of flattened endothelial cellsendothelial cells (resting on (resting on the the basal laminabasal lamina) lining the ) lining the lumen of the vessel lumen of the vessel

Subendothelial layerSubendothelial layer made made up of loose connective up of loose connective tissue. May have few tissue. May have few longitudinally arranged longitudinally arranged smooth muscle fiberssmooth muscle fibers

Beneath the subendothelial Beneath the subendothelial layer is an layer is an internal elastic internal elastic laminalamina, composed of elastin , composed of elastin (fenestrated elastic sheet), (fenestrated elastic sheet), separating the tunica intima separating the tunica intima from the tunica mediafrom the tunica media

Tunica MediaTunica Media Intermediate layerIntermediate layer Composed of Composed of smooth smooth

musclesmuscles, some elastic fibers, , some elastic fibers, type III collagen (reticular type III collagen (reticular fibers) and type I collagen.fibers) and type I collagen.

Large muscular arteries have Large muscular arteries have external elastic laminaexternal elastic lamina, , separating the tunica media separating the tunica media from the tunica adventitiafrom the tunica adventitia

Capillaries and postcapillary Capillaries and postcapillary venules do not have a tunica venules do not have a tunica media, however, media, however, pericytes pericytes replace the tunica media. replace the tunica media.

Tunica AdventitiaTunica Adventitia Outermost layerOutermost layer Composed of Composed of connective tissueconnective tissue containing containing

types I & III collagen, fibroblasts and types I & III collagen, fibroblasts and longitudinal elastic fiberslongitudinal elastic fibers

Blends into the surrounding connective Blends into the surrounding connective tissue.tissue.

N.B. N.B. Vasa vasorumVasa vasorum::are small arterioles in tunica adventitia are small arterioles in tunica adventitia

and the outer part of tunica media.and the outer part of tunica media.

They are more prevalent in the They are more prevalent in the

walls of veins than arteries – why?walls of veins than arteries – why?

Venous blood contains less oxygen and nutrients than Venous blood contains less oxygen and nutrients than arterial blood.arterial blood.

ELASTIC ARTERIESELASTIC ARTERIES

Examples: aorta, common carotid a., Examples: aorta, common carotid a., subclavian a., common iliac aa, pulmonary subclavian a., common iliac aa, pulmonary Trunk.Trunk.

Microscopic structure:Microscopic structure:

1.1.T. Intima:T. Intima:

*Endothelium.*Endothelium.

*Subendothelial C.T. *Subendothelial C.T.

*Internal elastic lamina:*Internal elastic lamina:

(not prominent)(not prominent)

(indistinct)(indistinct)

ELASTIC ARTERIES (Cont.)ELASTIC ARTERIES (Cont.)

2.2. T. Media: T. Media: it consists of:it consists of:A.A. Fenestrated elasticFenestrated elastic

membranes (sheets) (lamellae):membranes (sheets) (lamellae):The main component of T.M.The main component of T.M.

B. In between, there are:B. In between, there are:1.1. Smooth muscle cells: areSmooth muscle cells: are

less abundant & secreteless abundant & secrete

all other components in T.M.all other components in T.M.

2.2. Collagen fibers (type I collagen). Collagen fibers (type I collagen).

3.3. Reticular fibers (type III collagen). Reticular fibers (type III collagen).

4.4. Elastic fibers. Elastic fibers.

C. External elastic laminaC. External elastic lamina

ELASTIC ARTERIES (Cont.)ELASTIC ARTERIES (Cont.)

3.3. T. Adventitia:T. Adventitia:– Much thinner than T.M.Much thinner than T.M.– It is composed of loose C.T.It is composed of loose C.T.– Contains vasa vasorum → send branches to Contains vasa vasorum → send branches to

the outer part of T.M.the outer part of T.M.

MUSCULAR ARTERIESMUSCULAR ARTERIES(Medium-sized artery)(Medium-sized artery)

Examples: brachial, ulnar, renal.Examples: brachial, ulnar, renal. Microscopic structure:Microscopic structure:

1.1.T. Intima.:T. Intima.:– Endothelium.Endothelium.– Subendothelial C.T. layer.Subendothelial C.T. layer.– Internal elastic lamina:Internal elastic lamina:

• Is prominent.Is prominent.• Displays an undulating surface.Displays an undulating surface.• Occasionally it is duplicated (bifid internal elastic Occasionally it is duplicated (bifid internal elastic

lamina).lamina).

MUSCUALR ARTERIES (Cont.)MUSCUALR ARTERIES (Cont.)

2.2. T. Media.:T. Media.:

Thicker than T. AdventitiaThicker than T. Adventitia

or similar in thickness. or similar in thickness. A.A. Smooth muscle cells (SMCs): Smooth muscle cells (SMCs):

are the predominant component.are the predominant component.

B.B. In between there are: In between there are:• Elastic fibers.Elastic fibers.• Type III collagen fibers.Type III collagen fibers.• Type I collagen fibers.Type I collagen fibers.

C.C. External elastic lamina: may be identifiable. External elastic lamina: may be identifiable.

3.3. T. Adventitia.: loose C.T. T. Adventitia.: loose C.T.

MEDIUM-SIZED VEINMEDIUM-SIZED VEIN Thickness of the wall: thinner than the Thickness of the wall: thinner than the

accompanying artery.accompanying artery. T. Intima: no internal elastic laminaT. Intima: no internal elastic lamina T. Media:T. Media:

– Thinner than T. AdventitiaThinner than T. Adventitia– Consists of:Consists of:

• Fewer SMCs.Fewer SMCs.• Types I & III Collagen fibers.Types I & III Collagen fibers.

T. Adventitia: thicker than T. Media T. Adventitia: thicker than T. Media

MEDIUM-SIZED ARTERYMEDIUM-SIZED ARTERYAND VEINAND VEIN

VALVES OF VEINSVALVES OF VEINS Valve of a vein is composed of 2 leafletsValve of a vein is composed of 2 leaflets Each leaflet has a thin fold of the T. Intima.Each leaflet has a thin fold of the T. Intima. Components:Components:

– EndotheliumEndothelium– Core of C.T.Core of C.T.

BLOOD CAPILLARIESBLOOD CAPILLARIES

Diameter: usually 8-10 µm.Diameter: usually 8-10 µm. Microscopic structure:Microscopic structure:

1.1. Single layer of squamous endothelial cells.Single layer of squamous endothelial cells.

2.2. Basal lamina: surrounds the external surface Basal lamina: surrounds the external surface of the endothelial cells.of the endothelial cells.

3.3. Pericytes:Pericytes:• Have processes.Have processes.• Share the basal lamina of the endothelial cells.Share the basal lamina of the endothelial cells.

BLOOD CAPILLARIESBLOOD CAPILLARIESTypes:Types:

1- 1- Continuous blood capillariesContinuous blood capillaries

2- 2- Fenestrated blood capillariesFenestrated blood capillaries

a- with diaphragmsa- with diaphragms

b- without diaphragmsb- without diaphragms

3- 3- Sinusoidal blood Sinusoidal blood

capillariescapillaries

Continuous Blood CapillariesContinuous Blood Capillaries

Microscopic structure:Microscopic structure:– No pores or fenestrae in their walls.No pores or fenestrae in their walls.

Distribution:Distribution:– In muscles,In muscles,

nervous T., C.T.nervous T., C.T.

Fenestrated Blood Capillaries Fenestrated Blood Capillaries withwith DiaphragmsDiaphragms

Microscopic structure:Microscopic structure:– Their walls have pores (fenestrae).Their walls have pores (fenestrae).– These pores are covered by a poreThese pores are covered by a pore

diaphragm.diaphragm.

Distribution:Distribution:– In intestine, pancreas and endocrineIn intestine, pancreas and endocrine

glands.glands.

Fenestrated Blood CapillariesFenestrated Blood Capillaries without without DiaphragmsDiaphragms

Distribution:Distribution:– In renal glomerulus.In renal glomerulus.

FENESTRATED FENESTRATED CAPILLARYCAPILLARY

WITHWITH DIAPHRAGMS DIAPHRAGMS

Endothelium

FENESTRATED FENESTRATED CAPILLARYCAPILLARYWITHOUT WITHOUT

DIAPHRAGMSDIAPHRAGMS

Lumen of glomerularBlood capillary

Podocyte

SINUSOIDAL CAPILLARIESSINUSOIDAL CAPILLARIES Diameter: 30-40 µm.Diameter: 30-40 µm. Microscopic features:Microscopic features:

– They possess many large They possess many large

fenestrae without diaphragms.fenestrae without diaphragms.– They possess discontinuous endothelial cells.They possess discontinuous endothelial cells.– They possess discontinuous basal lamina.They possess discontinuous basal lamina.– Macrophages may be located in or along the Macrophages may be located in or along the

outside of the endothelial wall.outside of the endothelial wall. Distribution:Distribution:

– Red bone marrow, liver, spleen and certain Red bone marrow, liver, spleen and certain endocrine glands.endocrine glands.

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