Histology and Functions of Blood Vessels

WS O

Department of Anatomy, HKU

Objectives:

• Describe the basic histological structures and functions of endocardium, myocardium, epicardium and the conducting system of the heart.

• Define the basic structure and function of blood vessels.

• Distinguish the major differences between various types of vessels: elastic and muscular arteries, arterioles, venules, veins.

• Describe the various types of capillaries and define their functions.

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Functions of the Circulatory System

1. Carry blood

2. Transport of hormones,

components of the immune system, molecules required for coagulation, enzymes, etc.

3. Exchange nutrients, waste products, and gases

4. Regulate blood pressure

5. Directs blood flow

Organization of Heart & Blood Vessels

• Heart wall can be viewed as a three-layered structure Inner layer = endocardium Middle Layer = myocardium Outer layer = epicardium (part

of the pericardium)

• Blood and lymphatic vessel walls (except for capillaries) can also be viewed as three-layered structures Inner layer = tunica intima Middle layer = tunica media Outer layer = tunica adventitia

Endocardium

Epicardium

Myocardium

Tunica adventitia

Tunica media

Tunica intima

Heart Wall has Three Layers

Cardiac Muscle

Intercalated Disc

Conducting system of the heart• Consists of modified cardiac muscle and conducting fibers that are specialized for initiating impulses & conducting them rapidly through the heart

• Made up of three components:- sinoatrial (SA) node serve as thepacemaker

- atrioventricular (AV) node receive the wave of excitation from the cardiac muscle of the atria and pass the excitation on to the bundle of His

- Purkinje fibers are organized into a branched bundle (Bundle of His)which extends from the atrio-ventricular (AV) node, through the interventricular septum down to the apex of the ventricles

Purkinje fibers

Cardiac muscle

Tunica intima

Tunica media

Tunica adventitia

1. Smooth muscle cells, collagen fibers

2. Elastic fenestrated lamellae

3. External elastic lamina

Blood Vessel Wall has three layers1. Endothelial cell lining

2. Subendothelial layer

3. Internal elastic membrane

1. Mostly collagen fibers

2. Elastic fibers (not lamellae)

3. Fibroblasts and macrophages

4. Vasa vasorum

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Vessel Wall Tunica intima

– lines the blood vessel and is exposed to blood

– endothelium – simple squamousepithelium overlying a basement membrane and a sparse layer of loose connective tissue• acts as a selectively permeable

barrier• secrete chemicals that stimulate

dilation or constriction of the vessel• normally repels blood cells and

platelets that may adhere to it and form a clot

• when tissue around vessel is inflamed, the endothelial cells produce cell-adhesion molecules that induce leukocytes to adhere to the surface

– causes leukocytes to congregate in tissues where their defensive actions are needed

Tunica intima

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Vessel Wall

Tunica media

– middle layer

– consists of smooth muscle, collagen, and elastic tissue

– strengthens vessel and prevents blood pressure from rupturing them

– vasomotion – changes in diameter of the blood vessel brought about by smooth muscle

Tunica media

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Vessel Wall

• tunica externa (tunica adventitia)

– outermost layer – consists of loose connective

tissue that often merges with that of neighboring blood vessels, nerves, or other organs

– anchors the vessel and provides passage for small nerves, lymphatic vessels

– vasa vasorum – small vessels that supply blood to at least the outer half of the larger vessels• blood from the lumen is

thought to nourish the inner half of the vessel by diffusion

Tunica adventitia

Thin-walled vessel Thick-walled vessel

Tunica intima Tunica intima

Tunica media Tunica media

Tunica externa

Tunica externa

Differences of Vessel Wall

Large artery

Medium artery

Medium vein

Ascending aorta

Pulmonary trunk (artery)

Arteries

Elastic artery

Tunica intima

Tunica media

Tunica externa

Tunica intimaInternal elastic laminaTunica mediaExternal elastic laminaTunica externa

Arteriole

Muscular artery

Tunica intimaTunica media

Tunica externa

Basement membrane

Endothelium

Capillary

Tunica intima

Valve

Tunica media

Tunica externa

Tunica intimaValve

Tunica media

Tunica externa

Tunica intima

Tunica mediaTunica externa

Venule

Small to medium-sized vein

Large vein

Superiorvena cava

Inferiorvena cava

Descendingaorta

Veins

Comparison of Blood vessels

Blood Pressure Changes With Distance

Ao

rta

Arterio

les

Cap

illaries

Ven

ules

120

100

80

60

40

20

0

Syst

em

ic b

loo

d p

ress

ure

(m

m H

g) Systolic pressure

Diastolicpressure

Large arteries

Small arteries

Ven

a cava

Large veins

Small vein

s

Distance from left ventricle

• Due to increase in overall cross-sectional area of vessels in branching

Artery

Artery

Tunica intima Tunica media Tunica externa

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Structural Features of Blood Vessels

• Arteries– Elastic– Muscular– Arterioles

• Capillaries: site of exchange with tissues

• Veins: thinner walls than arteries, contain less elastic tissue and fewer smooth muscle cells– Venules– Small veins– Medium or large veins

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Elastic Artery• Elastic or conducting arteries

– Largest diameters, pressure high and fluctuates between systolic and diastolic. More elastic tissue than muscle.

– Relatively thin tunica intima & tunica adventitia, thick tunica media (40-60 distinct concentrically arranged elastic laminae)

– Include aorta, pulmonary arteries, common carotid arteries, common iliac arteries

Elastic arteries. Elastic laminae in the Tunica media gives these vessels elastic properties. They expand as the heart contracts (to modulate BP and store energy) and recoil during ventricular relaxation (to maintain more even pressure in large arteries)

Elastic Artery

Muscular Artery• Muscular or medium or distributing arteries

– Smooth muscle allows vessels to regulate blood supply by constricting or dilating

– Thick tunica media due to 25-40 layers of smooth muscle. Thin intima– Also called distributing arteries because smooth muscle allows vessels

to partially regulate blood supply to different regions of the body.– Mostly named arteries, such as brachial & femoral arteries but also

include some smaller unnamed arteries

• Smaller muscular arteries– Adapted for vasodilation and vasoconstriction.

External elastic membrane

Internal elastic membrane

Prominent in muscular artery

Muscular arteries. The thick muscular layer in the tunica media resists damage due to relatively high BP in these vessels. They regulate blood flow to different regions of the body.

Elastic artery(tunica media – elastic fibers)

Muscular artery(tunica media – smooth muscles)

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Arterioles

• Transport blood from small arteries to capillaries

• Smallest arteries where the three tunics can be differentiated

• Tunica media-one or more layers of smooth muscle cells

• Like small arteries, capable of vasoconstriction and dilation- metarterioles are short vessels that

link arterioles to capillaries- contain small muscle cells formprecapillary sphincters~ control blood flow to capillary bed~ alter BP by altering peripheral

resistance to blood flow –“peripheral resistance vessels”

Tunica media

Tunica adventitia

Endothelial cell

Arteriole

Arteriole H&E

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Capillaries• Thin walled single endothelial cells (simple squamous epithelium),

basement membrane and a delicate layer of loose C.T. • Specialized for exchange of substances -Many pinocytic vesicles

• Pericytes:– have their own basement membrane– may develop into smooth muscle cells (capillary → arteriole)

• Presence of tight-junction between endothelial cells – Blood-brain Barrier

• Tight junction is complete in brain capillaries • Very few or absence of pinocytic vesicles

Pericyte

Three Types of Capillaries

Sinusoids or Sinusoidal capillary

Pericyte

Erythrocyte

Basallamina

Intercellularcleft

Pinocytoticvesicle

Endothelialcell

Tightjunction

Continuous capillaries

• Have relatively think cytoplasmand the capillary wall continuous

• Presence of tight junctions between adjacent cells

• Less permeable to large molecules than other capillary types

• Cytoplasm contains pinocytoticvesicles

• Found in muscle, nervous tissue

Continuous capillary

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Fenestrated CapillaryCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Erythrocyte

Endothelialcells

Filtration pores(fenestrations)

Basallamina

Intercellularcleft

Nonfenestratedarea

• Have thin cytoplasm and capillary wall is perforated at intervals by pores (fenestrations)

• Materials can cross the cellthrough the fenestrations

• Found in kidney &endocrine glands

Fenestrated Capillary

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Sinusoid in LiverCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Macrophage

Sinusoid

Microvilli

Endothelialcells

Erythrocytesin sinusoid

Liver cell(hepatocyte)

• Larger in diameter than theother types

• have wide spaces betweenedges of adjacent cells

• Materials can move freelyin and out

• Found in liver, spleen, &bone marrow

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Aging of the Arteries

• Arteriosclerosis:general term for degeneration changes in arteries making them less elastic

• Atherosclerosis:deposition of plaque on walls

Vein

Vein

Tunica intima Tunica media Tunica externa

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Venules and Small Veins

• Venules drain capillary network. Endothelial cells and basement membrane with a few smooth muscle cells. As diameter of venulesincreases, amount of smooth muscle increases.

• Small veins. Tunica intima is thin; tunica media is thin with circular smooth muscle. Smooth muscle cells form a continuous layer. Tunica adventitia is well developed and is made of longitudinally arranged collagenouselastic fibers

Medium vein

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Medium and Large Veins

• Medium veins. Go-between between small veins and large veins.

• Large veins. Tunica intima is thin: endothelial cells, relatively thin layer of C.T and a few scattered elastic fibers. Tunica media is thin and poorly developed. Adventitia is thick and a predominant layer with spirally arranged collagen fibers, elasticlamellae, longitudinal smooth muscle

Large Vein

Veins

Comparison of Major Features of Artery & Veins

Arteries – carrying blood at high pressure• Smooth muscle and/or elastic

fibers/lamellae predominate over collagen fibers in tunica media

• Elastic fibers/lamellae are highlydeveloped in the tunica media

• Smooth muscles and other componentsare arranged circularly in tunica media –allows change in diameter to regulate blood pressure and blood flow

• No valves Veins - carrying blood at very low pressure• Collagen fibers are relatively abundant among

the smooth muscles in the tunica media• Collagen fibers and othe components arranged

longitudinally - prevent excess stretching of vessel wall

• Valves present; to prevent backflow

Lymphatic Vessels

• Larger than blood capillaries• Very irregular shaped• Basal lamina is almost absence• Endothelial cells do not form tight

junctions (allow entry of flluid)• Larger lymph vessels contain valves

A villus of the jejunum containin a lymph capillary. A lacteal is a lymphatic capillary that absorbs dietary fats in the villi of small intestien.

References

1. Hole’s Human Anatomy & Physiology, 13th Edition, David Shier, McGraw-Hill

2. Seeley’s Principles of Anatomy & Physiology, 2nd Edition, Philip Tate, McGraw-Hill

3. Anatomy & Physiology with Integrated Study Guide, 5th

Edition, Stanley E Gunstream, McGraw-Hill4. Histology, A Text and Atlas, 6th Edition, Michael H Ross &

Wojciech Pawlina, Wolters Kluwer/Lippincott Williams & Wilkins