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SMS 1053SMS 1053

Dr. Mohanad R. AlwanDr. Mohanad R. Alwan

Blood vessels2

Learning Objectives:

Compare and contrast the structure and function of

Arteries

Veins

Capillaries

Blood Vessels

Closed circulatory system

Arteries Arterioles Capillaries Venules Veins 3 tunics Lumen

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The Vessels

Functions: Distribution of blood Exchange of materials with tissues Return of blood to the heart

Structure: Most have the same basic structure:

– 3 layers surrounding a hollow lumen

General Structure of Blood Vessels

Arteries and veins are composed of three tunics:

tunica interna tunica media tunica externa

Capillaries are composed of endothelium.

Generalized Structure of Blood Vessels

Arteries and veins are composed of three tunics – tunica interna, tunica media, and tunica externa

Lumen – central blood-containing space surrounded by tunics

Capillaries are composed of endothelium with sparse basal lamina

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The Anatomy Of Blood Vessels

Layers:1. Tunica interna (intima):

• Endothelial layer that lines the lumen of all vessels.• In vessels larger than 1 mm, a subendothelial connective tissue

basement membrane is present

2. Tunica media:• Smooth muscle and elastic fiber layer, regulated by sympathetic

nervous system• Controls vasoconstriction/vasodilation of vessels

3. Tunica externa (adventitia):• Collagen fibers that protect and reinforce vessels• Larger vessels contain vasa vasorum

General Structure

The Vessels

1. Tunica Intima Innermost smooth layer Simple squamous epithelium Continuous with the endocardium Present in all vessels

The Vessels

2. Tunica Media layer of smooth muscle - circular

arrangement – contains elastin

Supplied by sympathetic division of the ANS

Depending on body’s needs – lumen is narrowed (vasoconstriction) or widened (vasodilation)

The Vessels

3. Tunica Externa (Adventitia) Thin layer of CT Elastic & collagen fibres

The Vessels

Types of Vessels:

Arteries – carry blood away from the heart

Veins – carry blood towards the heart Capillaries – the most important part of

the vascular system; site of exchange of materials

The Blood Vessels and the Cardiovascular System

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Arteries: blood from heart Strong & Elastic Conduct blood to capillaries Sphincters

Capillaries: exchange with cells Veins

Return blood to heart Valves

Histological Structure of Blood Vessels

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Make Up of Blood Vessels: Arteries and Arterioles

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Endothelium Elastic tissues

Rebounds Evens flow

Smooth muscles

Fibrous tissue Tough Resists stretch

Figure 15-2: Blood vessels

Artery with thick wall16

Connective tissue

Muscle layer

Endothelium

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ELASTIC ARTERY (AORTA)     Stained with orsein1 - tunica intima2 - tunica media3 - tunica externa

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Types of Blood vessels: Arteries

Elastic Arteries: Thick-walled arteries near the heart; the aorta

and its major branches. Large lumen allows low-resistance conduction of

blood.

Contain lots of elastin in all three tunics.

walls stretch and recoil to propel blood

Withstand and regulate large blood pressure fluctuations.

Allow blood to flow fairly continuously through the body

Muscular (Distributing) Arteries and Arterioles

Muscular arteries – distal to elastic arteries; deliver blood to body organs Have thick tunica media with more smooth

muscle and less elastic tissue Active in vasoconstriction

Arterioles – smallest arteries; lead to capillary beds Control flow into capillary beds via

vasodilation and constriction

Types of Blood vessels: Arteries

Muscular (distributing) arteries medium sized vessels tunica media more smooth muscle;

less elastin major area of vaso-constriction &

dilation to regulate blood flow

The Vessels

Arterioles (diameter of 0.3 mm or less) smallest arteries; lead to capillary beds. close to capillaries - single layer of

muscle spiralling around the endothelial lining

regulates blood flow to capillary

General Structure

Venous System: Venules

Are formed when capillary beds unite Allow fluids and WBCs to pass from the

bloodstream to tissues Postcapillary venules – smallest venules,

composed of endothelium and a few pericytes

Large venules have one or two layers of smooth muscle (tunica media)

Venous System: Veins

Veins are: Formed when venules converge Composed of three tunics, with a thin tunica

media and a thick tunica externa consisting of collagen fibers and elastic networks

Capacitance vessels (blood reservoirs) that contain 65% of the blood supply

Venous System: Veins

Veins have much lower blood pressure and thinner walls than arteries

To return blood to the heart, veins have special adaptations Large-diameter lumens, which offer little

resistance to flow Valves (resembling semilunar heart valves),

which prevent backflow of blood

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

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Make Up of Blood Vessels:Veins and Venules (Contrasted to Arteries)

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Thinner walls Larger diameter Closer to skin Less muscle Less elastic

Figure 15-3: Metarterioles

Anatomy of Vessels29

Comparison of Veins and Arteries

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Arteries: Veins:

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•They also have semi-lunar valves to stop the blood flowing backwards

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•This is a medium sized vein, recognizable as such by its scanty wall and the presence of a valve inside it. •The valve flaps are marked by arrows.

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Artery and a vein

Vascular Anastomoses

Merging blood vessels, more common in veins than arteries

Arterial anastomoses provide alternate pathways (collateral channels) for blood to reach a given body region If one branch is blocked, the collateral

channel can supply the area with adequate blood supply

Thoroughfare channels are examples of arteriovenous anastomoses

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A capillary wall is very thin and composed of (endothelium only) single layer of cells as it does not have to withstand high internal pressure.A capillary wall is often highly permeable, partly because its very thin and partly because of holes in and between cells in some capillaries (particularly those with high demand of exchange e.g endocrine glands)

Capillaries

Capillaries

Capillaries are the smallest blood vessels Walls consisting of a thin tunica interna, one cell

thick Allow only a single RBC to pass at a time Pericytes on the outer surface stabilize their

walls There are three structural types of

capillaries: continuous, fenestrated, and sinusoids

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lumen

endothelium(one cell thick)

cell

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A capillary bed

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Make Up of Blood Vessels: Capillaries

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Figure 15-16: Types of capillaries

Continuous Capillaries49

Continuous capillaries are abundant in the skin and muscles, and have: Endothelial cells that provide an uninterrupted

lining Adjacent cells that are held together with tight

junctions Intercellular clefts of unjoined membranes that

allow the passage of fluids

Continuous Capillaries50

Continuous capillaries of the brain: Have tight junctions completely around the

endothelium Constitute the blood-brain barrier

Continuous Capillaries51

Figure 19.3a

Fenestrated Capillaries52

Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys)

Characterized by: An endothelium riddled with pores (fenestrations) Greater permeability to solutes and fluids than

other capillaries

Fenestrated Capillaries53

Figure 19.3b

Sinusoids54

Highly modified, leaky, fenestrated capillaries with large lumens

Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs

Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues

Blood flows sluggishly, allowing for modification in various ways

Sinusoids55

Capillary Beds56

A microcirculation of interwoven networks of capillaries, consisting of: Vascular shunts – metarteriole–thoroughfare

channel connecting an arteriole directly with a postcapillary venule

True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed

Capillary Beds

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Figure 19.4a

Capillary Beds

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Figure 19.4b

Blood Flow Through Capillary Beds59

Precapillary sphincter Cuff of smooth muscle that surrounds each true

capillary Regulates blood flow into the capillary

Blood flow is regulated by vasomotor nerves and local chemical conditions, so it can either bypass or flood the capillary bed

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Capillary Exchange

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Diffusion:

Filtration:

Reabsorption:

Capillary Exchange

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At the arteriole end of a capillary, water moves out of the blood due to the force of blood pressure.

At the venule end, water moves into the blood due to osmotic pressure of the blood.

Substances that leave the blood contribute to tissue fluid, the fluid between the body’s cells.

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In the midsection of the capillary, nutrients diffuse out and wastes diffuse

into the blood. Since plasma proteins are too large to

readily pass out of the capillary, tissue fluid tends to contain all components of plasma except it has lesser amounts of

protein. Excess tissue fluid is returned to the

blood stream as lymph in lymphatic vessels.

Capillary Exchange

Capillary exchange

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Capillary exchange

At the arterial end of a capillary, the blood pressure is higher than the osmotic pressure; therefore, water tends to leave the bloodstream.

In the midsection, oxygen and carbon dioxide follow their concentration gradients.

At the venous end of a capillary, the osmotic pressure is higher than the blood pressure; therefore, water tends to enter the bloodstream.

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Capillary Exchange

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Blood Flow in Capillaries67

Blood moves slowly in capillaries because there are more capillaries than arterioles.

This allows time for substances to be exchanged between the blood and tissues.

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Major arteries and veins of the systemic circuit

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