blood vessels.2
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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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