Chapter 21
Peripheral
Circulation
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Arteries – Capillaries - Veins
General pattern of blood flow from heart to body tissues & back to
heart…
Arteries - leaving heart (generally O2 rich)
Capillary networks/beds - in tissues
Veins – return to heart (generally O2 poor)
exception: in pulmonary arteries/veins O2 status is reversed
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Arteries – General Info
As distance from heart increases, arteries
decrease in size
increase in branching
Types of arteries include:
Elastic  largest & closest to heart
Muscular  mid-range
Arterioles  smallest & furthest from heart
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Veins – General Info
As distance from heart decreases, merge to form larger veins thus:
increase in size
decrease in branching
Types of veins include:
venules  smallest & closest to capillary bed
small veins
medium veins
large veins
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Capillaries (simplest vessels)
ALL blood vessels have an inner lining of simple squamous epithelium.
(endothelium)
This innermost lining resting on a supporting basement membrane is
the entire structure for capillaries
Size of typical capillary = 7 – 9 μm
Interior lumen is sufficiently large to allow RBCs to pass single file
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Types of Capillaries
Continuous – no gaps in endothelium least permeable type
Fenestrated (windowed) – these have numerous gaps (fenestrae) in
capillary walls 70-100 μm in diameter (of gap). found in kidney,
choroid plexus of brain, intestine
Sinusoidal – largest type… also have fenestrae. Basement membrane
is reduced or absent.
Venous sinuses – specialized regions in spleen even larger than
sinusoidal capillaries – very large gaps in endothelium
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Typical Capillary Bed
Blood approaches the capillary bed in arterioles (smallest arteries)
Metarterioles & Thoroughfare channels
Entry is controlled by precapillary sphincters as blood flows into:
arterial capillaries  venous capillaries
venules
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Typical Blood Vessel Wall
Several layers (tunics) stacked in a tube within tube pattern
Tunica Adventitia – outermost layer – connective tissue
Tunica Media – muscle, elastic & collagenous tissues
(vasoconstriction & vasodilation occur here)
Tunica Intima – inner layer – may have some muscle & connective
tissue. Basement membrane supporting the inner lining of epithelium
(endothelium)
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Artery Wall Varies With Type
Elastic arteries – often called conducting arteries have less muscle &
more elastic fibers
designed to withstand changes in pressure
blood pressure here varies with systolic/diastolic fluctuations
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Very thin
Elastic network (more than other arteries)
some smooth muscle (less than other arteries)
Relatively thick
internal & external elastic membranes
not distinct layers (they’ve merged)
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Elastic Artery
Artery Wall Varies With Type
Muscular arteries – often called distributing arteries
Walls are quite thick primarily because of increased size of tunica
media (25-40 layers of smooth muscle)
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Thick T-Adventitia blending with
surrounding connective tissue
Thick tunica media
(25 – 40 layers smooth muscle)
Distinct internal elastic membrane
(unlike in elastic artery)
Muscular Artery
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Vein Walls
Veins are thinner than arteries.
Tunica intima consists of endothelium & a thin basement layer with a
sparse layer of elastic fibers
Tunica media has much less muscle than found in arteries
Tunica adventitia (connective tissue) is the predominant layer
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Valves within veins
Valves consisting of folded tunica intima prevent flow of blood away
from heart (backward) in veins.
These occur predominantly in medium sized & large veins especially in
lower extremeties
Varicose veins result when these valves fail and allow blood to flow
backward.
Phlebitis – inflamed veins resulting from damaged veins allowing
buildup of blood
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Aging & Damage to Arteries
Arteriosclerosis – degenerative changes to arteries rendering them less
elastic (‘hardening’)
Atherosclerosis – plaque deposits along walls of arteries. Initially a
fatty (cholesterol) deposit forms which can later be replaced with dense
connective tissue & may mineralize with calcium deposits.
Atherosclerosis – once a disease of middle age & elderly is now
occurring at MUCH younger ages as a result of the “convenience food”
American diet
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MOVIE
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BREAK
Two Circulatory Pathways
Systemic Circulation – supplies oxygenated blood to the body’s tissues
Pulmonary Circulation –reoxygenates depleted blood
Pulmonary trunk delivers O2 poor blood to lungs
Pulmonary veins (return O2 rich blood to heart)
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Aorta
Entryway from heart to systemic circulation
Ascending aorta – immediately off of heart
to upper right quadrant of body & right side of head
Aortic arch – left side of head & upper left quadrant of body
Descending aorta – proceeds downward through thoracic cavity & into
abdomen supplying all of body below heart
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Hepatic Portal System
Blood from most of the viscera (stomach, intestines, spleen) enters a
drainage system which passes through the liver.
Veins from these organs merge to form the Hepatic Portal Vein
(portal = door – ‘doorway to liver’)
Upon reaching the liver this blood is filtered in sinusoids (enlarged
capillaries) nutrients stored or modified. Blood detoxified
Liver drains (upward) as sinusoids merge to central veins then hepatic
veins. Hepatic veins join the inferior vena cava which enters heart
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Lymphatic System
At the capillary beds we have already examined, fluids leak out to the
tissues carrying O2 and nutrients
Much of this fluid returns to the circulation by way of the lymphatic
system.
Associated with the capillary beds are a second set of vessels, the
lymphatic capillaries.
These collect the fluid (lymph) leaked from the circulatory system and
drain it back toward the heart.
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Lymphatic Capillary Structure
Similar to circulatory capillary but some differences
lack basement membrane
simple squamous cells with overlapping edges are loosely joined
one way travel (only toward heart)
internal valves (similar to venous valves) to permit one-way travel
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Lymphatic Drainage
Lymph capillaries merge to form lymph vessels similar to veins
Structure:
elastic membrane around the endothelium (inner layer)
smooth muscle & elastic fibers (mid layer)
fibrous connective tissue (outer layer)
Movement of lymph influenced by:
contraction of lymph vessel smooth muscle
contraction of surrounding skeletal muscle
changing pressure in thorax due to breathing
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Lymph Nodes
Oval structures located in the lymph vessels which filter lymph
All fluid passing toward heart passes through at least one node
Cells here include macrophages (phagocytes) which detect & engulf
invading antigens, & lymphocytes (site of lymphocyte division)
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Thoracic Duct
The major collecting vessel for lymph from the lower half of the body
& left side of upper body.
This ascends alongside the abdominal/thoracic aorta & finally joins the
left subclavian vein to return lymph to the circulation.
In the upper abdomen this vessel is enlarged and is often referred to as
the cysterna chyli (cistern = tank)
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Right Lymphatic Duct
Collects lymph from the upper right quadrant of the body
enters right subclavian vein
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Laminar vs Turbulent Blood Flow
In an unobstructed blood vessel blood tends to flow in a laminar
fashion.
several layers of blood at different speeds:
Blood near center of vessel flows fastest
Blood against the inner wall of the vessel flows slower
Turbulence occurs when the flow is obstructed (as by a plaque)
This turbulence causes vibrations which can be heard (stethoscope)
these turbulent sites may indicate a site of potential thromboses
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Blood Pressure
The force exerted against vessel walls
measured in mm mercury (mm Hg)
Blood pressure cuff connected to a sphygmomanometer (pressure
sensing device. Listen with stethoscope.
At high pressure from cuff no sound is heard… (blood flow restricted)
as pressure releases slowly
First sound heard at systolic pressure. (blood pressure high enough to
overcome cuff pressure)
Following this, hear vibrations due to turbulent flow (Korotkoff’s
sounds)
Sound ceases at diastolic pressure
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