heart & blood vessels histology by dr. nabil, khouri
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Heart & Blood Vessels Histology
By Dr. Nabil, Khouri
The Cardiovascular System IV
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The Heart Wall
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The myocardium. Note the endocardial layer, which consists of endothelium supported by a
rather thick layer of sub-endocardial connective tissue (green). The ventricular lumen is indicated.
Lumen
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Cardiac muscle orientation
The tissues that comprise the myocardium, as well as the adjacent tissues of the endocardium and pericardium, are continuous, which means that the cardiac muscle is one single tissue that wraps around itself to form the heart.
The myocardial tissue in normal heart spirals up from the base to the apex, causing a series of clear intersections of cardiac muscle tissue.
The double spiral formation of the myocardial tissue allows a 60% increase in ejection fraction with a fiber shortening of 15%.
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Different Cardiac muscle layers & Orientations
Tricuspid & bicuspid Valves
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Cardiac Muscle Longitudinal Section
Cardiac muscle consists of Branched muscle cells with one centrally placed nucleus.
The Nuclei are oval, rather pale and located centrally in the muscle cell which is 10 - 15 µm wide.
Cardiac muscle exhibits cross-striations.
Cardiac muscle is for these reasons also called involuntary striated muscle.
X40 Magnification
cell nucleus
Intercalated Discs
One cell
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Cardiac Muscle
N
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Cardiac Muscle Cross section & Purkinje fibers
X40 Magnification
X20 Magnification
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This medium power view of the inner wall of the left ventricle shows a cluster of Purkinje fibers, that are partof the conduction system of the heart. The fibers run within the subendocardial connective tissue.
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At high power, the structure of individual Purkinje fibers can be seen. Much larger than normal cardiac muscle fibers, these cells still
contain centrally-placed nuclei and, although they do still contain myofibrillar elements, their cytoplasm appears somewhat vacuolated
Purkinje fibers
Cardiac Cells
Sub-endocardial CT
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Note the coronary vessels surrounded by epicardial fat. Can you determine which are arteries and which are veins
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Blood Vessels
Blood is carried in a closed system of vessels that begins and ends at the heart
The three major types of vessels are arteries, capillaries, and veins
Arteries carry blood away from the heart, veins carry blood toward the heart
Capillaries contact tissue cells and directly serve cellular needs
www.lab.anhb.uwa.edu.au/.../Vascular.htm
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Generalized Structure of Blood Vessels
Arteries and veins are composed of three tunics –
tunica externa tunica media tunica interna
Capillaries are composed of endothelium with sparse basal lamina
Lumen – central blood-containing space surrounded by tunics
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Tunics Tunica interne (tunica intimae)
Endothelial layer that lines the lumen of all vessels
In vessels larger than
1 mm, a sub-endothelial
connective tissue basement
membrane is present Tunica media
Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system
Controls vasoconstriction/ vasodilatation of vessels
Tunica externa (tunica adventitia) Collagen fibers that protect
and reinforce vessels Larger vessels contain vasa
vasorum
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Histological Structure of Blood Vessels
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Large Elastic (Conducting) Arteries
Thick-walled arteries near the heart; the aorta and its major branches Large lumen allow low-resistance conduction of blood Contain internal elastic lamina and fibers in all three
tunics Withstand and smooth out large blood pressure
fluctuations Allow blood to flow fairly continuously through the body
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Elastic (Conducting) Arteries
Tunica intima: Large arteries often have a large subendothelial layer. The border of the
intima is delineated by the internal elastic membrane. The internal elastic membrane may not be conspicuous because of the abundance of elastic material in the tunica media.
Tunica media:Both connective tissue and smooth muscle are present in the intima. It is
the thickest of the three layers. The smooth muscle cells are arranged in a spiral around the long axis of the vessel. They secrete elastin in the form of sheets, or lamellae, which are fenestrated to facilitate diffusion. These lamellae, and the large size of the media, are the most striking histological feature of elastic arteries. In addition to elastin, the smooth muscle cells of the media secrete reticular and fine collagen fibers and proteoglycans (all not identifiable). No fibroblasts are present.
Tunica adventitia:This is a relatively thin connective tissue layer. Fibroblasts arethe
predominant cell type, and many macrophages are also present. Collagen fibres predominate and elastic fibres (not lamellae) are also present. Blood vessels supplying the adventitia and outer media are also present, these are called vasa vasorum ("vessels of the vessels").
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{Media
Adventitia
{Intima
{
Vasa vasorumSmooth muscle cells
Elastic laminae
Endothelium
ELASTIC ARTERY
Collagen
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Elastic artery Aorta
el = elastic lamellaeend = endothelial cell nuclein = smooth muscle cell nucleiTA = tunica adventitiaTI = tunica intimaTM = tunica media
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Elastic fibers
Lumen
Endothelium
vasa vasorum
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Muscular 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
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Medium Arteries: Tunica intima:
The tunica intima is thinner than in large arteries, there are fewer smooth muscle cells and less elastic tissue. The outermost part of the intima is defined by a very prominent internal
elastic membrane (not obscured by elastic lamellae as in large arteries). The basement membrane of the endothelium may rest directly on the
internal elastic membrane, or be separated by a sub-endothelial layer of CT.
Tunica media: Smooth muscle cells predominate in the tunica media, and little elastic
material is present. As in large arteries, no fibroblasts are present. lastic fibres (few), collagen, and ground substance are produced by the
smooth muscle cells. In tissue preparation, the internal elastic membrane of the intima appears
wavy due to the contraction of the smooth muscle of the media. Tunica adventitia:
The main constituent of the adventitia is collagen fibres, secreted by fibroblasts. Elastic fibres are also present, a concentration of such fibres at the inner boundary of the adventitia is called the external elastic membrane.
The external elastic membrane is not as prominent as the internal, and as arteries get smaller (see small arteries, below) disappears much earlier.
The tunica adventita is relatively larger than in elastic arteries,
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VESSEL-WALL LAYERING
Tunica INTIMA
Endothelium
Lamina propria
Tunica MEDIA
Tunica ADVENTITIA
}Internal elastic lamina
This layering scheme works well for muscular arteries, but becomes strained when applied to other vessels, e.g., in regions of the vena cava there may be no T media; in arterioles, the intima has a convincing identity only in TEM
External elastic lamina
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Muscular Artery and Vein
vein
artery
tunica externa thickest layer of
vein
A
V
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Artery and vein
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Endothelial cell
Smallest ARTERIOLE
Smallest arteriole, is a capillary with smooth muscle cells wrapped around it, with modifications to the endothelial cells - less transport, more interaction with SMCs.
Smooth muscle cell SMC/ VSMC
Reticular fibers
Contraction regulates flow by need
For fast flow & non-stick, until clotting is needed
Controls passage through the wall
Helps control blood flow
Mechanical support
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Arteriole
arteriole (center) and an accompanying venule (right).
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arteriole and an accompanying venule.
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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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Capillary Structure
Figure 21.4
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Capillary
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Continuous Capillaries
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 capillaries of the brain:
Have tight junctions completely around the endothelium
Constitute the blood-brain barrier
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Continuous Capillaries
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Fenestrated Capillaries
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
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Fenestrated Capillaries
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Sinusoids
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
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Sinusoids
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An interconnected network of vessels consisting of Collateral arteries feeding an arteriole Metarterioles Arteriovenous anastomoses Capillaries Venules
Capillary Beds
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Capillary Beds
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
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Capillary Beds
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The Organization of a Capillary Bed
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Capillary
Capillary
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Collect blood from all tissues and organs and return it to the heart
Are classified according to size Venules Medium-sized veins Large veins
Veins
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Vein
tunica externatunica media
tunica interna
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{ Adventitia
{Intima
Bundles of longitudinal smooth muscle
Occasional circular SMC Numerous elastic fibers
LARGE VEIN Details
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Large vien
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Muscular vein
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Medium size veins The tunica Externa
consists of the endothelium and a thin subendothelial layer with smooth muscle cells among the connective tissue elements. A thin internal elastic membrane may or may not be present. (If present, it is not nearly as prominent as in arteries).
The tunica media is much thinner relative to that of an artery, and consists mostly of circularly
arranged smooth muscle but also contains collagen fibres. The tunicas intima and media therefore tend to be less distinct from one
another than is the case in arteries. The tunica adventitia is usually thicker than the media and is made up
mostly of collagen fibres. It may contain longitudinally oriented smooth muscle bundles. (Remember
gradations between the vessels of different sizes are continuous.)
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Medium size vein
Medium-sized vein with a much less compact muscle layer than you saw in the preceding arteries. The tunica media is indicated by bar "a". Bar "b" = adventitia, which is at least as wide as the media, and often even wider. There is no evident inner elastic membrane. (Blood in the lumen stains red here.) To the right, compare sizes and walls of one small artery (d) and two very small veins (c) and (e
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The Function of Valves in the Venous System Venules and medium-sized veins contain valves Prevent backflow of blood
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A Comparison of a Typical Artery and a Typical Vein
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