6.0 BLOOD VESSELS
a) Define and use properly the following words: Tunica intima, Tunica media, Tunica
adventitia, Arteries, Elastic artery, Elastin, Muscular artery, Arteriole, Capillaries,
Endothelial cell, Pinocytotic vesicles, Fenestrae, Diaphragms, Pericytes, Continuous
capillaries, Fenestrated capillaries, Sinusoids, Blood sinuses, Venules, Postcapillary
venules, Muscular venules , Small veins, Medium veins, Large veins, Circular sphincter
muscle , Arteriovenous anastomoses, Lymphatic vessels.
b) Describe and associate basic structure/function for all structures listed above and
identify specific locations for blood vessles, sinuses, sinusoids and lymphatics.
c) Identify by microscopy the cells, organelles, tissues, and organs listed above.
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I.
ORGANIZATION OF BLOOD VESSELS
Because of the pressure gradients created within the vessel due to the pumping action of the
heart and the contraction of skeletal muscles, the structure of each vessel will differ according
to the corresponding pressure.
A. TUNICA INTIMA
1. contains endothelium + basal lamina + loose CT ± internal elastic lamina
(membrane)
B. TUNICA MEDIA
1. contains smooth muscle ± elastic fibers ± outer elastic lamina, collagen fibers, and
ground substance
C. TUNICA ADVENTITIA
1. the outer layer
2. contains Connective Tissue, elastin and collagen, many cells: fibroblasts, mast cells,
plasma cells, fat, sometimes smooth muscle, nerve (nervi vasorum) & small blood
vessels (vasa vasorum), and lymphatic vessels
II.
ARTERIES
Elastic, Muscular, Arterioles
A. ELASTIC ARTERIES (Large)
1. Largest diameter, Aorta & branches
2. conduction tubes, aid the movement of blood
3. walls expand greatly during systole, and recoil during diastole due to the elastic
fibers.
4. T. intima:
a. thin, consist essentially of the endothelium
5. T. media:
a. thickest layer with concentric layers of elastic laminae
b. elastic recoil for continuous flow of blood
c. smooth muscle, collagen fibers
6. T. adventitia:
7. no distinct external elastic lamina, blends with surrounding CT
B. MUSCULAR ARTERIES (Medium)
1. There is NO sharp dividing line between large elastic and medium muscular arteries
2. They are often intermediate; the elastic material decreases and the smooth muscle
increases
3. But, there is always a prominent internal elastic and external elastic lamina
4. T. intima:
a. thin, Basal lamina may contact internal elastic lamina
5. T. media:
a. may be the thickest layer
b. mainly smooth muscle cells (circular or spiral)
c. some collagen & elastic fibers
d. NO fibroblasts.
6. T. adventitia
a. thick, collagen fibers primarily
b. vasa & nervi vasorum, lymphatics
c. External elastic lamina
C. ARTERIOLES
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1. The small arteries and arterioles are distinguished by the number of smooth muscle
cells in the tunica media
2. Arteriole = 1-2 layers; small artery = up to 8 layers
3. small arteries have an internal elastic lamina, but may be absent in arterioles.
III. CAPILLARIES
A. Capillaries are very small thin walled tubules arranged in a network to allow the exchange
of substances between blood and tissues. They are about 8 µm diameter, contain a single
layer of endothelium and basal lamina, and are joined by tight junctions.
B. The Endothelial cell-- contains few organelles, but the plasmalemma has pinocytotic
vesicles and sometimes fenestrae (60-80 nm) with diaphragms or thin protein membranes
(except in kidney)
C. Pericytes are sometimes found associated with endothelium. The pericyte is an
undifferentiated perivascular cell that may differentiate into smooth muscle and fibroblasts
and may be contractile.
D. CONTINUOUS CAPILLARIES
1. contains pinocytotic vesicles
2. serve for trans-epithelial transport
E. FENESTRATED CAPILLARIES
1. contain pores with or without diaphragm (thinner than plasmalemma)
2. without diaphragm found in kidney
F. SINUSOIDS
1. more permeable than capillaries
2. larger than capillaries, lack uniformity,
3. they are shaped to fill surrounding spaces
4. slows the blood flow
5. absence of or very little Basal lamina
6. Liver sinusoids are fenestrated
7. Can have unusual long parallel endothelial cells with discontinuous basal lamina; if
present called Blood sinuses; found in spleen
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IV. VEINS
A. The terms "small" "medium" and "large" have only relative meaning in any one animal.
Large veins of cat may be smaller than the medium veins of cow.
B. Venules (small)
1. "POSTCAPILLARY VENULES"
a. receive blood from capillary
b. have endothelium, Basal lamina and pericytes
c. Sensitive to vasoactive agents such as histamine and serotonin, which
cause movement of fluid and white cells into CT during inflammation and
allergic reactions. Also, these agents cause the vessels to constrict (i.e..,
narrow their lumens)
d. Important in lymph nodes
2. MUSCULAR VENULES
a. 1-2 layers smooth muscle
C. SMALL VEINS
1. 2-4 layers smooth muscle, circular, some CT
D. MEDIUM VEINS
1. typical 3 layers
2. T. intima (some elastic lamina)
3. T. media (much thinner than in arteries)
4. T. adventitia - CT elastic fibers
E. LARGE VEINS
1. T. intima (somewhat of an internal elastic lamina)
2. T. media (thin)
3. T. adventitia (thick, bundles of longitudinal smooth muscle in the vena cava and
jugular vein)
V. SPECIAL VESSELS
A. Organ specific for specialized functions.
B. Very thick
1. arteries found in the teat and coronary heart arteries
2. thick veins in the glans penis
C. Very thin arteries
1. are found in the brain.
2. Brain capillaries have little or no spaces thus preventing infectious agents from
passing from blood into brain
D. Circular sphincter muscle bundles are found in T. intima of special veins and arteries of
penis, ovary, & uterus
E. Arteriovenous anastomoses
1. blood does not always pass from arteries to capillaries and then to veins.
2. direct routes from arterioles to venules,
3. they have thick smooth muscle in T. media
4. Physiological function: to shunt blood where needed
5. feet, lips, nose, ears (for maximum cooling, heating)
6. Also play a role in bronchioles, where deoxygenated blood is mixed with oxygenated
blood. This decreases the amount of O2 reaching the tissue.
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F. Lymphatic vessels
1. Drain the excess fluid from the connective tissue back into the circulatory system.
2. Thin wall; only any endothelium
3. large lumen; typically collapses or misshapened
4. Filled with plasma; no RBC; few WBC.
COMPARISON OF BLOOD VESSELS
Elastic Arteries
T. media > T. adventitia
Elastic fibers +++
Large lumen
Large Veins
T. adventitia > T. media
Elastic fibers +
Large lumen
valves
Small Arterioles
Small lumen
Compact smooth muscle
Roundish
no IEL/EEL
Small veins
Larger lumen
Loose smooth muscle
Irregular shape
no IEL/EEL
valves
Large Arterioles
~3-6 layers
folded endothelium
T.media >T.adventitia
± ILE
no EEL
Arteries
smooth muscle ~6-10 layers
folded endothelium
Elastic fibers
IEL
EEL
Veins
2-6 layers
smooth endothelium
no IEL
no EEL
thicker blood
Capillary
Sinusoid
no muscle
> lumen
no muscle
Post-Capillary
Venule
> lumen
no muscle
Collecting
Venule
>> lumen
no muscle
Pericytes
no pericyte
Pericytes
Pericytes ++
Muscular
Venule
>> lumen
smooth muscle
(1-2)
no Pericytes
*IEL = Internal Elastic Lamina; EEL = External Elastic Lamina
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Lymphatics
> to >> lumen
no muscle
no Pericytes
valves
no RBCs