BIOL 1344 Lecture Notes - Lecture 3: Tunica Intima, Tunica Externa, Oncotic Pressure

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6 Feb 2017
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Chapter 20
Blood Vessels
Arteries
o Convey blood away from the heart to the capillaries
Capillaries
o Microscopic, porous blood vessels for substance exchange between blood and tissues
Veins
o Drain blood from capillaries and convey it back to the heart
General structure
o Lumen inside space of the vessel
o Tunics
Tunica intima/interna
Endothelium that faces the lumen
Thin subendothelial layer of areolar ct
Provide a smooth surface for the blood to move through the vessel
Tunica media
Circularly arranged layers of smooth muscle cells supported by elastic
fibers
Contraction results in vasoconstriction
Relaxation results in vasodilation
Tunica externa
Areolar ct that contains elastic and collagen fibers
Helps anchor the vessel to other structures
Some large blood vessels require a blood supply for the tunica externa
and is supplied by small arteries called vasa vasorum that extend
through the tunica externa
Companion vessels
o Arteries and veins the supply the same body region and are next to each other
o Arteries have a thicker tunica media, narrower lumen, more elastic/collagen fibers
Arteries are more resilient/resistant to changes in bp than veins
Arteries reai ope ee he there is’t lood preset
Veins have a thicker tunica externa, wider lumen, less elastic/collagen fibers
Veis are typially ollapsed if there is’t lood i the essel
o Capillaries only contain tunica intima and an underlying basement membrane
Types of arteries
o Elastic
Largest (2.5-1 cm)
Conducting arteries conduct blood from the heart to muscular arteries
Large proportion of elastic fibers (especially tunica media)
Allows the artery to stretch and accommodate blood when it is ejected
from the ventricle and helps to propel blood through the arteries during
ventricular relaxation
Aorta, pulmonary trunk, brachiocephalic, common carotid, subclavian, common
iliac
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Chapter 20
o Muscular
1-0.3 cm
Distributing arteries
Distribute blood to specific areas of the body
Thicker tunica media with multiple layers of smooth muscle cells
Internal/external elastic lamina separates the tunica media from the tunica
externa
Better vasoconstriction/vasodilation but not as able to stretch as elastic arteries
Brachial, anterior tibial, coronary, inferior mesenteric
o Arterioles
Smallest 0.3 mm 10 um
Fewer than 6 layers of smooth muscle in tunica media
Larger arterioles have all three tunics but the smallest may only have a tunica
intima with a single layer of smooth muscle
Vasomotor tone always in a slightly contracted state
o Capillaries
Smallest blood vessels ~ 1mm in length, 3-10 um in diameter
Connect arterioles to venules
Erythrocytes travel in single file (Rouleau)
Solely endothelial layer of cells on a basement membrane
Categorized based on degree of permeability
Continuous
o Most common
o Form complete, continuous lining around the lumen that rests
on a complete basement membrane
o Tight jutios do’t for a oplete seal ad hae gaps alled
intercellular clefts
o Materials move in and out via membrane transport processes
Diffusion, pinocytosis
o Materials move through intercellular clefts via diffusion and
bulk flow
Fenestrated
o Composed of a complete, continuous lining of endothelial cells
and a basement membrane
o Thin areas of the cells are called fenestrations (pores)
Small enough to prevent formed elements to pass
through but allows smaller plasma proteins
Found where there is a great amount of fluid transport
between capillaries and interstitial tissues
Small intestines
Aqueous humor of the eye
Choroid plexus of the brain
Endocrine glands
kidneys
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Chapter 20
Sinusoids
o Incomplete lining of endothelial cells with large gaps and the
basement membrane is incomplete or absent
o Found in red bone marrow and some endocrine glands for
facilitation of hormones into the blood
o Commonly have a reddish colorf
o Capillary Beds
Fed by a metarteriole
Proximal part is encircled by smooth muscle cells
Distal part (thoroughfare channel) has no smooth muscle
Thoroughfare channel connects to a postcapillary venule
True capillaries branch from the metarteriole and make up the bulk of the
capillary bed
A smooth muscle ring called the precapillary sphincter controls blood
flow
o When relaxed blood flows into the true capillaries
o When contracted blood flows to the postcapillary venule
o Sphincters cycle contraction and relaxation at about 5-10
cycles/min (vasomotion)
o Only a quarter of the capillary beds are open at a time
60,000 miles of capillaries and only about 250300 mL of blood (5% of
total volume)
Perfusion the specific amount of blood entering the capillaries per unit
time per gram of tissue
o Venules
Smallest veins 8-100 um in diameter
Companion vessels with arterioles
Merge to form veins
o Small, Medium, Large Veins
Becomes a vein when the diameter is greater than 100 um
Contain valves that are primarily tunica intima and strengthened by
elastic/collagen fibers
Similar to semilunar valves of the heart
o Systemic Vein Reservoirs
Largest percentage of blood is in systemic circulation (70%)
Pulmonary circulation (18%)
Heart circulation (12%)
o Simple Pathway
One major artery delivers blood to the organ or body region then branches into
smaller arteries and split into a capillary bed. A venule drains the blood and
merges with other venules to become a vein to drain the region or organ
End arteries arteries that provide only one pathway through which blood can
reach an organ
o Alternative Pathways
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