Histo Block I: Capillaries, venules, veins Flashcards
WHat does the wall of capillaries consists of?
Capillaries consist only of endothelium with its basement membrane. There is no discernable (distinct) tunica media or tunica adventitia.
Usually link arterioles to venules
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micrograph of capillary in cerebellum
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Micrograph of capillary with pericyte
How is the lumen of capillaries?
The lumen is only one erythrocyte wide or smaller (~5 -10 microns)
What cells are associated with capillaries?
May have associated cells called pericytes (Rouget cells) which sit inside capillary basement membrane
What is the function of pericytes?
Pericyes help to regulate molecular exchange across capillary & can differentiate into endothelial or smooth muscle cells upon injury.
Function of capillaries
The thin wall allows easy exchange of molecules across the wall. Are approximately 50,000 miles of capillaries in body. They are the major site of movement of gases, metabolites, & waste products between the blood and the CT, i.e., body tissues.
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Capillary in longitudinal section
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capillary in cross section
What are the types of capillaries?
- Continuous capillaries
- Fenestrated capillaries
- Discontinuous (sinusoidal) capillaries
describe continuous capillaries
- no fenestrations(holes or pores) through the cytoplasm of the endothelial cell or discontinuities in or between cells.
- Tight junctions are usually present between the endothelial cells
describe fenestrated capillaries
- Presence of holes (fenestrations) through cytoplasm of capillaries.
- Diaphragms are present across the fenestrations in all cases except for kidney glomerular capillaries
describe discontinuous sinusoidal capillaries
- Distinct discontinuities (gaps) between endothelial cells.
- Fenestrations with diaphragms may also be present. Discontinuous basal lamina
How is the endothelium of continuous capillaries?
Endothelium is a continuous uninterrupted layer around the lumen with many tight junctions and without either fenestrations or discontinuities
What is the function of the continuous capillaries?
- Endothelium can be thick (200-300 nm in width) or thin (100 nm in width) depending upon selective permeability properties of wall. Tightly controls and regulates the passage of molecules to and from the lumen and the tissues. (least permeable)
How can molecules pass in continuous capillaries?
Molecules can pass across by diffusion, active transport, or by the transcellular pathway involving pinocytosis or receptor mediated endocytosis
Where can we find continuous capillaries?
lung, muscle, CNS, thymic cortex. Blood-brain-barrier, Blood-thymic barrier
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micrograph of
capillary in cerebellum
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EM of continuous capillary
how is the endothelium of fenestrated capillaries?
Typically a thinner endothelium which contains membrane lined transcellular channels (fenestrae) with diameters of 60 – 80 nm which run though the cytoplasm
-Fenestrae have a diaphragm everywhere except glomerular capillaries
-Fenestrae can be randomly distributed through the cell – may be many or few
What is the function of fenestrated capillaries?
Fenestrated capillaries are much more permeable than continuous capillaries and are found in areas where rapid exchange is needed e.g. endocrine glands and kidney glomerulus
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EM of fenestrated (arrows) capillary
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EM of wall of fenestrated capillary. The arrows point to the fenestrae with diaphragms
How is the endothelium of sinusoidal capillaries?
The endothelium is thin and irregular with large gaps and discontinuities between the cells which can be hundreds of nm in size.
-Lumen is often slightly larger than other capillaries (9 -12 um)
-Often find macrophages associated with these gaps
What is the function and location of sinusoidal capillaries?
Are the most porous and leaky of the capillaries
Found in areas where extremely free and open exchange of molecules across the capillary wall is required e. g. liver sinusoids, lymph node sinusoids and bone marrow sinusoids
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EM of sinusoidal capillary in liver
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Scanning EM of splenic sinusoid
On what does the control of the rate of movement of substances into and out of capillaries in a particular organ depends upon?
- Relaxation or contraction of the smooth muscle in arterioles, metarterioles, and the pre-capillary sphincters
- What type of capillaries are predominant in that tissue or organ
- The density of capillary beds
- The presence and frequency of metarteriole thoroughfares and arterial venous shunts
Whta does the Relaxation or contraction of the smooth muscle in arterioles, metarterioles, and the pre-capillary sphincters do?
can control whether blood enters a particular capillary bed or capillary
which capillaries are less permeable?
continuous capillaries are less permeable than fenestrated capillaries, which are less permeable than sinusoidal capillaries
what does higher density of capillaries do?
can allow for increased exchange of molecules. This is related to organ function, e.g., many capillaries in liver, kidney, cardiac & skeletal mm; fewer in dense CT.
what does The presence and frequency of metarteriole thoroughfares and arterial venous shunts do?
These allow the blood to bypass the capillary beds and is important in determining the rate of exchange of the capillaries within the tissue
How is the wall of postcapillary venules?
have a wall similar to capillaries but are larger in diameter (diameter up to 50m). Thus greater than 1 erythrocyte in diameter.
What structure is associated with postcapillary venules?
Often have associated pericytes (pericytic venule)
Why are postcapillary venules important for?
Are low pressure vessels and are important for reabsorption of fluids from the CT
what is the principal site for transfer of white blood cells between blood & CT?
postcapillary venules
which vasoactive agents are present in postcapillary venules?
histamine and serotonin which can increase permeability of venules and loss of fluid into the adjacent CT
where are postcapillary venules present?
They often are present accompanying arterioles
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Arteriole and venule
how is the tunica media y muscular venules?
1-2 layers smooth muscle in T. media. 50-100 um diameter Often accompany larger arterioles.
how is the wall of muscular venules?
Have thinner more irregular walls and are often more flattened than arterioles; consistent with lower pressure in lumen.
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Muscular venule and arteriole
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Muscular venule and arteriole
How many layers of smooth muscle are present in small veins?
3 or more layers of smooth muscle in tunica media and a diameter of 100 um (0.1 mm) to 1 mm.
Wall structure is thicker and more distinct than in muscular venules
Are frequently more flattened than accompanying small arteries
How is the tunica adventitia in small veins?
Tunica adventitia is becoming more defined
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Small vein and accompanying small artery
WHat are the measurements of medium veins?
Medium veins are greater than 1 mm but less than 10 mm in diameter
describe the tunica intina in the medium veins
The tunica intima is relatively thin consisting of the endothelial cells and their basal lamina as well as a small amount of subendothelial connective tissue
describe the tunica media in medium veins
The tunica media has 3 or more layers of smooth muscle, but generally not as many layers as in muscular arteries or as well organized as in muscular arteries
how is the internal elastic membrane in medium veins?
There is no distinct internal elastic membrane or distinct external elastic membrane. This differs from the muscular arteries where these are prominent
how is the tunica adventitia in medium veins?
The tunica adventitia is typically the thickest layer in the wall of the medium vein.
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medium vein
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muscular artery
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muscular artery and medium vein
Where can we find valves?
common in veins in the folds of the tunica intima; Are folds of the tunica intima. They are lined on their surface with endothelium.
what are the functions of valves in veins?
particularly in the veins in the lower part of the body, especially the legs where they prevent backflow of blood
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valves in veins
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medium vein with valve
Which veins can be classified as large veins?
Veins greater than 10 mm diameter (subclavian, hepatic portal, and vena cavae
describe the tunica adventitia of large veins and what can we find there?
Thick tunica adventitia with longitudinally oriented bundles of smooth muscle.
collagen fibers, elastic fibers, and large bundles of longitudinally oriented smooth muscle cells. Vasa vasorum are typically present in the tunica adventitia
describe the tunica intima of large veins
Endothelium and some subendothelial CT and scattered smooth muscle cells. Valves are common
describe the tunica media of large veins
Relatively thin with some circularly oriented smooth muscle cells. Much thinner than tunica adventitia
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large vein
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large vein (vena cavae)
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large vein (vena cavae)
