Vessel II Flashcards
What are the microvasculature?
Metarterioles
Capillaries
Postcapillary venules
What is the function of metarterioles?
Regulate blood flow into capillaries
Serve as bypass route when they shut off blood to a capillary bed
Can coordinate contraction of sphincters to form a bypass route to postcapillary venules
What is the function of capillaries (in a capillary bed)?
Gatekeepers monitoring access to tissues
Facilitate 2 way fluid exchange tissues - passive diffusion and active transport across endothelium via transcellular and paracellular pathways
What is the function of postcapillary venules?
Receive blood from capillary bed
Primary sites of WBC migration into tissues
What is the flow through metarterioles?
Arterioles to metarterioles to capillaries
What are the components of metaarterioles?
Highly vasoactive
Tunica intima - endothelium with BM, no subendothelium
Tunica media - 1 discontinuous layer of smooth muscle, no EEL
Tunica externa - absent
What do metarterioles use to regulate flow into capillaries?
Precapillary sphincters constrict the entrance to capillaries - tunica media
When contracted - narrows lumen and close off entrance to capillary
When relaxed - lumen is patent and blood flows into capillary
What are the characteristics of capillaries?
Small
Form capillary beds
Tunica intima - endothelim with BM, no subendothelium
Tunica media - absent, capillaries have pericytes just outside tunica intima (pericytes are similar to smooth muscle cells)
Tunica externa - always absent
What are the functions and structure of pericytes?
Stellate cells derived from mesenchymal cells
Contractile
Have their own external lamina that merges with endothelial BM
Surround capillaries and help modulate blood flow through them - during contraction, they tightly hug the capillary to reduce or cut off blood flow
Stem cells can differentiate into fibroblasts, smooth muscle cells, and endothelial cells - blood vessel formation
What are the types of blood capillaries?
Continuous - most common, least leaky
Fenestrated - small fenestrations going the entire way through endothelial cells’ cytoplasm, BM solid. Has subclass: with or without diaphragms. Mildly leaky
Sinusoidal (sinusoids) - largest capillary with very large fenestration in endothelial cells and their BM, very leaky
What do all three blood capillaries lack?
Tunica media and tunica externa
What kind of walls do continuous capillaries have? Where are they located and what makes them have the walls that they do?
Solid walls
Least leaky
BM solid
Lack fenestrations
Endothelial cells strongly bound to each other
Located in: CNS, lungs, skeletal muscle tissue, CT, and exocrine glands
What are characteristics of continuous capillaries in TEM?
No fenestrations in or between cells or in BM
Endothelial cells have marginal folds that help WBCs find the margin between adjacent endothelial cells
Numerous pinocytotic vesicles (form via pinocytosis) that help transport substances across endothelium. Vesicles may merge to form transient channels across cell cytoplasm
Pericytes usually visible
What kind of walls do fenestrated capillaries have and what makes they have this characteristic? Where are they located?
Endothelial cells have fenestrations - permanent windows through cytoplasm that allow fluid to pass between lumen and tissues without having to enter cell via endocytosis, cross its cytoplasm, and be exocytosed
Fenestrations may have molecular diaphragms - temporarily seal off fenestrations in order to limit fluid transfer
BM lacks fenestrations
Most fenestrated capillaries have diaphragms
Locations: Kidneys or endocrine glands
What are the functions and structures of molecular diaphragms in fenestrated capillaries?
Diaphragm usually spans fenestration
Molecular barrier that have negative ionic charge
Negatively charged plasma proteins cannot cross the diaphragm
Fenestrated capillaries located in places where rapid fluid exchange occurs between blood and tissues
1 molecule wide
Where are fenestrated capillaries with diaphragms located?
Intestines, most endocrine glands, kidney (except glomeruli)
Where are fenestrated capillaries without diaphragms located?
Glomeruli of kidneys
Is fluid transfer faster through fenestrations or transient channels?
Fenestrations
What are the characteristics of sinusoidal capillaries?
Largest type of capillary
Very leaky - endothelial cell fenestrations are large, large gaps between adjacent cells, and BM is absent or incomplete
Very large gaps between cells and in BM - allows RBCs and WBCs to exit the bloodstream and allows very rapid fluid transfer
Located in areas of very rapid fluid exchange and where blood cells need quick in/out - red marrow, spleen, liver, lymph nodes, cortex of adrenal gland
What are the structure and function of postcapillary (pericytic) venules?
First venules after capillary bed
May have pericytes
Receive blood from capillary bed (some exchange of metabolites) and primary site of WBC migration into tissues
Tunica intima: endothelium + BM, no subendothelium
Tunica media: absent but may have pericytes
Tunica externa: absent
Where do WBCs primarily leave the bloodstream to the tissues?
Leave at the postcapillary venules
Endothelial cells of these venules have marginal folds to help WBCs find the edges of adjacent cells - cells spread the folds and unlock junctions between cells
What is the structure and function of muscular venules?
Receive blood from capillaries and postcapillary venules and drain into small veins
Venules accompany arterioles
Tunica intima: endothelium + BM, no subendothelium
Tunica media: may have 1-2 isolated smooth muscle cells
Tunica externa: thin layer of CT extends around the entire circumference (lacks vasa vasorum and nervi vasorum)
What is the structure and function of small veins?
Receive blood from venules and drain into medium veins
Small veins accompany small arteries
Have valves that prevent backflow
Tunica intima: endothelium + BM, no subendothelium
Tunica media: smooth muscle cells form 1 layer which may be discontinuous or complete
Tunica externa - thin layer of CT extends around entire vessel circumference
When muscle relaxes, vein refills. When muscle contract, vein is squeezed
What is the structure and function of medium veins?
Receive blood from small veins and drain into large veins
Medium veins accompany muscular (medium) arteries
Large lumen and thin walls
Have valves
Location: jugular vein
Tunica intima: endothelium + BM; thin subendothelium
Tunica media: smooth muscle in 2-3 complete layers, EEL often visible, may have 1-2 elastic lamellae or elastic fibers
Tunica externa: thickest layer, elastic and collagen fibers present, vasa/nervi vasorum
What is the structure and function of large veins?
Receive blood from medium veins and drain into large veins or the R atrium of the heart
Accompany elastic (large) arteries
Large lumen and thin walls
Lack valves - do not run between skeletal muscles and thus are not compressed by them
Locations: IVC, portal vein, common iliac vein
Tunica intima: thick
Tunica media: smooth muscle form 3+ layers around entire circumference, elastic lamellae, in IVC and SVC - transition from smooth to cardiac muscle approaching the heart
Tunica externa thickest layer, sooth muscle in longitudinal bundles, both collagen and elastic fibers, vasa and nervi vasorum present
What are vessels with atypical structure?
Coronary arteries
Great saphenous vein
What is the structure and function of the coronary arteries?
Coronary arteries are first two branches off the aorta - means they must deal with higher BP, blood volume compared to muscular arteries
Tunica intima: thick and progressively thickens with age, lots of elastic fibers
Tunica media: any layers of smooth muscle
Tunica externa: collagen I and elastic fibers are relatively loose
What is the structure and function of the great saphenous vein?
Tunica intima: endothelium + BM, subendothelium, and poorly develop IEL
Tunica media: 2-3 substantial (thick) layers of smooth muscle bundles, which are perpendicular to each other. Inner layer has longitudinal bundles. Outer layer has circumferential layers. Gives them some peristalsis ability
Tunica externa: Typical for medium sized vein
