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
