2.7A. Functional organization of microcirculation and its control. Flashcards
(22 cards)
I. Basics
1. What is microcirculation?
Refers to circulation of the blood in the smallest blood vessels in the human body (ex: terminal arterioles, capillaries, metarterioles and venules), which separate the arterial and venous system
I. Basics
2. Route of microcirculation
Arterioles (input) ↔ capillaries ↔ venules (output)
I. Basics
3. Function of microcirculation
exchange of products
II. Functional organization of microcirculation
1. What are 6 components of microcirculation?
- Terminal arterioles (<20μm)
- Metarterioles
- Precapillary sphincters
- Capillaries
- (postcapillary) venules
- AV-shunt
II. Functional organization of microcirculation
2. Characteristics of Terminal arterioles (<20μm)
- the vessel on the arterial side of the microcirculation
- composed of (1) single layer of SMCs (2) thin adventitia layer (3) endothelium
II. Functional organization of microcirculation
3. Characteristics of Metarterioles
- SMCs are discontinuous and NOT innervated
- Capillaries originate from here
-> Exchange of H2O and other solutes
II. Functional organization of microcirculation
4. Characteristics of Precapillary sphincters
- 1 smooth muscle cell
- Are responsible to local tissue conditions
-> Relaxation/contraction of precapillary sphincter modulate tissue blood flow (which capillaries are open/closed)
II. Functional organization of microcirculation
5. Characteristics of Capillaries
- Smallest blood vessels in human body
- Receives arterial blood from terminal arterioles
- Exchange site of nutrients and cellular waste product
between blood and tissues - Site of fluid exchange between vascular and interstitial
compartments - No smooth muscle – only endothelial cell
-> Basal membrane
-> Have pores – filled with H2O
-> Total surface of capillaries in body = 5000 – 7000 m2
II. Functional organization of microcirculation
6. Characteristics of (postcapillary) venules
- Carry blood back into the vein
- Have a discontinuous SMC layer that allows for control of local blood flow
- They may also exchange some solute across their walls
II. Functional organization of microcirculation
7. Characteristics of AV-shunt
- Direct link with arteriole and venule
- NO CAPILLARIES
- A lot of SMCs -> SYM control
-> Only in skin circulation (responsible for thermoregulation)
III. Types of capillaries
1. What are the 4 types of capillaries
1) Continuous capillaries
2) Fenestrated capillaries
3) Sinusoidal (discontinuous) capillaries
4) Tight capillary
III. Types of capillaries
2. Characteristics of Continuous capillaries
- Most abundant (ex: muscle, fat, nervous tissue)
- Cells are joined by tight junctions
- Has small pores
- Pinocytotic vessel -> pinocytotic channel (permeability of wall for H2O and other soluble particles)
III. Types of capillaries
3. Characteristics of Fenestrated capillaries
- Found in GI-tract, endocrine + exocrine glands, kidney
- Has fenestrae (intracellular perforations)
-> increased pore diameter
-> increased capillary permeability = more permeable than continuous type
III. Types of capillaries
4. Characteristics of Sinusoidal (discontinuous) capillaries
- Found in liver, spleen, bone marrow
- Have open spaces between endothelial cells
-> very permeable (cells cross easily) - Sometime permits passage of blood cells
III. Types of capillaries
5. Characteristics of Tight capillary
- Found in brain and retina
- No pores, no H2O-filled structures
- Allows only highly-regulated transcellular transport (ex: BBB)
IV. Autoregulation
1. Characteristics of autoregulation
- Intrinsic ability of a body part to maintain a constant blood flow despite changes in perfusion pressure
- A passive process that occurs in absence of neural and hormonal influences
- Tendency of vascular smooth muscles to contract when stretched
IV. Autoregulation
2. Mechanism of autoregulation
Tendency of vascular smooth muscles to contract when stretched:
- Stretch induces opening of stretch-activated, non-selective cation channels in the VSCMs
-> depolarization
-> L-type Ca2+-channel activation
-> [Ca2+]↑
-> muscle contraction (myogenic response)
-> vasoconstriction
V. Vasomotion
1. What is Vasomotion?
- Vascular SM in arterioles undergoes cyclic contractions and relaxations (oscillations)
V. Vasomotion
2. What is mechanism of Vasomotion?
Mechanism: the cyclic change of Ca2+-signal, synchronized across the SM in the arteriole wall
- This improves flow in the periphery
-> giving blood a little push
-> makes flow through the periphery more efficient
- Oscillations also prevents a ‘’latch state’’, which is otherwise common in smooth muscle due to having low levels of ATP
VI. Plug and bolus flow
1. What are characteristics of Plug and bolus flow
- Capillaries have a smaller diameter than RBC (cap: 6μm, RBC: 7μm)
- The RBCs undergo severe deformation while they travel though the capillary
- Each RBC is a ‘’plug’’ followed by a plasma ‘’bolus’’ between another RBC
- RBC does not ever come into contact with the wall, because of the wall’s glycocalyx layer, which covers the internal surface of the endothelium:
+) If glycocalyx is present, hematocrit is lower (less RBC’s out of overall layer)
+) If glycocalyx is absent, hematocrit goes up
VI. Plug and bolus flow
2. Why doesn’t RBC come into contact with the capillary wall>
RBC does not ever come into contact with the wall, because of the wall’s glycocalyx layer, which covers the internal surface of the endothelium:
- If glycocalyx is present, hematocrit is lower (less RBC’s out of overall layer)
- If glycocalyx is absent, hematocrit goes up
VII. Flow: impact of WBCs
- How can WBCs impact the blood flow in capillaries?
- WBCs are even larger than RBCs, but still make it pass through the capillaries
- Slower flow, frequently blocks in the capillary that has to build up a pressure gradient to push it through
