2.7A. Functional organization of microcirculation and its control. Flashcards
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
