Week 7: Microcirculation Flashcards
What is the purpose of micro circulation?
to ensure eqilibirum with the external and internal environments
it is part of the alimentary system
examples: pulmonary, gastrointestinal, renal, and bodily microcirculation
Macro- micro transition:
How are the arteries and veins connected?
Arterial branches → arterioles → capillaries → venules → venous branches
What is the graph representing optimal parameter profiles
Branching
book
Coupling with the microflow environment
book
What is the microcirculatroy unit?
The unit that connects the arterioles and the venules
What is the input and output of the microcirculatory unit?
input: arteriole
output: venule
What is the “throughfare channel” and what does it allow?
a channel which allows blood to flow freely between an arteriole and venule
can carry nutritive flow → metarteriole, capillary network
can carry nonnutritive flow → shunt
What is the direction of flow in the microcirculatory unit?
arteriole → venule
(within the capillary, the spatio-temporal pattern is more complex)
What offers a means of overall control of flow?
the smooth muscle layer in the wall of the arteriole
What offers the means of local control?
precapillary sphincters
Graph displaying the different vessels v.s. pressure
The pressure is autoregulated (graph)
How is the diameter of the vessels adjusted?
vasomotion
the oscillation of vascular tone (diameter) with frequencies in the range from 1-20 min and is seen in the most vascular beds
Where does the oscillation of vasomotion originate?
vessel wall
(seen in vivo & vitro)
What must be present in order for vasomotion to occur?
cellular oscillator
can be modeled as a strong of events forming a feedback loop
In order to get macroscopic oscillations of blood vessels ____
the oscillations in individual smooth muscle cells must be synchronized
What are the physiological roles of vasomotion?
-
improve flow in the periphery of circulation
- flow is better through an oscillating diameter rather than a steady one
-
make peripheral flow distribution more efficient
- by ↑ the synchronization
-
↑ the reactivity of a blood vessel
- avoiding the “latch state” (state of prolonged force in SM due to low ATP)
- pattern alters in disease
Describe the flow of RBC and plasma.
- flow in two phases in arteries and arterioles
- in the capillaries their flow behavior separates int plug and bolus flows (i.e. single file
- the plasma bolus is being stirred while flowing which provokes gas and meterial exchange betwen capillary lumen and wall
Why can a RBC of 7 micra in diameter fit into a capillary of smaller diameter (5 micra) ?
special shape and remarkable membrane flexibility
Describe the flow of white blood cells.
- flow is much slower than that of RBC
- tend to interfere with microcirculatory flow
- therefore WBC have impact of microcirculatory flow
What is the impact of glycocalyx on flow?
- the bushlike extracellular matrix structure penetrates the plasma column, capturing its edge
- acts as a stagnant cell-free plasma layer
- size of this layer effects microcirculatory hematocrit
- molecular composition is complex and assists in multiple homeostatic functions
What is responsible for the formation of ESL (endothelial surface layer) ?
interaction between soluble plasma components and the membrane-bound glycocalyx
Discuss the ESL in regards to thickness
ESL thicker than glycocalyx
- thickness of ESL ↑ with vessel diameter
- relative thickness vise versa
- therefore the ESL is able to withstand the sheer stress of flowing blood and exclude RBC
What are the effects of ESL on hemodynamic parameters in the microcirculation?
effects the flow resistance and hematocrit because of the presence of cell-free layer and thickness
What is the result of the loss of dynamics?
↓ phase separation in the macro-micro transition dimension of the circulation
What is the result of the slower dynamics?
- lesser slipping over the ESL
- results in ↑ cellular interaction
- results in ↑ microcirculatory viscosity
the combination of loss of dynamics and slow dynamics results in the verge of collapse
What are capillary types defined by?
their endothelial lining
Name examples of continuous capillary type
tight: brain
retina
intercellular clefts: skeletal muscle
myocardium
smooth muscle
skin
Name examples of fenestrated capillary type
GI mucosa
glands
certain renal vessels
Name examples of discontinuous capillary type
liver
bone marrow
adrenal cortex
spleen
Different ways of passive endothelium transcapillary exchange
Describe transcellular exchange
- mechanism: difussion
- what does it transport? → gases (O2, CO2, Xe)
→ lipid soluble molecules
→ water via aquaporin-1 channel
- depends on diffusion properties of the surrounding tissues
Describe paracellular (small pore pathway) exchange
- mechanism: diffusion
- what does it transport? → water soluble (lipophobic) molecules
→ small polar molecules ( < albumin-size)
- via small water-filled pathways (pores): → intercullular clefts
→ interendothelial junction complex
→ gaps
→ fenestrae
What version do we use of Fick’s Law of diffusion?
What is flow limited transport?
- for small molecules
- there is rapid equilibrium between the plasma and the tissue
- only blood flow determines how much molecule can be delivered to the tissue per unit time
What is diffusion limited transport?
- when diffusion becomes restricted and there is no equilibrium between the plasma and the tissue
What are the complicating factors of diffusion?
- attractions between molecules of solute and solvent
- surface charge
- pore structure
- glycocalyx
What is the paracellular (large pore pathway) exchange
mechanism: (very slow) diffusion
what does it transport? protein sized molecules
via: large waterfilled pathways (large pores)
→ large clefts
→ gaps
→ fenestrae
What is transcytosis?
mechanism: vesicular transport NOT diffusion
- translocation of macromolecules
- flux is NOT governed by Fick’s Law
What are the exchange pathways, mechanism, and structures of the exchange of fluid
exchange pathways: transcellular, paracellular
mechanism: transcellular = diffusion
paracellular = convection
structure: transcellular = AQP1 channels
paracellular = clefts, fenestrae, gap junctions
Fluid exchange
book
What are the homeostatic functions of the endothelium?
- regulates adhesion, rolling, and migration of leukocytes tho the endothelium
- maintains vessel lining that prevents formation of blood clots (thrombi)
- supresses the proliferation of VSM
- regulates VSM tone
What are 2 examples of vasoactive molecules produced by endothelium?
- Prostacyclin
- Nitric Oxide
Name some properties of Prostacyclin
- released in response to shear stress
- formed from arachidonic acid (AA) by the actions of cyclooxygenase (CycOx) and protacyclin synthase (PGl2Syn)
Function:
- prevent platelet aggregation
- prevent platelet adhesion to endothelium
- relaxes adjacent smooth muscle via ↑ cAMP
Name some properties of Nitric Oxide
- released in response to shear stress
- can also be stimulated by Ach
- stimulated endothelium releases NO
Function:
- ↑ cGMP level which produces relaxation of VSM by ↓ [Ca++] sensitivty of the myofilament
Draw the individual and net pressures in the capillary and in the interstitium
What is the equation for capillary colloid osmotic pressure? List common reflection coefficients
Draw the diagram representing the factors responsible for filtration and absorption across the capillary wall
