MICROCIRCULATIONMICROCIRCULATION
FUNCTIONAL ANATOMY
Microcirculation is circulation of the blood through the smallest
vessels of the body – arteriols, capillaries and venules.
(20-50 µm)
(>50 µm)
(4 - 9 µm)
The principal function of the microcirculation is to permit the transfer
of substances (water, solutes, gases) between the vascular system
and the tissues.
STRUCTURE OF VESSEL WALLSTRUCTURE OF VESSEL WALL
The total area of all the
capillary walls in the body
exceeds 500 m2.
The capillary wall is about
1 µm thick.
The rate of blood flow in
capillaries is 0.2 - 1 mm/s.
1
2
3
Transit time from arterial to
venular end of a capillary
is 1 - 2 seconds.
4
UULTRASTRUCTURE OF CAPILLARYLTRASTRUCTURE OF CAPILLARY
Lumen
Fenestrationes or pores
Endothelial cell
Endothelial cell
Basement membrane
Nucleus
5-10 µm
Intercellular cleft
Vesicles
MOVEMENT OF FLUID
ACCROSS CAPILLARY WALL
resorption
filtration
diffusion
Effect of oncotic
pressure
Effect of hydrostatic
pressure
The diffusion, filtration and resorption of water across capillary wall occur through
Intercellular clefts, pores and fenestrations.
OSMOTIC PREASURE
osmotic
(oncotic)
pressure
osmotic
(oncotic)
pressure
hydrost.
pressure
PRESSURE GRADIENTS ACROSS THE WALL
OF CAPILLARY
37 17
INTERSTITIAL SPACE
arteriole venule
filtration resorption
capillary
Net filtration pressure:
Pc- Pi = 36 mmHg
Net resorption pressure:
π c- π i = 25 mmHg
Net filtration pressure:
Pc - Pi = 16 mmHg
Net resorption pressure:
π c- π i = 25 mmHg
Pc
Pc
Pi = 1
πi ≈ 0
π c= 25
CAPILLARY HYDROSTATIC PRESSURE Pc = 37 - 17 mmHg
INTERSTITIAL HYDROSTATIC PRESSURE Pi = 1 mmHg
CAPILLARY ONCOTIC PRESSURE πc = 25 mmHg
INTERSTITIAL ONCOTIC PRESSURE πi ≈ 0 mmHg
EXCHANGE OF FLUID VIA CAPILLARIES
([Pc − Pi] − σ [πc − πi]) - effective filtration pressure
Net filtration pressure Net resorption pressure
STARLING'S EQUATION
Kf - Filtration coefficient
Jv - NET FLUID MOVEMENT ACROSS CAPILLARY WALL
Pc - capillary hydrostatic pressure
Pi - interstitial hydrostatic pressure
πz - capillary oncotic pressure
πi - interstitial oncotic pressure
σ - coefficient permeabilty
CAUSES OF INCREASED INTERSTITIAL FLUID
VOLUME (EDEMA)
2
4
Increased capillary permeability
3
1
Reduced lymph drainage
MOVEMENT OF SOLUTES ACCROSS
CAPILLARY WALL
• DIFFUSION - if there is, for a certain solute, a concentration difference
between the plasma and interstitial space the solute diffuses across the capillary
wall. Lipid-soluble molecules (e.g. O2 ,CO2) move across the capillary wall directly
while lipid insoluble molecules (e.g. ions, urea) move across the capillary wall by
Intercellular clefts, pores or fenestrations.
• SOLVENT DRAG - The dissolved particles are dragged through the
capillary wall along with filtered and reabsorbed water.
!!! TO REMEMBER !!!
Four forces known as Starling forces determine net fluid movement
across the capillary membranes.
Pc= Capillary Pressure Tends to move fluid out of the capillary.
Pi= Interstitial Fluid Pressure Tends to move fluid into the capillary.
πc = Plasma Colloid Osmotic Pressure Tends to cause osmosis of fluid
into capillary.
πi = Interstitial fluid colloid osmotic pressure Tends to cause osmosis of fluid
out of the capillary
Effective filtration pressure = ((Pc-Pi) – (πc- πi))
The diffusion is the key factor in providing exchange of gases, substrates
and waste products between the capillaries and the tissue cells.
CAUSES OF EDEMA DEVELOPMENT:
Capillary Pressure - Pc (↑hydrostatic pressure, heart failure)
Plasma Proteins (nephrotic syndrome)
Capillary Permeability - Kf (infections)
Lymph drainage- πi (lymphatic blockage)