Microcirculation
Basics
bloodflow to cap bed controlled by arterioles, pre-cap sphincters, shunts
caps have very little blood volume
are largest pressure drop in CV system
parallel flow resistance - individual cap is high resistance, low when in parallel
slows blood down for efficient exchange
result of high total cross-sectional area of caps
Flow = (velocity)(area); if
Ý
area,
ß
velocity
Nutrient Exchange
diffusion: for lipid soluble substances
pores: water soluble substances
Fick’s Law: J=PS(C
o
-C
i
) or F = consumption / arterial-venous difference
J: subst. moved per unit time (mol/sec)
P: permeability (L / m
2
·
sec)
S: surface area (m
2
)
C
o
: concentration outside cap (mol/L)
C
i
: concentration inside cap (mol/L)
flow limited diffusion: for small molecules diffusion limited by rate that bloodflow can supply substance
diffusion limited diffusion: diffusion across caps is limiting factor
pinocytosis
Forces Determining Exchange
capillary hydrostatic pressure: affected by arterial and venous systems, mostly venous
i.e. LV failure leads to pulmonary edema through pulmonary venous pressure
plasma colloid osmotic pressure
interstitial hydrostatic pressure
interstitial colloid osmotic pressure
In Summary:
osmotic opposes hydrostatic
Starling Equilibrium combines all these:
Q
f
= k[(P
c
+
p
i
) - (P
i
+
p
p
)]
k: capillary membrane filtration constant
P: hydrostatic pressure
p
: osmotic pressure
if Q > 0 then get filtration
reflection coefficient: reletive impediment of movement of a substance through the capillary membrane (water = 0 ; albumin = 1)
filtration: net movement of fluid out of capillary
reabsorption: net movement of fluid into capillary
filtration and reabsorption rarely is balanced because osmotic and hydrostatic forces rarely balanced
edema: caused when lymphatic system cannot keep up with filtration
lymph flow increased by anything that increases filtration i.e. muscle activity
in capillaries:
Ý
area
Þ
ß
velocity
in vessels:
ß
area
Þ
Ý
velocity