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capillaries
10–40×109
sites of exchange
walls thin: <1 µm
intercellular “clefts”
membrane vesicles
vescicular channels
“pores”
liver
gut tube
glomerular fenestrae
not in the brain
small diameter: 7 µm
extensive branching: no cell is more than 100 µm away
surface area of about 600 m2, only 250 mL of blood
intermittency of flow
metarterioles
precapillary sphincters
slow average velocity of flow
calculation of the cross-sectional areas:
Acapillary = π × r2
= 3.14 × ((7.5 ÷ 2) µm)2
= 3.14 × 14.06 µm2
= 44.2 µm2
Atotal = Ncapillaries × Acapillary
= 35×109 × 44.2 µm2
= 1.5×1012 µm2
= 1.5 m2
Aaorta = π × r2
= 3.14 × ((2.5 ÷ 2) cm)2
= 3.14 × 1.56 cm2
= 4.9 cm2
= 4.9×10-4 m2
Capillary exchange
diffusion across capillary wall for solute exchange
lipid-soluble substances
water-soluble substances
permeability
concentration gradient
no carrier-mediated transport systems
interstitial “fluid”
fluid filtration
| Type of pressure | Pressure value |
|---|---|
| capillary blood pressure | 30 mm Hg → 10 mm Hg |
| blood colloid osmotic pressure (oncotic pressure) | 28 mm Hga |
| interstitial-fluid hydrostatic pressure | −3 mm Hg |
| interstitial-fluid colloid osmotic pressure | 8 mm Hg |
a 19 mmHg from plasma proteins; 9 mmHg from Donnan effect
Starling equilibrium
net fluid filtration of 2 ml/min
filtration coefficient: 0.01 ml/min/mm Hg/100 g
lymphatic system and interstitial fluid return
structure of lymphatic capillaries
muscle contractions and valves
rate of lymph flow
interstitial fluid pressure
lympahtic pump
functions
return of excess filtered fluid
defense against disease
transport of absorbed fat
return of filtered protein
edema
reduced concentration of plasma proteins
increased permeability of capillary walls
inflammation, blisters, hives
increased venous pressure
blockage of lymph vessels
elephantiasis
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