Starling forces
[Ref: BL8:p166-169;WG21:p593-595]
Starling forces
Qf = k[(HPc-HPi)-(OPi-OPc)]
- Qf = fluid movement
- k = filtration coefficient
- HPc = hydrostatic pressure of capillary
- HPi = interstitial pressure of capillary
- OPc = oncotic pressure of capillary
- OPi = oncotic pressure of interstitum
Hydrostatic pressure is the principle force in capillary filtration
Values
- k (for the whole body)
= 0.0061mL/min/100g/mmHg
Arterial vs venous end of capillary bed
Average values of hydrostratic pressure (derived from measurements in human skin)
[Both BL8 and WG21]
- HPc = 32mmHg at the arterial end
- HPc = 15mmHg at the venous end
- mean Hpc = 25mmHg
Values for normal tissues
- HPi = zero or slightly negative
- OPc = 25-28mmHg
- OPi = zero or slightly negative
Values for lung
- HPc = 8-10mmHg (mean capillary pressure)
- OPi = 16-20mmHg (due to proteins leaked into interstitium)
- HPi = close to zero
- OPc = 25mmHg
Thus, in pulmonary alveolar capillaries,
- Fluids are osmotically drawn out of capillary by plasma proteins that leak through the endothelium.
[BL8:p169]
[WG21:p664] Fluids are drawn into the capillaries to keep alveoli free of fluid (no mention of OPi)
Values for kidney
- HPc = 45 mmHg at both afferent and efferent arterioles
- HPi = 10 mmHg
- OPc
= 20 mmHg @ Afferent
= 35 mmHg @ Efferent
* Due to filtration of protein-free fluid
- OPi = 0 mmHg
At afferent end, Driving pressure
= 45-10-20
= 15mmHg
At efferent end, driving force
= 45-10-35
= 0 mmHg
NB:
According to [KB2:p80]
- HPc
= 60 mmHg @ Afferent
= 58 mmHg @ Efferent
- HPi = 15 mmHg
- OPc
= 21 mmHg @ Afferent
= 33 mmHg @ Efferent
- OPi = 0 mmHg
Cerebral circulation
[KB2:p80]
Capillary membrane in cerebral capillaries is relatively impermeable to most of the low molecular weight solutes as well as plasma proteins
--> All exert osmotic forces
One mOsm increase in osmotic gradient
--> 17-20 mmHg increase
--> Small changes in tonicity has marked effect on cerebral volume
Reflection coefficient
... relative impediment to the passage of a substance through the capillary wall
Reflection coefficient
* for water = 0
* for albumin = almost 1
Reflection coefficient determines the oncotic pressure (directly proportional)
* [BL8:p167]
Net flow flux
= k x Net driving force
= k x {(HPc-HPi) - reflection coefficient x (OPc-OPi)}
* [KB2:p78]
Role of albumin
Albumin is the most significant plasma protein in determining oncotic pressure
- Osmotic pressure due to albumin molecules
- Negative charge retenting cations
- Binds to small number of chloride ions
Also see Osmotic pressure and oncotic pressure
Other notes
Diffusion vs filtration
- Diffusion is the most important means for water and solute transfer across the endothelium
Movement of water across capillary walls due to:
- Filtration and absorption
= 0.06mL per min per 100g tissue
- Diffusion
= 300mL per min per 100g tissue
Permeability and MW
- Molecules larger than 60,000 MW do not penetrate the endothelium.
- Molecules smaller than 60,000 MW penetrate at a rate inversely proportional to their size.
- For small molecules (e.g. water, NaCl, urea, glucose), the capillary pores offer little restriction to diffusion (low reflection coefficient) --> flow limited