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