3. Physiology
        3.2. Cardiovascular
            3.2.3. Peripheral circulation
3.2.3.2. Starling forces

Starling forces

[Ref: BL8:p166-169;WG21:p593-595]

Starling forces

Qf = k[(HPc-HPi)-(OPi-OPc)]

Hydrostatic pressure is the principle force in capillary filtration

Values

Arterial vs venous end of capillary bed

Average values of hydrostratic pressure (derived from measurements in human skin)

[Both BL8 and WG21]

Values for normal tissues

Values for lung

Thus, in pulmonary alveolar capillaries,

[WG21:p664] Fluids are drawn into the capillaries to keep alveoli free of fluid (no mention of OPi)

Values for kidney

At afferent end, Driving pressure
= 45-10-20
= 15mmHg

At efferent end, driving force
= 45-10-35
= 0 mmHg

NB:

According to [KB2:p80]

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

Also see Osmotic pressure and oncotic pressure

Other notes

Diffusion vs filtration

Movement of water across capillary walls due to:

 

Permeability and MW

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