Starling Forces and Filtration

The balance of hydrostatic and colloid osmotic (oncotic) pressures between capillary plasma and interstitial fluid, causes fluid to be filtered out of a capillary at the arteriolar end and re-absorbed at the venous end. These forces acting to move fluid in and out of a capillary are sometimes referred to as Starling forces.

In a capillary hydrostatic pressure (PC) falls from 33 mmHg at the arterial end to 15 mmHg at the venous end. Interstitial hydrostatic pressure (PIF) can vary from 9 to -9 mmHg, depending on the tissue. In solid tissues it is usually near zero or slightly positive (1 mmHg). Loose areolar connective tissue, such as in the epidural space, tends to have a negative hydrostatic pressure.

Colloid osmotic pressure in the capillaries (nC) is 25 mmHg, while in the interstitial fluid colloid osmotic pressure (nIF) is usually zero. The Starling forces are summarized in Figure CR.22.

The pressure acting to force fluid out of the capillary is made up of the hydrostatic pressure in the capillary and the colloid osmotic pressure in the interstitial fluid:

Similarly the pressure acting to force fluid back into the capillary is made up of the interstitial hydrostatic pressure and the colloid osmotic pressure in the capillary:

The resultant pressure gradient is the difference between outward and inward pressures (Figure CR.22):

Figure CR.22 Starling forces

The rate of filtration is proportional to this pressure gradient and is given by: Rate of nitration - [<P( + *„=MI\,= + x K

where:

K = filtration coefficient of isotonic fluids (i.e. volume rate filtered per unit of pressure) and is about 0.01 ml/min/mmHg/100 g tissue at 37° C.

Assuming an interstitial hydrostatic pressure (Pff) of 1 mmHg, and an interstitial colloid osmotic pressure (rcff) of zero, it can be seen that at the arterial capillary end:

Pressure gradient = -(1+25)

Therefore, pressure gradient acts outwards, filtering fluid out of the capillary. While at the venous capillary end: Pressure gradient = 15 - (] + 25)

Here the pressure gradient is negative acting inwards, re-absorbing fluid back into the capillary.

The total volume of fluid filtered through the capillaries is dependent on capillary blood flow, but estimates have been made of between 1 and 3 litres per h. The filtered fluid exceeds the amount of fluid reabsorbed by about 10%, the difference being absorbed by the lymphatic system.

Capillary Filtration Equilibrium

In a capillary there is normally a dynamic equilibrium between the fluid filtered out, the fluid re-absorbed and the fluid absorbed by lymphatics. Disturbance of this equilibrium either dehydrates tissue or makes it oedematous. The equilibrium can be disturbed by various changes. Some examples are shown in Figure CR.23.

FACTORS DISTURBING CAPILLARY FILTRATION EQUILIBRIUM

Change

Effects

Vasodilatation pressure

proximal capillary hydrostatic pressure filtration

Intravascular volume

proximal capillary hydrostatic pressure filtration

Vasoconstriction

i proximal capillary hydrostatic pressure i filtration

X Intravascular volume

i proximal capillary hydrostatic pressure i filtration

Hypo-albuminaemia

i capillary colloid osmotic pressure filtration i reabsorption

Venous pressure

distal capillary hydrostatic pressure i reabsorption

Figure CR.23

Figure CR.23

The Lymphatic System

The functions of the lymphatic system include:

• Drainage of interstitial fluid—the total lymph drainage for an adult is about 2-4 litres per 24 H

• Return of 'leaked' protein in the interstitial fluid to the systemic circulation. Lymph from most tissues contains a protein concentration of 20 g/l

• Absorption of particles, proteins and other large molecular weight molecules accumulated in inflamed tissues

• Absorption of protein and fat derived from metabolism and gastro-intestinal absorption. The protein concentration is two-to-three times higher in lymph draining from liver and intestine

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Responses

  • brigitte fuerst
    What causes disturbance of starling forces?
    2 years ago
  • eve
    Which Starling force or forces supports filtration?
    2 years ago
  • angela
    Which starling force helps filtration?
    1 year ago
  • peter
    Which starling force or forces support(s) filtration?
    1 year ago
  • EDWIN
    How is starling forces linked to negative fluid balance?
    5 months ago

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