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PH-FD06

Oedema and fluid movement

How fluid moves between capillaries, tissues and lymphatics, and the four ways the balance fails: raised capillary pressure, low plasma protein, leaky vessels and blocked lymph drainage.

By the end, you can

  • Describe the body’s fluid compartments and the forces that move fluid across the capillary wall.
  • Explain how raised venous pressure, low albumin, increased permeability and lymphatic obstruction each cause oedema.
  • Distinguish a transudate from an exudate, and pitting oedema from lymphoedema.
  • Explain why oedema collects in dependent parts of the body.
  • Relate oedema in the lungs, brain and body cavities to its effects on function.
  • Recognise the classic appearances, classifications and tests of oedema and effusions, and explain how diuretics, compression and other treatments act on fluid movement.

What it is

Oedema is an excess of fluid in the interstitial space, the tissue fluid between cells. It forms when more fluid leaves the capillaries than the capillaries and lymphatics return. Fluid collecting in a body cavity is an effusion (pleural, pericardial) or ascites (peritoneal).

4 types to tell apart
Raised hydrostatic pressure
Higher pressure in capillaries pushes out more fluid, as in heart failure, venous obstruction or salt and water overload. The fluid is a low-protein transudate.
Low plasma oncotic pressure
Too little albumin to hold fluid in the vessels, as in nephrotic syndrome, liver failure or severe malnutrition. Also a transudate.
Increased permeability
Leaky vessels let protein and cells out, as in inflammation, burns and sepsis. The fluid is a protein-rich exudate.
Lymphatic obstruction
Lymph cannot carry fluid and protein away, as after lymph node removal or in filarial infection: lymphoedema.

US spelling: oedema → edema, litres → liters.

Normal function

Fluid moves out of capillaries and back

About 60% of a young adult man’s body weight is water (less in women and older people). Roughly two-thirds is inside cells; of the third outside, about three-quarters is interstitial fluid and a quarter is plasma.

At the capillary, two main forces act. Hydrostatic pressure, from the heart’s pumping, pushes fluid out: in the usual teaching values about 35 mmHg at the arteriolar end of a capillary, falling to about 15 mmHg at the venular end. Plasma oncotic pressure, about 25 mmHg and mostly from albumin, which cannot easily cross the wall, draws fluid back. These are the Starling forces.

Slightly more fluid leaves than returns. Lymphatic capillaries, like the beside the of a villus, collect the excess with any leaked protein and return it to the veins, a few litres a day. In current understanding, many capillaries filter along most of their length, so lymph returns most of the filtrate.

Point to it on the model
Fluid exchange at the capillarySchematic
NormalCapillary plasmaRich in albumin
  • Filtration: hydrostatic pressure (from Capillary plasma to Interstitial fluid)
  • Return: plasma oncotic pressure (from Interstitial fluid to Capillary plasma)
NormalInterstitial fluidLittle protein
  • Excess fluid and protein drain (from Interstitial fluid to Lymphatic capillary)
NormalLymphatic capillaryReturns to the veins

Simplified Starling model; in many capillaries filtration continues along most of their length and lymph returns most of it.