Physiology chapters
Body fluid compartments and oedema
How water is divided between the body's compartments, how they are measured, the Starling forces that move fluid across capillaries, and the causes and mechanisms of oedema, including pulmonary oedema.
- Total body water is about 60% of body weight: two thirds intracellular (40%) and one third extracellular (20%); the extracellular part is about three quarters interstitial and one quarter plasma.
- Fluid moves across capillaries by Starling forces: capillary hydrostatic pressure pushes it out; plasma oncotic pressure (mainly albumin) pulls it in; lymph returns the excess.
- Oedema comes from high capillary pressure, low plasma protein, leaky capillaries or blocked lymphatics.
On this page
Overview
Water is the largest part of the body. It is divided into compartments separated by cell membranes (between cells and the fluid around them) and by capillary walls (between plasma and interstitial fluid).
Understanding these compartments and the forces that move water between them explains oedema, dehydration and the choice of IV fluids.
Body fluid compartments
In an average 70 kg adult man:
| Compartment | Share of body weight | Volume (approx.) |
|---|---|---|
| Total body water | 60% | 42 L |
| Intracellular fluid (ICF) | 40% (two thirds of TBW) | 28 L |
| Extracellular fluid (ECF) | 20% (one third of TBW) | 14 L |
| Interstitial fluid (incl. lymph) | About 15% (three quarters of ECF) | About 11 L |
| Plasma | About 5% (one quarter of ECF) | About 3 L |
A small amount is transcellular fluid (CSF, synovial, peritoneal, intraocular fluid).
The "60-40-20" rule: total body water 60%, intracellular 40%, extracellular 20% of body weight.
Blood volume is about 7% of body weight (about 5 L): plasma plus cells.
Body water is lower in women, the elderly and the obese, because fat holds little water. It is higher in infants (about 70–75%).
Composition
| Ion | ECF | ICF |
|---|---|---|
| Main cation | Na+ | K+ |
| Main anion | Cl−, HCO3− | Phosphates, proteins |
Plasma and interstitial fluid are almost the same, except that plasma has much more protein.
Measuring the compartments
Indicator dilution principle: a known amount of a marker is injected; once it has mixed, its concentration is measured.
Volume = amount injected ÷ concentration after mixing (correcting for any lost).
| Compartment | Marker |
|---|---|
| Total body water | Deuterium oxide (heavy water), tritiated water, antipyrine |
| Extracellular fluid | Inulin, mannitol, sucrose, radioactive sodium or chloride |
| Plasma | Evans blue dye, radio-iodinated albumin |
| Red cell volume | Chromium-labelled red cells |
ICF and interstitial fluid cannot be measured directly:
- ICF = total body water − ECF.
- Interstitial fluid = ECF − plasma volume.
Osmosis between compartments
Water moves freely across cell membranes to keep the osmolarity of ICF and ECF equal (about 280–300 mOsm/L).
- Isotonic fluid (0.9% saline): stays in the ECF; cell size unchanged.
- Hypotonic fluid: water moves into cells; cells swell.
- Hypertonic fluid: water leaves cells; cells shrink.
Starling forces across capillaries
Four pressures decide whether fluid leaves or enters the capillary:
| Force | Direction | Typical value (approx.) |
|---|---|---|
| Capillary hydrostatic pressure | Out of the capillary | About 17–30 mmHg (higher at the arterial end) |
| Interstitial hydrostatic pressure | Usually slightly negative, so it pulls fluid out | About −3 mmHg |
| Plasma oncotic (colloid osmotic) pressure | Into the capillary | About 28 mmHg |
| Interstitial oncotic pressure | Out of the capillary | About 8 mmHg |
Net filtration = (capillary hydrostatic − interstitial hydrostatic) − (plasma oncotic − interstitial oncotic).
Normally, slightly more fluid leaves than returns. The extra (about 2–3 L per day) is returned to the blood by the lymphatics.
Plasma oncotic pressure, mainly from albumin, is the main force holding fluid inside capillaries.
Oedema
Oedema is too much fluid in the interstitial space. It has four main mechanisms:
| Mechanism | Examples |
|---|---|
| Raised capillary hydrostatic pressure | Heart failure, venous obstruction (deep vein thrombosis), excess salt and water retention (kidney failure) |
| Low plasma oncotic pressure (low albumin) | Nephrotic syndrome, liver cirrhosis, malnutrition (kwashiorkor) |
| Increased capillary permeability | Inflammation, burns, allergy, sepsis |
| Blocked lymphatics | Filariasis, cancer, lymph node removal or radiotherapy |
Safety factors against oedema
The body tolerates moderate changes before oedema appears because:
- Interstitial pressure is normally negative; it must rise before fluid can collect easily.
- Lymph flow can increase many-fold.
- Protein is washed out of the interstitium as filtration rises, lowering interstitial oncotic pressure.
Systemic oedema in heart failure and kidney disease
When cardiac output falls, the kidneys sense poor perfusion and activate the renin-angiotensin-aldosterone system and ADH. Salt and water are retained; blood volume and venous pressure rise; fluid leaks into tissues. This is a vicious circle.
In some kidney diseases the problem starts with the kidney retaining salt and water. Blood volume expands (vascular overfill), capillary pressure rises and oedema follows. In nephrotic syndrome, low albumin lowers oncotic pressure (sometimes called underfill), and the kidneys then retain salt in response.
Pitting oedema in the legs and lower back is typical of systemic fluid overload; it is gravity-dependent.
Pulmonary oedema
The lungs are normally kept dry: pulmonary capillary pressure is low (about 7 mmHg), well below plasma oncotic pressure (about 28 mmHg), and lymph drainage is strong.
How it happens: most often left heart failure or mitral stenosis, which raise left atrial and pulmonary capillary pressure. Fluid first fills the interstitium, then the alveoli. In acute cases, it appears when pressure rises above about 25–30 mmHg (the oncotic pressure). Damage to capillary walls (pneumonia, toxic gases, ARDS) also causes it.
What you see: breathlessness, especially lying flat; crackles; frothy pink sputum; low oxygen.
In chronic high left atrial pressure, the lymphatics enlarge, so patients tolerate higher pressures (up to about 40–45 mmHg) before frank pulmonary oedema.
Expect: the 60-40-20 rule and volumes, markers for each compartment (inulin for ECF, Evans blue for plasma, deuterium for TBW), the main ions of ECF and ICF, the four Starling forces and which pulls fluid in, the four mechanisms of oedema with examples, and the pressure threshold for pulmonary oedema.
Quick revision
- TBW 60%: ICF 40%, ECF 20% (interstitial 15%, plasma 5%).
- Markers: deuterium (TBW), inulin (ECF), Evans blue (plasma).
- ECF: Na+ and Cl−. ICF: K+ and phosphate.
- Starling: hydrostatic out, oncotic (albumin) in; lymph removes 2–3 L/day.
- Oedema: high capillary pressure, low albumin, leaky capillaries, blocked lymph.
- Pulmonary oedema: left heart failure; above about 25–30 mmHg acutely.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What percentage of body weight is water?
About 60% in a young adult man: 40% inside cells and 20% outside cells.
Which marker measures extracellular fluid volume?
Inulin, or other substances that cross capillaries but not cell membranes, such as mannitol.
What causes oedema in nephrotic syndrome?
Loss of albumin in the urine lowers plasma oncotic pressure, so fluid leaves capillaries; salt retention by the kidney adds to it.
Why does left heart failure cause pulmonary oedema?
It raises left atrial and pulmonary capillary pressure above plasma oncotic pressure, forcing fluid out of the capillaries into the lungs.