Starling forces at the capillary !
- Mechanism
- Exam priority: high
Blood pressure pushes fluid out of capillaries and blood proteins pull it back in; the lymph carries away what is left over.
Updated 2026-11-01
Also called: Capillary filtration, Oncotic pressure
See it in 3D: Thoracic duct · Cisterna chyli
What it is
Fluid moves across the capillary wall according to the balance of hydrostatic and oncotic (colloid osmotic) pressures, the Starling forces. Capillary blood pressure is the main force pushing fluid out; plasma proteins, chiefly albumin, hold fluid in. Normally a small net filtration occurs and the lymphatics return it to the blood.
How it works, step by step
- Arteriolar tone sets how much of the arterial pressure reaches the capillary.
- Capillary hydrostatic pressure pushes water and small solutes out through clefts and pores.
- Plasma proteins cannot cross easily (σ near 1), so their oncotic pressure pulls water back.
- The balance favours slight filtration along most of the capillary.
- Filtered fluid enters the lymphatics, aided by negative interstitial pressure and muscle pumping, and returns to the veins via the thoracic duct.
Starling forces along a capillary
Arteriolar NFP +11 mmHgVenular NFP −6 mmHgFiltration 3.6Reabsorption 1.1Net filtration 2.5 a.u. (lymph capacity 7.5)
Normal (classic model): fluid leaves at the arteriolar end, some returns at the venular end and the lymph returns the rest.
Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.
- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema
Equations
- Jv
- net fluid flux (out is positive)
- Kf
- filtration coefficient (permeability × area)
- Pc, Pi
- capillary and interstitial hydrostatic pressure
- πc, πi
- plasma and interstitial oncotic pressure
- σ
- reflection coefficient for proteins (0–1)
Typical values in a skeletal muscle capillary (vary by tissue and source)
| Parameter | Value | Note |
|---|---|---|
| Capillary hydrostatic pressure | ≈ 30–35 mmHg arterial end, ≈ 10–15 mmHg venous end | |
| Plasma oncotic pressure | ≈ 25–28 mmHg | |
| Interstitial hydrostatic pressure | ≈ −3 to 0 mmHg | slightly negative in loose tissue |
| Interstitial oncotic pressure | ≈ 3–8 mmHg | |
| Serum albumin | ≈ 3,5–5,0 g/dL |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Low albumin (cirrhosis, nephrotic syndrome, malnutrition such as kwashiorkor) lowers πc and causes generalized edema.
- Severe dengue causes plasma leakage by raising capillary permeability (low σ, high Kf), which can lead to shock.
Test yourself
Think of the answer, then tap the question to see it.
Why does venous pressure affect capillary filtration more than arterial pressure?
Precapillary arterioles absorb most of the arterial pressure, while there is little resistance between capillaries and veins, so a rise in venous pressure is passed straight back to the capillary.
Why does a patient with cirrhosis and serum albumin of 2 g/dL develop edema?
Low albumin lowers plasma oncotic pressure, so the force holding fluid in the capillary falls and net filtration exceeds lymph drainage.
Why do the alveoli normally stay dry despite a very large capillary surface?
Pulmonary capillary pressure is low (about 7 mmHg) and lymph flow is efficient, so net filtration stays small.
In inflammation or severe dengue, which Starling term changes and with what effect?
Capillary permeability rises (higher Kf, lower σ), so proteins leak out, the oncotic gradient falls and fluid leaves the plasma.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- How edema forms
- How the lymphatic system works
- Flow, pressure and resistance: Poiseuille and turbulent flow
- Heart failure physiology
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 20.3 Capillary Exchange (OpenStax (Rice University), 2022); Lymphedema (StatPearls (NCBI Bookshelf), 2025); Hepatic Cirrhosis (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 16). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.