Starling's law of capillaries - Capillary exchange

Starling’s Law of Capillaries – Summary
Starling’s Law describes how fluid moves in and out of capillaries based on the balance of hydrostatic and oncotic (osmotic) pressures.

Key Starling Forces:

Capillary Hydrostatic Pressure | Pc | Pushes fluid out of the capillary into tissues
Interstitial Hydrostatic Pressure | Pi | Pushes fluid into the capillary (usually negligible)
Capillary Oncotic Pressure | πc | Pulls fluid into the capillary (due to plasma proteins like albumin)
Interstitial Oncotic Pressure | πi | Pulls fluid out of the capillary (usually low)

Net Filtration Pressure (NFP):
{NFP} = (P_c - P_i) - (\pi_c - \pi_i)

Positive NFP (arterial end) → Filtration (fluid leaves capillary)
Negative NFP (venous end) → Reabsorption (fluid enters capillary)



ApproximateTypical Values Table (mmHg):

| Pc (Capillary Hydrostatic) |Arterial End ~35 |Venous End ~15 |
| Pi (Interstitial Hydrostatic) |Arterial End ~0 |Venous End ~0 |
| πc (Capillary Oncotic) |Arterial End ~25 |Venous End ~25 |
| πi (Interstitial Oncotic) |Arterial End ~1 |Venous End ~1 |

NFP Arterial: (35-0) - (25-1) = 11 mmHg (Filtration)
NFP Venous: (15-0) - (25-1) = -9 mmHg (Reabsorption)