Capillary exchange (Starling forces)

The Starling equation. Net fluid movement out of a capillary (Jv) equals Kf × [(Pc − Pi) − σ(πc − πi)].

Pc is capillary hydrostatic pressure and Pi is interstitial hydrostatic pressure. πc is plasma oncotic pressure, about 70–80% of which comes from albumin, and πi is interstitial oncotic pressure. Kf is the filtration coefficient, which reflects the permeability and surface area of the wall. σ is the reflection coefficient for proteins, close to 1 in most capillaries.

Typical systemic values (Guyton). Pc is about 30 mmHg at the arteriolar end and about 10 mmHg at the venular end. Pi is about −3 mmHg in loose subcutaneous tissue. πc is about 28 mmHg and πi about 8 mmHg.

The net outward pressure is therefore about +13 mmHg at the arteriolar end, so fluid filters out. At the venular end it is about −7 mmHg, so fluid is reabsorbed. On average there is a small excess of filtration. Across the whole body this excess, a few litres a day, is returned to the blood as lymph.

The revised Starling principle. More recent work shows that the endothelial glycocalyx makes the effective oncotic gradient act across the glycocalyx itself. In most steady-state tissues, sustained reabsorption at the venular end is small, and the lymphatics remove most of the filtrate.

Special capillary beds. In glomerular capillaries, Pc is about 50–60 mmHg, so filtration continues along their whole length. In pulmonary capillaries, Pc is low (about 7–10 mmHg), which keeps the alveoli dry. In the intestinal and peritubular capillaries, net reabsorption occurs.

Diffusion. For solutes, diffusion across the capillary wall moves far more material than bulk flow does.