Edema: Two Pressures

Why fluid leaks into the tissues · Week 1
At every capillary, two forces fight over the fluid. Hydrostatic pressure pushes water out of the vessel. Oncotic pressure, created by albumin, pulls it back in. Tip that balance with too much push or too little pull. Then fluid floods the tissue as edema.

A capillary & the tissue around it

The orange arrows push fluid out, which is the hydrostatic force. The albumin inside pulls it back, which is the oncotic force. Watch the tissue when the balance breaks.
Pick a scenario to see the balance.
Pick a scenarionormal · too much push · too little pull

What's happening

↑ Hydrostatic (push)

Too much pressure inside pushes fluid out. Causes include heart failure, venous congestion, and fluid overload.

↓ Oncotic (pull)

There is not enough albumin to pull fluid back. Causes include liver failure, malnutrition, and nephrotic syndrome.
Two ways to flood the tissue: turn the push up, which is hydrostatic, or turn the pull down, which is oncotic. Either way, net fluid moves into the interstitium → edema. Albumin is the key protein holding water in the vessel.