At the capillaries, fluid, nutrients and wastes move between the blood and the tissues. A balance of pushing and pulling forces decides how much fluid leaves the vessel and how much comes back.
Two main forces
Hydrostatic pressure, the blood pressure inside the capillary, pushes fluid out into the tissues. Oncotic pressure, made mostly by albumin in the plasma, pulls fluid back in.
Normal balance
Slightly more fluid leaves the capillaries than returns. The lymphatic system collects this extra fluid and returns it to the blood through large veins near the heart.
Pressure and protein
High capillary pressure pushes out more fluid, as in heart failure or a deep vein clot. Low albumin, from liver disease, kidney protein loss or malnutrition, lets less fluid return.
Leaks and blockage
Inflammation, sepsis and burns make capillaries leaky, so fluid and protein escape. Blocked lymph flow, after lymph node removal or from a tumor, also causes swelling.
| Mechanism | Example | Typical edema |
|---|---|---|
| High capillary pressure | Heart failure, DVT | Pitting, in low-lying areas or one leg |
| Low albumin | Cirrhosis, nephrotic syndrome | Pitting, widespread, face and abdomen |
| Leaky capillaries | Sepsis, burns, allergy | Widespread or local swelling |
| Lymph blockage | After lymph node removal | Firm, often non-pitting |
At the bedside
Press over the shin or ankle for a few seconds and see if a dent remains. Swelling of one leg suggests a local cause such as a deep vein clot; swelling of both legs suggests a body-wide cause such as heart, kidney or liver disease.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).