Interstitial Fluid: The Plasma That Leaves Capillaries to Bathe Cells

From Blood Plasma to Interstitial Fluid

When hydrostatic pressure forces plasma across capillary walls into the tissue space, it becomes interstitial fluid (tissue fluid).

Composition

• Water, electrolytes, nutrients, oxygen, signaling molecules.
• Very little protein (capillary barrier retains most albumin).

Function

Bathes cells → enables exchange of nutrients, wastes, and gases between blood and cells. The lymphatic system returns excess fluid and proteins to the circulation.

Clinical Note

Edema = excess interstitial fluid (due to increased hydrostatic pressure, decreased oncotic pressure, lymphatic obstruction, or inflammation).