Fluid continuously moves between the capillary lumen and tissue interstitium, driven by the balance of hydrostatic and oncotic pressures described by Starling's equation. In health, this maintains tissue fluid balance; imbalance leads to edema (when capillary hydrostatic pressure or vascular permeability rises) or dehydration.
Calculate net filtration pressure using typical values for hydrostatic and oncotic pressures in arteriolar and venular ends of capillaries. Apply Starling's equation to clinical scenarios like liver disease, malnutrition, and inflammation.
You already understand that blood is a complex fluid containing plasma proteins, cells, and dissolved solutes, and that water moves by osmosis from regions of low solute concentration to regions of high solute concentration. At the capillary level, these principles govern a continuous exchange of fluid between the blood and the surrounding tissues — a process that delivers nutrients, removes waste, and maintains tissue fluid balance every second of your life.
Two opposing forces drive fluid movement across the capillary wall. Hydrostatic pressure is the physical pressure of blood pushing outward against the capillary wall, which tends to force fluid out of the capillary into the interstitial space. Oncotic pressure (also called colloid osmotic pressure) is the osmotic pull exerted by plasma proteins — primarily albumin — that are too large to cross the capillary wall, and this force tends to pull fluid back into the capillary. The Starling equation formalizes this balance: net filtration pressure equals the difference between hydrostatic pressures (capillary minus interstitial) minus the difference between oncotic pressures (capillary minus interstitial). When the net pressure is positive, fluid filters out; when negative, fluid is reabsorbed.
In a typical capillary, pressures shift along its length. At the arteriolar end, capillary hydrostatic pressure is relatively high (around 35 mmHg) because blood has just arrived from the arteriole. This exceeds the inward oncotic pull (~25 mmHg), so the net force pushes fluid out — filtration dominates. As blood flows toward the venular end, hydrostatic pressure drops (to about 15 mmHg) because fluid has been lost and resistance has dissipated, while oncotic pressure stays roughly constant (plasma proteins are concentrated by the fluid loss). Now oncotic pressure exceeds hydrostatic pressure, and fluid is pulled back in — reabsorption dominates. The result is that most of the filtered fluid returns to the capillary, and the small excess is collected by the lymphatic system.
When this balance is disrupted, the clinical consequence is edema — excess fluid accumulation in the interstitial space. Consider the mechanisms: if capillary hydrostatic pressure rises (as in heart failure, where venous congestion backs up into capillaries), more fluid is pushed out than can be reabsorbed. If plasma oncotic pressure falls (as in liver disease or malnutrition, where albumin synthesis drops), the inward pull weakens and fluid leaks out. If capillary permeability increases (as in inflammation or burns, where histamine and other mediators widen the gaps between endothelial cells), proteins escape into the interstitium, reducing the oncotic gradient. Each of these scenarios disrupts a different term in the Starling equation, but all produce the same result: fluid accumulates where it should not, and tissues swell.