Starling Forces: Capillary Exchange, Hydrostatic and Oncotic Pressure

Starling forces describe the pressures that push fluid out of a capillary into the interstitial space and pull it back in. Two pressures push fluid out and two pull it in, and the balance sets the direction of net fluid movement.

The four pressures

Oncotic pressure (colloid osmotic pressure) is the osmotic pull created by large molecules, mainly albumin, that cannot cross the capillary wall. It is the same idea as osmotic pressure in the colligative properties notes, except that only the impermeant proteins count.

Net filtration pressure

Net filtration pressure = (P_c - P_i) - (π_c - π_i)

A positive value means net filtration out of the capillary. A negative value means net reabsorption into it.

Worked example

Typical textbook values: at the arteriolar end P_c = 35 mmHg, at the venous end P_c = 15 mmHg. Take P_i = 0 mmHg, π_c = 25 mmHg, π_i = 0 mmHg (approximate teaching values).

• Arteriolar end: (35 - 0) - (25 - 0) = +10 mmHg, so fluid leaves.
• Venous end: (15 - 0) - (25 - 0) = -10 mmHg, so fluid returns.

Hydrostatic pressure falls along the capillary because of friction and fluid loss, while oncotic pressure stays roughly constant. Filtration therefore dominates at the start and reabsorption at the end. In reality a little more fluid is filtered than reabsorbed, and the lymphatic system returns the excess, along with leaked protein, to the blood.

Edema: when the balance fails

Edema is excess interstitial fluid. Each cause maps to a Starling term.

• Higher capillary hydrostatic pressure: heart failure or venous obstruction backs pressure up into the capillaries.
• Lower plasma oncotic pressure: liver failure (less albumin made), malnutrition or nephrotic syndrome (albumin lost in urine).
• Increased capillary permeability: inflammation lets protein leak out, raising π_i.
• Lymphatic blockage: filtered fluid and protein cannot drain.

Link to the kidney

The glomerulus uses the same forces. High glomerular hydrostatic pressure drives filtration, and the rising oncotic pressure of the remaining plasma opposes it. The peritubular capillaries then reabsorb fluid because oncotic pressure beats their low hydrostatic pressure, as covered in the nephron note.

Common traps

• Confusing oncotic with hydrostatic. Hydrostatic is mechanical pressure; oncotic is osmotic pull from protein.
• Thinking albumin crosses easily. Its failure to cross is exactly what creates oncotic pressure.
• Forgetting the lymphatics. They explain why filtration slightly exceeds reabsorption without constant swelling.

One-minute recap

Net filtration = (P_c - P_i) - (π_c - π_i). Hydrostatic pressure pushes fluid out and falls along the capillary; oncotic pressure from albumin pulls fluid in and stays steady. Edema arises from high P_c, low π_c, leaky capillaries or blocked lymph.