Capillaries & Tissue Fluid

Hydrostatic Pressure ($P_h$): This is the physical pressure exerted by the blood against the capillary walls, generated by the heart's pumping action, which pushes fluid out of the capillary.

Oncotic Pressure ($\pi$): Also known as colloidal osmotic pressure, this is created by large plasma proteins (like albumin) that cannot pass through the capillary wall, exerting an 'inward pull' on water.

Arterial End Dynamics: At the start of the capillary bed, $P_h$ (approx. $4.3$ kPa) is greater than $\pi$ (approx. $3.3$ kPa), resulting in a net outward force that creates tissue fluid.

Ultrafiltration: This process filters out water, ions, and small nutrients into the intercellular spaces, while red blood cells and large proteins remain in the vessel.

Pressure Drop: As blood moves toward the venous end, hydrostatic pressure drops significantly (to approx. $1.6$ kPa) due to friction and the loss of fluid volume.

Constant Oncotic Pressure: Because large proteins remain trapped in the blood, the oncotic pressure stays relatively constant throughout the capillary length.

Net Inward Flow: At the venous end, the inward osmotic pull ($\pi$) exceeds the outward hydrostatic pressure ($P_h$), causing most of the water to move back into the capillary by osmosis.

Waste Removal: This return flow carries metabolic waste products, such as carbon dioxide and urea, from the cells back into the bloodstream for excretion.

Identify the Gradient: Always determine which pressure is dominant. If $P_h > \pi$, fluid moves OUT. If $\pi > P_h$, fluid moves IN.

Protein Permeability: A common exam trap is suggesting proteins leave the capillary. Remember: large plasma proteins are too big to pass through the endothelium.

Edema Analysis: If an exam question mentions swelling (edema), look for causes like high blood pressure (increased $P_h$) or starvation/liver disease (decreased $\pi$ due to low protein).

Calculation Check: Net Filtration Pressure ($NFP$) can be calculated as $(P_{h,cap} - P_{h,tissue}) - (\pi_{cap} - \pi_{tissue})$. Usually, tissue pressures are assumed to be near zero.