At the start of a capillary bed (closest to arterioles),...

At the start of a capillary bed (closest to arterioles), ultrafiltration moves
a. proteins into the capillary
b. interstitial fluid into the capillary
c. proteins into the interstitial fluid
d. water, ions, and small solutes into interstitial fluid

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Key Concepts

Starling Forces

This concept describes the balance between hydrostatic pressure and oncotic (colloid osmotic) pressure that governs the movement of fluids between the capillaries and the interstitial space. At the arteriole end of the capillary bed, higher hydrostatic pressure pushes water, ions, and small solutes out into the interstitial fluid, while the lower oncotic pressure at this end contributes less to retaining fluid within the vessels.

Capillary Ultrafiltration

Ultrafiltration in the microcirculation refers to the process where water along with small solutes is filtered across the capillary wall into the interstitial fluid. This movement is primarily driven by the hydrostatic pressure at the arterial end of the capillary bed, making it a key mechanism in the exchange of substances between blood and tissues.

Capillary Permeability

Capillary permeability explains the ability of substances to pass through the capillary wall. The structure of capillaries allows small molecules like water, ions, and small solutes to exit via ultrafiltration, while larger molecules such as proteins are generally retained unless there is an increase in permeability due to pathological conditions.

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