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Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis.
Bulk flow is used by small, lipid-insoluble solutes in water in order to cross the the capillary wall and is dependent on the physical characteristic of the capillary. Continuous capillaries with their tight structure reduce bulk flow, fenestrated capillaries permit a larger amount and discontinuous capillaries allow the largest amount of bulk flow.
The movement of materials across the capillary wall is dependent on pressure and is bi-directional depending on the net filtration pressure derived from the four Starling forces.
When moving from the bloodstream into the interstitium bulk flow is termed filtration, which is favoured by blood hydrostatic pressure and interstitial fluid oncotic pressure. Alternatively when moving from the interstitium into the bloodstreatm the process is termed re-absorption and is favoured by blood oncotic pressure and interstitial fluid hydrostatic pressure.
Modern evidence shows that in most cases venular blood pressure exceeds the opposing pressure, thus maintaining a positive outward force. This indicates that capillaries are normally in a state of filtration along their entire length.
The kidney is a major site for bulk flow transport. Blood that enters the kidneys is filtered by nephrons, the functional unit of the kidney. Each nephron begins in a renal corpuscle, which is composed of a glomerulus containing numerous capillaries enclosed in a Bowman's capsule. Proteins and other large molecules are filtered out of the oxygenated blood in the glomerulus and pass into
Bowman's capsule and the tubular fluid contained within. Blood continues to flow around the nephron until it reaches another capillary rich region termed the peritubular capillaries, where the previously filtered molecules are reabsorbed from the tubule of the nephron.
Tubular re-absorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. Re-absorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium, and then the transport of these substances from the interstitium into the bloodstream