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Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis.
Bulk Flow Process
Bulk flow is used by small, lipid-insoluble solutes in water to cross the the capillary wall and is dependent on the physical characteristics of the capillary. Continuous capillaries have a tight structure reducing bulk flow. Fenestrated capillaries permit a larger amount of flow and discontinuous capillaries allow the largest amount of flow.
When moving from the bloodstream into the interstitium, bulk flow is termed filtration, which is favored by blood hydrostatic pressure and interstitial fluidoncotic pressure. When moving from the interstitium into the bloodstream, the process is termed reabsorption and is favored 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 Kidneys and Bulk Flow
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 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 the peritubular capillaries, where the previously filtered molecules are reabsorbed from the tubule of the nephron.
Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. Reabsorption 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