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Of all the fluid filtered at the arterial end of the capillary bed, about (85%) (15%) is reabsorded at the venous end.
Capillary Exchange -- Capillary exchange refers to exchange of material between the blood and tissues in the capillaries .
Capillary Exchange Mechanism -- There are commonly three mechanisms which facilitate the capillary exchange .These are -
Capillary Exchange Diffusion
Transcytosis
Bulk Flow
1 - Capillary Exchange Diffusion -- Diffusion is most widely mechanism , allows the flow of small molecules across capillaries .The process depends on the difference between interstitium and blood due to which molecules moving to low concentrated area from high concentration areas .Due to this difference -
Glucose and Oxygen from blood to the tissues.
Carbon di Oxide from the tissues in to the blood .
2 - Transcytosis -- This mechanism due to which large , lipid - insoluble substances at both sides of membranes.
This process causes movement of macromolecules across the interior of a cell through vesicles .
Due to this mechanism substances endocytosed from endothelial cell in to lipid vesicle and exocytosed from lipid vesicle to endothelial cell .
3 - Bulk Flow --By this mechanism small lipid insoluble solutes in water cross the capillary wall .Production of four Starling forces decides the modulate capillary dynamics .
The net filtration pressure decides the movement of the materials across the wall .
Filtration and Reabsorption by Kidney-- Capillary fluid movement occurs due to diffusion ( colloid osmotic pressure ) , transcytosis and filtration .
Filtration -- Movement of proteins or other large molecules from the blood in to the interstitium , this movement is favored by blood hydrostatic pressure and interstitial fluid oncotic pressure .
Reabsorption -- Movement of proteins or other large molecules from the interstitium in to blood . The movement of this type is favored by oncotic pressure and interstitial fluid hydrostatic pressure .
With the advancement of techniques it has been explained that in most cases , venular blood pressure exceeds the opposing pressure results in to a positive outward force .This shows that capillaries are normally in a state of filtration along their entire length .
Kidney is major site for bulk flow transport and the structural and functional unit of kidney is Nephron .Each nephron begins in a renal capsule composed of a glomerulus containing numerous capillaries enclosed in Bowman's capsule .
In all vascular beds of the body filtration is greatest at arterial end of capillaries .As the blood moves along the length of capillary the rate of fluid filtrate decreases but in some capillaries reabsorption can occur at the venular end of the capillaries and at small post capillary venules .
Blood enters the kidney is filtered by nephrons .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 .
Reabsorption -- It is two step process beginning with the active extraction of substances and passive extraction of substances .
Tubular Reabsorption occur due to solutes and water are removed from the tubular fluid and transported in to the blood .
The extraction of substances from the tubule fluid in to the renal interstitium and then the transport of these substances from the interstitium in to blood stream .
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