The blood colloid osmotic pressure BCOP is the 2nd force opposing filtration. It is mainly due to the presence of proteins eg albumin, globulins etc. These proteins normally cannot pass through the endothelial-capsular membrane and so remain within the glomerular capillaries.
Osmotic pressure is the pressure required to prevent the net movement of water into a solution containing solutes when the solutions are separated by a selectively permeable membrane. The greater the concentration of solutes in a solution, the greater its osmotic pressure. The plasma proteins suspended in blood cannot move across the semipermeable capillary cell membrane, and so they remain in the plasma.
As a result, blood has a higher colloidal concentration and lower water concentration than tissue fluid. It therefore attracts water. We can also say that the BCOP is higher than the interstitial fluid colloidal osmotic pressure IFCOP , which is always very low because interstitial fluid contains few proteins.
Thus, water is drawn from the tissue fluid back into the capillary, carrying dissolved molecules with it. This difference in colloidal osmotic pressure accounts for reabsorption. The normal unit used to express pressures within the cardiovascular system is millimeters of mercury mm Hg. When blood leaving an arteriole first enters a capillary bed, the CHP is quite high—about 35 mm Hg. Gradually, this initial CHP declines as the blood moves through the capillary so that by the time the blood has reached the venous end, the CHP has dropped to approximately 18 mm Hg.
In comparison, the plasma proteins remain suspended in the blood, so the BCOP remains fairly constant at about 25 mm Hg throughout the length of the capillary and considerably below the osmotic pressure in the interstitial fluid. The net filtration pressure NFP represents the interaction of the hydrostatic and osmotic pressures, driving fluid out of the capillary. Since filtration is, by definition, the movement of fluid out of the capillary, when reabsorption is occurring, the NFP is a negative number.
NFP changes at different points in a capillary bed. Recall that the hydrostatic and osmotic pressures of the interstitial fluid are essentially negligible. Thus, the NFP of 10 mm Hg drives a net movement of fluid out of the capillary at the arterial end. At this point, there is no net change of volume: Fluid moves out of the capillary at the same rate as it moves into the capillary. Near the venous end of the capillary, the CHP has dwindled to about 18 mm Hg due to loss of fluid.
Because the BCOP remains steady at 25 mm Hg, water is drawn into the capillary, that is, reabsorption occurs. Figure 1. Net filtration occurs near the arterial end of the capillary since capillary hydrostatic pressure CHP is greater than blood colloidal osmotic pressure BCOP. The major force pushing fluid along and out of the capillary is blood hydrostatic pressure.
The major force driving fluid back into a capillary is the osmotic pressure of the blood. In brief, if blood hydrostatic presure is greater than blood osmotic pressure then fluid moves out of the capillary. If blood Osmotic pressure is greater than blood hydrostatic pressure, then fluid moves into the capillary. Over the length of a 'typical' capillary, blood hydrostatic pressure falls so that the balance shifts from a driving force for efflux to a driving force for influx.
Read on for more details and to understand why the glomerular capillaries in Bowman's capsule are slightly different. The main driving force for filtration is the hydrostatic pressure of the blood. The Starlings forces in renal glomerular capillaries are slightly different from those elsewhere in the body. Hydrostatic pressure remains high along the whole length of the capillary and so the balance of the Starling forces is always towards fluid efflux from the capillary.
It would be pointless to filter the blood at the arterial end of the capillary if most of the filtrate came back with a fall in hydrostatic pressure before it reached the far end.
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.
Tubular Secretion : Diagram showing the basic physiologic mechanisms of the kidney and the three steps involved in urine formation. Learning Objectives Explain the process of filtration and reabsorption in capillaries. Key Points Bulk flow is a process used by small lipid-insoluble proteins to cross the capillary wall.
Capillary structure plays a large role in the rate of bulk flow, with continuous capillaries limiting flow and discontinuous capillaries facilitating the greatest amount of flow. When moving from the blood to the interstitium, bulk flow is termed filtration. When moving from the interstitium to the blood, bulk flow is termed re-absorption.
0コメント