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.300 300 100 300Lower panel: It shows theNacountercurrent exchange rela-H2O H2OH2O600 600 400 600 tionship between Henle s loopNa and collecting duct.See textfor further details.RedrawnH2O H2OH2O900 900 700 900after Pitts (1966).NaH2O H2OH2O1200 1200 1000 12001200 1200 1200Because of the U-like shape of the loop, the fluid in both limbs flow in acountercurrent way (opposite directions) so that there is a multiplicatoryeffect that, in the end, becomes a longitudinal gradient.(Figure 2.53, upper panel, drawn horizontally) explains the build-upprocess step by step: At stage 1, say that fluid gets into the loop filling itfully at the same concentration, entering via the descending limb and ex-iting along the ascending portion.Thereafter, at stage 2, due to the basicactive transport of sodium, a 200 mOsm/L is transversely generated allalong the loop (say that the descending side rises its concentration to 400mOsm/L while the other side lowers it to 200 mOsm/L).However, flowcontinues and a moment later, stage 3 depicts the situation when fresh 138 Understanding the Human Machinefluid at 300 mOsm/L gets in shifting the whole column by a certainlength.Again the active mechanism restores the osmolar difference(stage 4), but observe that the region of the bent (right side in the figure,right upper panel) begins to increase its concentration creating a differ-ence with respect to the entrance.Successive fluid shifts followed alwaysby the basic transversal osmolar gradient build-up plus the countercurrentflow give rise to a longitudinal much larger gradient between the en-trance (at 300 mOsm/L) and the medullar region (reaching there 1,2001,400 mOsm/L).The mechanism is most interesting and ingenious, somuch that it can be qualified as outstanding.A neat design of the GreatEngineer, indeed! Rash (1984) simulated with a microprocessor this mul-tiplying phenomenon, which may even have technological applications.With the present technology, such simulation should be easier and a realchallenge for biomedical engineering students.2.4.5.2.Countercurrent exchangerThe medullo-cortical concentration gradient, however, tends to equili-brate, as a ball placed on a ramp tends to roll down unless something isdone either to prevent it or at least to partially brake it.Another mecha-nism is needed to conserve such gradient.The vasa recta of the peritubu-lar capillaries that run parallel to Henle s limbs take care of that function(Figure 2.53).Notice the specialization of the medullary nephrons.Bloodgets into the vasa recta at 300 mOsm/L and, as it moves down in theirdescending branches, water traverses the capillary walls because of theosmotic pull from outside while active osmotic particles get into theblood, also due to a small transverse gradient.As blood goes up follow-ing the ascending limbs of the vasa recta the opposite shifts take place,that is, water gets into blood and solutes go out because at any level theascending side has a slightly higher concentration than the descendingone.Thus, the countercurrent exchanger reduces excessive loss of osmo-tically active solutes from the inner medulla.Blood in the vasa recta re-move sodium and water.Loss of the medullocortical osmotic gradientwould be disastrous for animal or human life.This mechanism is similar to the countercurrent heater exchangerswidely used in industrial plants, it has been amply studied both theoreti-cally and experimentally and its better efficiency has been fully demon-strated as compared to heater exchangers with parallel streams in the Chapter 2.Source: Physiological Systems and Levels 139same direction.Penguins have in their legs a circulatory arrangement ofthe same kind that helps them in keeping a temperature gradient from thetrunk to the feet.Does it make the feet warmer? No, but the upper legs,the abdomen and the upper body better conserve body temperature.Theextremities of the sloth also have the same circulatory arrangement, how-ever, its function is unknown, especially if we recall that they live intropical regions, like NE Brasil.Another stimulating subject for the in-quisitive mind.2.4.5.3.Osmotic exchangerThe distal convoluted tubules and the collecting ducts (more the latterthan the former) regulate the final osmotic urine adjustment by the actionor not of the antidiuretic hormone (ADH), which, as mentioned before,controls their wall permeability to water.Most of the water is recoveredand only a minor amount is excreted, either as hypotonic or hypertonicurine (measured with respect to the plasmatic 300 mOsm/L), and depend-ing on the hydration degree of the subject.Sodium is also recoveredalong this final pathway along with other ions (such as phosphate andbicarbonate).Thus, the final urine equilibrates with the hypertonic inter-stitium of the renal medulla and papillae.The major osmotically active constituents of urine are sodium and chlo-ride ions and urea.The osmolar clearance may be calculated from a for-mula derived from the clearance definition given above, that is,[Uosm]VCosm =(2.100)[Posm]In this expression, [Uosm] represents the collected urine osmotic concen-tration, V the collected volume in a given period of time and the denomi-nator stands for the plasma osmolarity.In words, it is defined as the vol-ume of plasma per unit time completely cleared of osmotically activesolutes or, also, as the osmolar excreted load per unit of plasmatic os-motic concentration.The interested student can find details and mecha-nisms in the literature.2.4.6.Renal Blood FlowLet us go back to Figure 2.51 writing now the continuity equations fornodes Gl and Ca, 140 Understanding the Human Machine¦[AA]- F[Px]-¦'[AE]= 0 (2.101)'¦´[AE]-S[Kx]+ R[K"]-¦[V]= 0 (2.102)xSolving for Æ [AE] the two previous equations, equating, and consideringequation (2.88), the following is easily obtained,¦[AA]- ¦[V ]= E[Ux](2 [ Pobierz caÅ‚ość w formacie PDF ]
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.300 300 100 300Lower panel: It shows theNacountercurrent exchange rela-H2O H2OH2O600 600 400 600 tionship between Henle s loopNa and collecting duct.See textfor further details.RedrawnH2O H2OH2O900 900 700 900after Pitts (1966).NaH2O H2OH2O1200 1200 1000 12001200 1200 1200Because of the U-like shape of the loop, the fluid in both limbs flow in acountercurrent way (opposite directions) so that there is a multiplicatoryeffect that, in the end, becomes a longitudinal gradient.(Figure 2.53, upper panel, drawn horizontally) explains the build-upprocess step by step: At stage 1, say that fluid gets into the loop filling itfully at the same concentration, entering via the descending limb and ex-iting along the ascending portion.Thereafter, at stage 2, due to the basicactive transport of sodium, a 200 mOsm/L is transversely generated allalong the loop (say that the descending side rises its concentration to 400mOsm/L while the other side lowers it to 200 mOsm/L).However, flowcontinues and a moment later, stage 3 depicts the situation when fresh 138 Understanding the Human Machinefluid at 300 mOsm/L gets in shifting the whole column by a certainlength.Again the active mechanism restores the osmolar difference(stage 4), but observe that the region of the bent (right side in the figure,right upper panel) begins to increase its concentration creating a differ-ence with respect to the entrance.Successive fluid shifts followed alwaysby the basic transversal osmolar gradient build-up plus the countercurrentflow give rise to a longitudinal much larger gradient between the en-trance (at 300 mOsm/L) and the medullar region (reaching there 1,2001,400 mOsm/L).The mechanism is most interesting and ingenious, somuch that it can be qualified as outstanding.A neat design of the GreatEngineer, indeed! Rash (1984) simulated with a microprocessor this mul-tiplying phenomenon, which may even have technological applications.With the present technology, such simulation should be easier and a realchallenge for biomedical engineering students.2.4.5.2.Countercurrent exchangerThe medullo-cortical concentration gradient, however, tends to equili-brate, as a ball placed on a ramp tends to roll down unless something isdone either to prevent it or at least to partially brake it.Another mecha-nism is needed to conserve such gradient.The vasa recta of the peritubu-lar capillaries that run parallel to Henle s limbs take care of that function(Figure 2.53).Notice the specialization of the medullary nephrons.Bloodgets into the vasa recta at 300 mOsm/L and, as it moves down in theirdescending branches, water traverses the capillary walls because of theosmotic pull from outside while active osmotic particles get into theblood, also due to a small transverse gradient.As blood goes up follow-ing the ascending limbs of the vasa recta the opposite shifts take place,that is, water gets into blood and solutes go out because at any level theascending side has a slightly higher concentration than the descendingone.Thus, the countercurrent exchanger reduces excessive loss of osmo-tically active solutes from the inner medulla.Blood in the vasa recta re-move sodium and water.Loss of the medullocortical osmotic gradientwould be disastrous for animal or human life.This mechanism is similar to the countercurrent heater exchangerswidely used in industrial plants, it has been amply studied both theoreti-cally and experimentally and its better efficiency has been fully demon-strated as compared to heater exchangers with parallel streams in the Chapter 2.Source: Physiological Systems and Levels 139same direction.Penguins have in their legs a circulatory arrangement ofthe same kind that helps them in keeping a temperature gradient from thetrunk to the feet.Does it make the feet warmer? No, but the upper legs,the abdomen and the upper body better conserve body temperature.Theextremities of the sloth also have the same circulatory arrangement, how-ever, its function is unknown, especially if we recall that they live intropical regions, like NE Brasil.Another stimulating subject for the in-quisitive mind.2.4.5.3.Osmotic exchangerThe distal convoluted tubules and the collecting ducts (more the latterthan the former) regulate the final osmotic urine adjustment by the actionor not of the antidiuretic hormone (ADH), which, as mentioned before,controls their wall permeability to water.Most of the water is recoveredand only a minor amount is excreted, either as hypotonic or hypertonicurine (measured with respect to the plasmatic 300 mOsm/L), and depend-ing on the hydration degree of the subject.Sodium is also recoveredalong this final pathway along with other ions (such as phosphate andbicarbonate).Thus, the final urine equilibrates with the hypertonic inter-stitium of the renal medulla and papillae.The major osmotically active constituents of urine are sodium and chlo-ride ions and urea.The osmolar clearance may be calculated from a for-mula derived from the clearance definition given above, that is,[Uosm]VCosm =(2.100)[Posm]In this expression, [Uosm] represents the collected urine osmotic concen-tration, V the collected volume in a given period of time and the denomi-nator stands for the plasma osmolarity.In words, it is defined as the vol-ume of plasma per unit time completely cleared of osmotically activesolutes or, also, as the osmolar excreted load per unit of plasmatic os-motic concentration.The interested student can find details and mecha-nisms in the literature.2.4.6.Renal Blood FlowLet us go back to Figure 2.51 writing now the continuity equations fornodes Gl and Ca, 140 Understanding the Human Machine¦[AA]- F[Px]-¦'[AE]= 0 (2.101)'¦´[AE]-S[Kx]+ R[K"]-¦[V]= 0 (2.102)xSolving for Æ [AE] the two previous equations, equating, and consideringequation (2.88), the following is easily obtained,¦[AA]- ¦[V ]= E[Ux](2 [ Pobierz caÅ‚ość w formacie PDF ]