FUNCTIONS OF THE LOOP OF HENLE AND THE DISTAL NEPHRON the tunctions of this portion* of the nephron are Concentration of urine

load. [ N.B (i) TG feed back must not be confused with glomerulo tubular balance. (ii) The TG feed back, thus, also controls the glomerular blood flow and may cause renal autoregul ation] FUNCTIONS OF THE LOOP OF HENLE AND THE DISTAL NEPHRON the tunctions of this portion* of the nephron are Concentration of urine. Further reabsorplion of Na+, C!- and water. Acidification of urine Secretion of K+ and some drugs Some of these functions are Interdependent. Therefore, concentraticn of urine, electroyte and water absorption will be constiered together Concentration of urine. Countercurrent multiplier exchange system Some facts are restated at the onset. These are; 1. There are two classes of nephrons. Most of the (about 85% in man) nephrons belong to the class of cortical nephrons, whereas the rest (about 15%) belong to the class of juxta rnedullary nephrons. 2. The anatomical features (fig.8.1.2.) of these two class of nephrons are fairly different. The important differences are: (a) the loop of Henle in the Juxta medullary nephron, very long and the U-bend of the loop lies at the tip of the renal papilla, whereas the loops of Henle of the corticaI nephrons are short and end in the parts of the medulla dose to the cortex (that is, superficial parts of the meduia). (b) the blood vessels also show differences in the cortical nephrons, the efferent arteriole from the glomerulus breaks up into peritubular capillaries which remain in the cortex but n case ofthe juirla medullary nephrons, the efferent arteriole floes not break up in peritubular capillaries; instead, it gives rise to long delicate walled vessels (which are basically capillaries) called vasa recta (flg. 8.2.4) which follow the course of me loop of Henle and remain within the medulla. (c) the juxta medullary nephrons have, even in their ascending limb of the loop or Henle, a thin segment (fig 8.1.2). The actual mechanism of concentration of the tubular fluid (concentration of the urine) occurs by a process called counter current multiplier exchange system. The details are as follows A term, distal segment has been used In this book and means, loop of Henle + distal nephron (p. 465) 1. At its beginning, the descending limb the loop of Henle receives, from the pars recta of the proximal tubule, a fluid which is isotonic with plasma. The approximate composition of the fluid is : osmolalita 300 m osmcl/kg, NaC12 80 m mol\kg, urea 20 mmol\kg Fig. 8. 24. To show the difference of vascular supply between cortical and juxta medullary nephrons. 2. In the interstitium, that is in the pentubular spaces (ie, space which lies in between the tuoules) of medulla, the osmokality of the fluid, at this zone, called the outer zone of medulla, (fig. 8.2.5.) is between 300 to 600 mosm\kg Further deep into the medulla, that is, in the deeper zone, the osmolality rises more and more, till it becomes maximum at the level of the'U-bend where it beomes nearly 1200 mosmol/kg. Therefore, the medullary osmolality continuously rises from the outenmost zone to the deeper zone, and this phenomenon s called "medullary gradient 3. Concentration of the tubular fluid occurs rather spectacularly in the juxta medullary nephrons alone, therefore, the events which will be deseribed below occur only typically in a loop of Henle of a Juxtamedullary nephron but somewhat smilar events (Out In a law key) also occur in the cortical nephrons. 4. Assume that the flow of the fluid in the descending Iimb (of Henle s loop) is slopped for a while. The osrnolalty of the fluid in the peritubulr space (medeulary inter-stitium) is higher than that or the tubular fluid, Further, the descendig limb is freely permeable to water, but not to the electrolytes (Na* and CI-) As a result, the water moves out from the descending Iimb to enter the peritubular space and the fluid of the descending limb becomes concentrated in electrolytes (fig. 8.2 .5.). Then the fluid in the descending limb resumes movement and moves forward and, as s result, another small quantity of isotonic fluid from pars recta Is delivered into the descending Iimb. at the next moment, the forward movement of The fluid stops again and the process of concentration as mentioned above is repeated. By repeated repetitions of the whole process, there is intense concentration of the fluidi in the descending tubule, the fluid becoming more and more concentrated as it moves more and more to the U turn (fig 8,2.5). As the interstrtium is also becoming more and more hyperosmo laI (the meduIary gradient, see above), The outward movement of water continues. This mechanism results in multiplication of the effect that isr it causes multiplication (- intensification or amplification) of the concentration. further, the fluid In the ascending and descending limb moves In the opposite (countercurrent) direction. Hence the whole process is called countercurrent multiplier- system Fig 8 .2 .5 Medullary gradient Because of presence of this gradient, tubular fluid tn me descending loop of Henle Becomes very concentrated in sodlum and. as a result, the Na removal from the ascending limb involves much less energy, 5 Nest the fluid enters the ascending Iimb. As we are distussing juxtamedulary nephron. this part of the loop of Henle is also a thin segment. At this segment ie in the ascending limb, the tubule becomes impermeable to water but permeable o Na* and C-. The Na* is actively transported out from The ascending limb to the fluid of trie peritubular space which Is already rich In Na* However, thanks to the countercurrent multiplier system mentioned above, the Na* concentration within the ascending limb near the U bend b also (qiute heavy and so the work done (for transporting the Na* against concentration gradient) is not too great. Minimisation of work done for the active. Transport of the Na* from the ascending limb to the peritubutar space appears, on teleological ground. To be one of the fundamental aims of the counter current mutiplication. 6. Thus, the tubular fluid now. eventually, reaches the thick part of the ascending limb of the loop of Henle Na* and Cl- ions continue to be transported out from this part also, to the pentubular space bul this part e also totally impermeable to water (so water remains within the tubule) The results in hypolonicity of the fluid and the fluid delivered from the thick ascending Iimb to the distal nephron, in a juxtameduiary nephron, Is always hypotonic. The Na+ reabsorption at this segment follows. The Clreabsorption. Thus, the Cl reabsorption here, is pimary and Na* reabsorption is secondary (see 'chloride driven Na transport' earlier in this chapter) Similar events occur In the cortical nephrons also but as the length af the loop of Henle in the cortical ne phrons is short, the