THE GLOMERULUS 917 The glomerular \201 lter See Figs 13.3 and 13.4. The glomerular \201 lter is composed of four layers: Charged endothelial glycocalyx. \200 Endothelial fenestrations. \200 The glomerular bas ement membrane. \200 The inter-podocyte slit diaphragm. The inter-podocyte slit diaphragm offers a mesh of interlocking proteins and lipids, includ ing nephrin, podocin, CD2AP, and podocalyxin. Podocyte dysfunction (at the heart of many proteinuric nephropa- thies) impairs both the slit diaphragm and foot process adhesion. Filtration is principally determined by molecular size and, to a much lesser extent, by charge. The glomerular barrier is very permeable to water. Substances with a MW <5,000Da are freely \201 ltered (unless albumin-bound in plasma). Larger molecules are partially \201 ltered, with \201 ltration fraction depend- ing on both size and charge. Negatively charged molecules have a lower \201 ltration fraction th an similarly sized cationic molecules. Albumin (MW 61kDa) is polyanionic and scarcely \201 ltered under nor- mal circumstances. Approximately 70,000g passes through the glomeru- lus every 24h, of which just 7g (0.01%) a ppears in the \201 ltrate (and this is usually reabsorbed further down the nephron). CHAPTER 13 Renal physiology 918 Extra-glomerular matrix Mesangial cells Efferent arteriole Macula densa Afferent arteriole Glomerular basement membrane Fenestrated endothelium Podocytes Foot process Urinary space Parietal epithelium e u b u t l l a m x o r P i Fig. 13.2 Glomerular structure. Blood enters via the afferent arteriole and leaves via the efferent arteriole. The capillaries have a fenestrated endothelium. The visceral epithelium of Bowman \200\231 s capsule consists of podocytes, the foot processes of which (with the GBM) cover the capillaries and the mesangium. At the vascular pole, the visceral epitheliu m re \202 ects into the parietal epithelium, which itself becomes the proximal tubule at t he urinary pole. US, urinary space . Reproduced from Fogo, A, Kashgarian, M (2011). Diagnos tic Atlas of Renal Pathology , 2nd edn. With permission from Elsevier. THE GLOMERULUS 919 PO E MM GBM M Fig. 13.3 Schematic demonstrating the \201 ltration barrier. The glomerular capillary has a fenestrated endothelium (E). The capillary is surrounded by the GBM, which deviates to c over the mesangial cells (M). The interdigitating foot processes of the podocyte (PO), sepa rated by the slit diaphragm, cover the GBM and form the \201 nal barrier to \201 ltrati on. Reproduced with permission from Davison AMA, Cameron JS, Grunfeld J-P et al (eds) (2005 ). Oxford Textbook of Clinical Nephrology, 3rd edn. Oxford, Oxford University Press. Fig. 13.4 Scanning EM of rat glomerular capillaries. The capillary is covered by branching podocytes. The primary (P) and secondary (F) processes interdigitate, separated by slit dia phragms, and cover the entire surface of the GBM. Reproduced with permission from Davison A MA, Cameron JS, Grunfeld J-P et al (eds) (2005). Oxford Textbook of Clinical Nephrology, 3r d edn. Oxford, Oxford University Press. CHAPTER 13 Renal physiology 920 Regulation of GFR Introduction GFR is dependent on the net hydrostatic and colloid osmotic pressure gradients between glomerular plasma and the \202 uid in Bowman \200\231 s space . This will be determined by: \200 Renal blood \202 ow (RBF): Renal blood \202 ow represents 20 \200\223 25% of cardiac output (1.25L/ min). Two to three million nephrons produce an ultra \201 ltrate of plasma (125mL/min or 7 170L/day). Most water and solutes are all reabsorbed into the tubular epithelium. Glomerular structure ( \201 ltration surface area and permeability). \200 Transglomerula r capillary pressure (afferent \200\223 efferent tone). \200 Plasma oncotic pressure. Physiological regulation Autoregulation Preserves renal blood \202 ow and GFR despite var iations in systolic BP. Predominantly effected by the afferent arteriole: local stretch rec eptors quickly adjust afferent arteriolar tone via a \200\230 myogenic re \202 ex \200 \231 . Afferent and arteriolar tone Controls intra-glomerular pressure and \202 ow: \200 Incre ased afferent arteriolar tone (vasoconstriction) leads to reduced \202 ow and reduced pressure within the glomerulus. Increased efferent arteriolar tone (vasoconstriction) leads to reduced \202 ow and increased pressure within the glomerulus. Tubuloglomerular feedback TGF allows tubular \202 ow sensing to change GFR: Cl \200\223 delivery to the juxtaglomerular apparatus is sensed at the macula densa ( b p. 921). i dist al Cl \200\223 delivery l afferent arteriolar vasoconstriction l d GFR (medi- ators includ e adenosine, adenosine triphosphate, thromboxane, NO, and A2). The importance (and beauty) of this mechanism can be appreciated if large quantities of Cl \200\223 (and thus Na + ) a re pathologically delivered to the distal tubule (e.g. non-oliguric ATN). Through TGF, d re nal blood \202 ow leads to d GFR, safeguarding against profound diuresis and volume deple tion. In other words, TGF basically aims to avoid volume depletion/ overload if GFR i or d . Systemic factors \200 The sympathetic nervous system (noradrenaline/norepinephrine) is a key mediator of vasoconstriction of the afferent arteriole. Thus, in systemic hypotension ( l i sympathetic activity), renal blood \202 ow is reduced (allowing blood to be diverted to the brain and heart). Noradrenaline also stimulates production of renin and A2. A2, with its potent vasoconstrictive effects at the efferent arteriole (with much weaker vasoconstrictor effects at the afferent arteriole), will assist in the ma intenance of GFR ( (cid:2) this is why ACE-I drop REGULATION OF GFR 921 transglomerular capillary pressures). Renin also increases with sodium depletion l i A2. Vasodilator prostaglandins (PGE 1 , PGE 2 , and prostacyclin) are also important ( (cid:2) hence the need to avoid NSAIDs when renal blood \202 ow is compromised). Other factors: endothelins (esp. ET 1 ) are vasoconstrictors (efferent > afferent). ANP and BNP cause afferent dilatation. NO relaxes both the afferent and efferent arteriole and inc reases renal blood \202 ow. The juxtaglomerular apparatus \200 The juxtaglomerular apparatus has a regulatory role. I t consists of a specialized area of cells that are intimately related to the distal nephron and to the adjacent afferent and efferent arterioles and glomerulus. It comprises the macula densa, extra-glomerular mesangium, and arterioles (the last part of the afferent and the \201 rst part of the efferent). The macula densa is situated toward the end of the thick ascending limb of the loop of Henle (i.e. in the distal nephron) (see Fig. 13.1). The cells of the macula densa are closely packed with large nuclei and attached to a baseme nt membrane that is intimately related to the extra-glomerular mesangium. They contain larg e amounts of NO and cyclo-oxygenase. Modi \201 ed smooth muscle cells, called granular cells, surround the afferent arteriole. \200\230 Granular \200\231 refers to conspicuous cytoplasmic granules containing renin th at is ready for exocytosis into the surrounding interstitium. Granular cells have dense sym pathetic innervation. Tubular Cl \200\223 is sensed in the macula densa as part of tubuloglomerular feedback ( b p. 920). This instigates changes in glomerular blood \202 o w and GFR via arteriolar haemodynamics. Renin release is also controlled by this system, al though the exact mechanisms are unclear. CHAPTER 13 Renal physiology 922 Tubular function: overview Introduction The tubular epithelium is a single cell layer. The cells have a distinct polar- ity (determined by their cytoskeleton). Their luminal (apical or tubular) surface is exposed to \201 ltrate and their basolateral surface to interstiti al \202 uid (and capillary beds). Speci \201 c transporters are located in the mem- bran es at each of these surfaces. There is a tight junction between cells near the luminal side . Solute and water movement may be passive via paracellular routes (regulated via the tight junction) or active via transcel- lular routes (regulated via the membrane transporters). This movement may be from tubular \202 uid to blood (reabsorption) (see Table 13.1) or fr om blood to tubular \202 uid (secretion). The four sections of the renal tubule are described on b p. 914. Basic concepts \200 The tubules: Reabsorb the majority of the \201 ltrate (mainly the proximal convoluted tubule (PCT)). Given that 7 180L of plasma is \201 ltered each day, an enormou s amount of reabsorption is required. Regulate solute and water balance (PCT, loop of Henle (LH), distal tubule (DT), and collecting ducts (CD)). \200 Regulate acid \200\223 base balance (DT an d CD). Table 13.1 Sites of reabsorption of the major ions in the nephron (%) Na + 65 K + 65 HCO 3 \200\223 80 Ca 2+ 70 PCT Loop 25 30 10 \200\223 15 20 DT and CD 0 \200\223 10 0 \200\223 5 0 \200\223 5 10 Passive transportation occurs via: Simple diffusion down a concentration or charge gradient. \200 Carrier-mediated diffusion where a speci \201 c membrane carrier assists transportation across the cell membrane. A speci \201 c membrane channel. Active transportation refers to an ion being moved against a concentration, or charge, grad ient. It requires energy (from enzymatic hydrolysis of ATP). The most important active tran sporter is the sodium pump (Na + K + -ATPase). Another example of an active transporter in the tubule is the H + ATPase (excretes H + in the distal nephron). TUBULAR FUNCTION: OVERVIEW 923 The sodium pump (Na + K + -ATPase) \200 Transfers three Na + from inside a cell in exchan ge for two K + (this imbalance means it moves a net positive charge). In the kidney, it is found exclusively at the basolateral (capillary) surface of the tubule and plays a prime role in tubular reabsorption. \200 It keeps intra cellular Na + concentration very low (10 \200\223 30mmol/L) and K + concentration very high ( 7 150mmol/L). Na + will then want to move back into the cell down the electrochemical gradient that has been created \200\224 this occurs either through speci \201 c sodium channels or via carrier proteins. These carrier proteins provide an opportunity for co-transportation of other ions into the cell (e.g. K + and Cl \200\223 ) or countertransport out of the cell ( e.g. H + and Ca 2+ ) against concentration, or charge, gradients, i.e. to take advantage of the movement of Na + . Glomerulo-tubular balance The process whereby a change ( i or d ) in GFR is compensated for by a corresponding change in absorption by the rest of the nephron. This prevents changes in the \201 ltered load having major consequences for solute excretion. For example, the amount of sodium reabsorbed in each nephron segment is, in general, proportional to the amo unt of sodium delivered to that segment. The mechanisms underlying this are poorly understo od. CHAPTER 13 Renal physiology 924 The proximal convoluted tubule Introduction \200 The PCT reabsorbs the bulk of sodium, ch loride, bicarbonate, glucose, amino acids, urate, water ( 7 60%), and low molecular weight proteins (e.g. B 2 microglobulin). Appreciable amounts of Ca 2+ and PO 4 are also reabsorbe d here (see Table 13.1). It is not homogenous \200\224 it consists of two parts, a convoluted (cortical) section and a straight (medullary section). The \200\230 average \200\231 PCT is 7 14mm long and offers a large surface area (due to the villous-like arrangement at the apical surface of epithelial cells). The normal kidney contains around 1,000,000 glomeruli; this equates to a potential surface area for reabsorpt ion of >50m 2 . The PCT is rich in mitochondria. \200 The PCT is able to reabsorb up to 65% of the 24,000 mmol of Na + and 180L of water \201 ltered per day. Sodium and chloride \200 In the early part of the PCT, most sodium is reabsorbed via spec i \201 c transporters (see Fig. 13.5), coupled with absorption of glucose and organic mole cules. A further transporter exchanges Na + with H + ions. \200 The energy for these processes c omes from the sodium gradient into the cell, itself generated by the Na + K + -ATPase ( b p. 923). The gap junctions between epithelial cells are slightly leaky, so the PCT is highly permeable to water. In the early PCT, chloride (an excess is generated when sodium is absorbed with other anions or molecules) is reabsorbed via the paracellular route. Further along the PCT, Na + and H + are exchanged, while Cl \200\223 is exchanged for another base (e.g. formate, bicarbonate, oxalate) (see Fig. 13.6). The base a nd H + are then reabsorbed, so the net result is reabsorption of NaCl. Cl \200\223 leaves the cell in exchange for K + or via speci \201 c chloride pumps. Potassium In this area of the nephron, K + is mostly reabsorbed down a concentration gradie nt via the paracellular space. Bicarbonate (see also b p. 818) The PCT reabsorbs 90% of \201 ltered HCO 3 but does not acidify the urine. HCO 3 reabsorption is accomplished by mea ns of H + secretion. Dissociation of carbonic acid (H 2 CO 3 ) within epithelial cells yiel ds an H + ion that enters the \201 ltrate via the Na + /H + exchanger in the luminal mem- brane (see Fig. 13.5). The H + combines with HCO 3 in the \201 ltrate to form carbonic a cid, which is rapidly converted to CO 2 and H 2 O by carbonic anhydrase. This recycling of H + means there is no net acid excretion at this point. CO 2 enters the cell by simple diff usion and is converted back to carbonic acid to start the cycle again. Within the cell, the HCO 3 gener- ated from carbonic acid dissociation leaves via the basolateral surface in ex change for Cl \200\223 or in association with Na + . THE PROXIMAL CONVOLUTED TUBULE 925 Calcium Mainly paracellular reabsorption across the tight junction down a concen- tration ( and charge) gradient. Phosphate Co-transported into the cell along with Na + . 2 Inhibited by PTH \200\224 meani ng that PTH is phosphaturic. Glucose, amino acids Both are co-transported into the cell along with Na + . The sodium-glu cose co-transporter 2 (SGLT2) has become a novel therapeutic target in diabe- tes (inhibiti on l d glucose reabsorption l glucose excretion). The SGLT2 inhibitor dapagli \202 ozin (S E: weight loss, dehydration) is approved for use in T2DM. Lumen Interstitium 3 Na+ Na+ Glucose Organic ions ATP 2 K+ Na+ H++ HCO3 \200\223 \200\223 HCO3 Na+ \200\223 HCO3 H+ + H2CO3 H2CO3 CA CA CO + H O H O + CO 2 2 2 2 Cl \200\223 H2O Fig. 13.5 Major pathways of solute reabsorption in the early part of the PCT. CA, carbonic anhydrase. Adapted with permission from Greger R (1999) New insights into the molecular mec hanism of the action of diuretics. Nephrol Dial Transplant 14 : 536 \200\223 40. Lumen Interstitium 3 Na+ Cl \200\223 Base \200\223 2 K+ Na+ Cl \200\223 H+ Fig. 13.6 Major pathways of solute reabsorption in the late part of the PCT. Adapted with p ermission from Greger R (1999) New insights into the molecular mechanism of the action of d iuretics. Nephrol Dial Transplant 14 : 536 \200\223 40. CHAPTER 13 Renal physiology 926 The loop of Henle Introduction The prime function of the loop of Henle is to establish a gr adient of osmo- lality in the renal interstitium and 6 the tubular \202 uid. This allows the con- centration of urine to be varied widely. Structure \200 The loop dives deep into the renal medulla and then back out into the cortex. It consists of a thin descending limb, a thin ascending limb (in some nephrons), and a thick ascending limb. There are important solute and water permeability di fferences along the loop. The macula densa ( b p. 921) is found close to the thick ascending limb. Capillaries serving the loop accompany it on its journey. These vasa recta have a bespoke \200\230 hairpin \200\231 arrangement, as a \200\230 standard \200\231 capillary network would simply allow the medullary osmotic gradient to dissipate through equilibration with capillary blood. This U-shaped organization allows water loss and solute entry in the descending limb to be offset by water entry and solute loss in the ascending. This passive exchange of water and solutes perpetuates the hypertonic extracellular environment of the medulla that is crucial for water homeostasis. It is refer red to as \200\230 countercurrent exchange \200\231 . The thick ascending limb contains cells rich in mitochondria, enabling the active transport ation of electrolytes. The reabsorption of Na + , K + , Cl \200\223 , Ca 2+ continues in t his section of the nephron, and the majority of Mg 2+ is recovered here. Solute transportat ion (See Fig. 13.7) \200 The descending limb is impermeable to Na + , but water moves int o the interstitium down the osmotic gradient. The result is a high luminal Na + and Cl \200\223 concentration. Na + reabsorption is passive in the thin ascending limb (Na + and Cl \200\223 move into the interstitium down a concentration gradient) and active in the thick ascending limb. The active process is again driven by the basolateral Na + K + -ATPase (the low intr acellular Na + concentration it generates allows Na + entry from the lumen). This Na + enters principally through the Na + , K + , Cl \200\223 , Cl \200\223 (NKCC) co-transporter, which is unique to this section of the nephron (and the target of loop diur etics) ( b p. 792). Na + then exits the cell on the basolateral side through the Na + K + -ATPase, while Cl \200\223 and K + exit through a co-transporter. In addition, K + re-enters the lumen throug h a luminal K + channel (ROMK channel) \200\224 a recycling process that is necessary to p revent K + availability from becoming a limiting factor for the operation of NKCC. This movement of K + back to the lumen also keeps it electrically net positive, which facil itates the passive paracellular reabsorption of Na + (as well as K + , Ca 2+ , NH 4 + , and Mg 2+ ). THE LOOP OF HENLE 927 Lumen Interstitium 3 Na+ Na+ ATP 2 Cl \200\223 K+ 2 K+ K+ Cl \200\223 K+ Ca2+ Mg2+ Na+ Fig. 13.7 Transportation mechanisms in the thick ascending limb of the loop of Henle. The N KCC co-transporter is the key. Potassium can \200\230 leak \200\231 back into the lumen v ia the ROMK channel, rendering the lumen positively charged. This charge gradient facilitat es passive reabsorption of Ca 2+ , Mg 2+ , and more Na + via the paracellular route. CHAPTER 13 Renal physiology 928 The loop of Henle: the countercurrent system \200 Allows the concentration of urine to be varied, according to physiological circumstances. The U shape of the loop and its capillary network, the differences in permeability (to Na + and water) between the descending and ascending limbs, and the active reabsorption of Na + in the thick ascending limb all underpin the countercurrent system (see Fig. 13.8). The system maintains an interstitial osmotic gradient that increases from the renal cortex ( 7 290mOsmol/kg) to the tip of the medulla ( 7 1,200mOsmol/kg). The thin and thick ascending limbs are both impermeable to water. \200 Despite this water impermeability, Na + is reabsorbed in this segment. This reabsorption of Na + (and Cl \200\223 ), but not water, leaves the tubular \202 uid dilute and hypotonic ( l this regi on is often called the diluting segment). So: Isotonic \202 uid ( 7 290mOsmol/kg) enters the loop from the PCT. \200 As it progresses down the descending limb, it encounters medullary interstitial \202 uid of increasing tonicity (the interstitium is hypertonic be cause of Na + reabsorption without water in the thick ascending limb). The descending limb is permeable to water (but not Na + ), so water \202 ows out, down an osmotic gradient, into the concentrated milieu of the interstitium. As this occurs, the tubular \202 uid progressively equilibrates osmotically with its surroundings and becomes more and more hypertonic as the loop descends into the medulla. The effect is to concentrate the luminal \202 uid so that, at the deepest part of the loo p, both the luminal and interstitial osmolality reach up to 1,200mOsmol/kg. In the ascending limb, Na + (and Cl \200\223 ) are absorbed (via the NKCC transporter) wit hout water, generating an osmotic gradient of 7 200mOsmol/kg between lumen and interstitium at any given level. By the time the luminal \202 uid reaches the distal tubule, it is dilute and hypotonic. It would remain dilute as it passes through the distal nephron and collecting duct, were it not for the action of vasopressin (ADH) ( b p. 933). \200 High plasma osmolality l ADH r elease l i permeability of collecting duct to water l i urine osmolality (up to 7 1,200mOsmol/kg). Low plasma osmolality l ADH release suppressed l d permeability of collecting duct to water l d urine osmolality (down to 7 200mOsmol/kg). THE LOOP OF HENLE: THE COUNTERCURRENT SYSTEM 929 Box 13.1 Role of urea \200 Urea also makes an important contribution to the maintenance o f a hypertonic medullary interstitium. The tubules are relatively impermeable to urea, meaning that a signi \201 cant amount is d elivered to the collecting duct. Then, vasopressin-dependent water reabsorption increases l uminal urea concentration further. However, the vasopressin-sensitive transporters UT-A1 and UT-A3 allow urea to escape into the medullary interstitium. Although some re-enters the luminal \202 uid or equilibrates with the capillary network, the net result is an increase in interstitial tonicity. Na+ H2O Na+ H2O Na+ Na+ Cortex Isotonic 290 290 290 100 Hypertonic Outer medulla Na+ Na+ H2O 600 600 600 400 Inner medulla H2O Na+ 900 900 700 900 Hypertonic 1200 1200 Fig. 13.8 Countercurrent multiplication by the loop of Henle (all units are mOsmol/kg). The thin descending limb is water-permeable. The tubular \202 uid becomes hypertonic (the in terstitium is hypertonic). In the ascending limb, NaCl is absorbed (via the NKCC transporte r) without water, generating an osmotic gradient of 7 200mOsmol/kg at any given level. Coun tercurrent \202 ow in the two limbs of the loop multiplies this gradient longitudinally. The result is a hypertonic interstitium in the inner medulla (urea also contributes to this hypertonicity \200\224 see Box 13.1 ). Thus, \202 uid entering the distal tubule is hyp otonic. Further Na + reabsorption in the distal tubule can make the \202 uid still more h ypotonic. If ADH is not present, the collecting duct is impermeable to water, and dilute ur ine is passed. If ADH is present, the collecting duct becomes permeable to water, and the u rine becomes hypertonic as it passes through the medulla. Reproduced with permission from D avison AMA, Cameron JS, Grunfeld J-P et al. (eds)(2005). Oxford Textbook of Clinical Nephro logy, 3rd edn. Oxford: Oxford University Press. CHAPTER 13 Renal physiology 930 The distal nephron Introduction The \201 ne-tuning of solute and water reabsorption occur s in the distal nephron. The distal convoluted tubule The distal convoluted tubule (DCT) is impermeable to the passi ve move- ment of Na + and Cl \200\223 and water (vasopressin does not affect water absorp- tion here). This allows large concentration gradients to develop when necessary (see Fig. 13.9). Sodium 7 5% of the \201 ltered Na + is reabsorbed in the DCT. The thiazide-sensitive Na + Cl \200\223 co-transporter (NCCT) is the major route. Further Na + is absorbed by Na + /H + exchange, and additional Cl \200\223 by Cl \200\223 / HCO 3 \200\223 exchange. H + an d HCO 3 \200\223 then combine in the lumen to form CO 2 and H 2 O (the CO 2 can be reabsor bed and recycled). The energy for the action of the NCCT co-transporter is again derived fr om Na + K + -ATPase, as the resulting electrochemical gradient permits Na + reabsorption in to the cell. Calcium Absorbed via a speci \201 c epithelial Ca 2+ channel under the in \202 u- enced o f PTH and calcitriol. Thiazide diuretics block the NCCT co-transporter. They also enhance Ca 2+ absorption (exact mechanisms unknown), reducing calcium excre- tion in the urine. Hypokalaemia occurs, as th ere will be an increase in Na + delivery to the collecting ducts and 6 additional uptake vi a ENaCs ( b p. 931). This will increase activity of the basolateral Na + K + -ATPase, and t he resulting intracellular K + will then move into the lumen and be lost in the urine. Defects in the NCCT co-transporter also underlie Gitelman \200\231 s syndrome ( b p. 800). Lumen Interstitium 3 Na+ Na+ ATP Cl \200\223 2 K+ K+ Cl \200\223 K+ Fig. 13.9 Distal tubular sodium and chloride reabsorption occur predominantly via the NCCT co-transporter. THE DISTAL NEPHRON 931 The collecting duct \200 Two cell types are found in the collecting duct. Principal cells are responsible for Na + (and water) reabsorption and K + excretion. Intercalated cells se crete H + ( A -intercalated cells) or HCO 3 \200\223 ( B -intercalated cells). Changes in permeability of the collecting duct allow concentration of urine under the control of vasopressin (ADH). Principal cells ( 7 65% of cells) (See Fig. 13.10) Sodium Reabsorption from the lumen occur s via a speci \201 c Na + trans- porter, the epithelial sodium channel (ENaC), and it then leaves the cell on the basolateral side via the Na + K + -ATPase. (see Fig. 13.10). Althou gh only 7 5% of \201 ltered Na + is reabsorbed here, this is the main site of body Na + r egulation (see Box 13.2). In contrast to Na + reabsorption higher in the nephron, it is pre dominantly the electrical, rather than concentration, gradient that drives Na + from the lu men into the cell (as the luminal Na + concentration may be as low as 5mmol/L by this stage of the nephron). Na + K + -ATPase generates a net negative charge within the cell, and Na + moves down this charge gradient via ENaC into the cell. As the tubu- lar \202 uid becom es more negatively charged, it favors K + movement into the lumen. Potassium Enters principal cells on the basolateral side via the Na + K + -ATPase and then move into the lumen (along the favorable charge gradient discussed previously) via speci \201 c aldosterone-sensitive K + channels (aldosterone promotes K + excretion). High urinar y \202 ow rates help to maintain low intraluminal K + concentrations, allowing the gradi- ent to persist and the channel to operate. Hence, hypovolaemia can l hyperkalaemia. Box 13.2 Control of Na + excretion \200 Aldosterone , after binding its mineralocorticoid receptor, increases the number of open ENaC channels, regulating Na + absorption (and excr etion). It also increases the number and activity of the Na + K + -ATPase (it can double th e surface area of the basolateral membrane). Atrial natriuretic peptide ( ANP ) also acts on ENaC, with i ANP l inactivation of ENaC. Na + delivery: d delivery l i ENaC activity, thereby reducing Na + loss in the urine (and preventing volume depletion). Vasopressin (ADH) may also be important through an increase in ENaC numbers and activity. Locally produced PGE 2 decreases ENaC activity. 2 Amiloride and triamterene block ENaC, thu s reducing both Na + reab- sorption and K + excretion. Spironolactone inhibits the effect o f aldoster- one on its receptor, with similar effects on Na + and K + . CHAPTER 13 Renal physiology 932 Intercalated cells These cells are essential to acid \200\223 base homeostasis. There are two func- tional and morphological types. A -intercalated cells are tall, columnar epi- the lial cells, containing a luminal H + ATPase that enables them to secrete protons. B -interc alated cells are \202 atter cells that contain a basolateral H + ATPase and a luminal Cl \200\223 HCO 3 exchanger (called pendrin) that enables them to excrete base, The A -intercalated cells transportation H + ions out of the cell into the tubu- lar \202 uid. This is aldosterone-sensitive (aldosterone l i H + excretion). H + is generated (alon g with HCO 3 \200\223 ) from the dissociation of carbonic acid via carbonic anhydrase. HCO 3 \200\223 is then returned to the circulation in exchange for Cl \200\223 ions via a di fferent basolateral Cl \200\223 HCO 3 co-transporter (called AE1 protein) (see Fig. 13.11) . Metabolic acidosis converts the collecting tubule from a state of HCO 3 secretion to HCO 3 absorption (and 6 H + secretion) that involves a phe- notypic shift of B -intercalated cell s to A -intercalated cells. The role of the kidney in acid \200\223 base regulation is fur ther discussed on b p. 818. In circumstances of severe acidosis or d K + , intercalated cells also express an H + /K + ATPase, similar to that responsible for gastric acid secre- tion. This allows additional H + secretion in exchange for K + . Interstitium Lumen 3 Na+ ENaC Na+ ATP ALDO RECEPTOR 2 K+ K+ K+ Fig. 13.10 The principal cell in the collecting tubule. Lumen Interstitium H2CO3 CA H+ H+ + HCO3 \200\223 \200\223 HCO3 Cl \200\223 ATP Fig. 13.11 Acid secretion in the A -intercalated cell in the collecting tubule. H + /K + AT Pase is an alternative route for secretion of H + into the lumen. CA, carbonic anhydrase. THE DISTAL NEPHRON 933 Vasopressin (ADH) Vasopressin, or antidiuretic hormone (ADH), is synthesized in the hypotha lamus and released by the posterior pituitary in response to rising plasma osmolality (sens ed by hypothalamic osmoreceptors) and/ or signi \201 cant hypovolaemia (sensed by arterial baroreceptors and atrial stretch receptors). Vasopressin acts via V 1 receptors to stimulate thirst and systemic vasoconstriction. In the kidney: \200 Vasopressin binds V 2 receptors located on the baso lateral membrane of principal cells in the collecting duct. It acts to reabsorb water in the collecting duct and 6 to concentrate the urine. \200 The duct becomes more permeable to water by the translocation of speci \201 c water channels (aquaporin-2) to the luminal membrane. Aquaporin-2 is stored in intracellular vesicles ready for membrane insertion (the basolater al membrane is already water-permeable by virtue of aquaporin-3 and -4). The high interstitial osmolality, maintained by the countercurrent system, facilitates easy movement of water ( b p. 928). Vasopressin also increases the permeability of the collecting duct to urea ( b p. 929). Defects in aquaporin-2 structure and function underlie X-linked nephrogenic diabetes insipidus ( b p. 789). Increased intraluminal Ca 2+ concentration interferes with aquaporin-2 membrane insertion. This explains the observation that hypercalcaemia causes de fective urinary concentrating ability, polyuria, and dehydration. High plasma osmolality l ADH release l binds V 2 receptors l aquaporin-2 translocation l i permeability of collecting duct to water l i urine osmolality up to that of the inner renal medulla (up to 7 1,200mOsmol/kg) l production of a concentrated urine. Low plasma osmolality l ADH release suppressed l no aquaporin-2 translocation l collecting duct impermeable to water l d urine osmolality as low as that of tubular \202 uid enterin g the cortical collecting duct (down to 7 200mOsmol/kg) l production of a weak urine. 935 Chapter 14 Appendices Insertion of temporary haemodialysis catheters 936 Preparing renal patients for theater 942 Acute coronary syndrome (ACS) in CKD 946 Plasma exchange 950 Useful resources 954 CHAPTER 14 Appendices 936 Insertion of temporary haemodialysis catheters Indications Temporary haemodialysis catheter s should be temporary \200\224ideally no line should remain in situ for longer than a few days. The risk of infection increases signi \201 cantly if in place for longer \200\224con sider switching to a tun- nelled line if continuing access is required ( b p. 296). Routes of insertion \200 The right internal jugular vein is the preferred route. It is super \201 cial, technically easy to cannulate (in a \200\230virgin \200\231 neck), and joins the SVC in a straight line. The left internal jugular is an alternative, but the guidewire and line have to curve in order to reach the SVC. The risk of malposition and malfunction (and perforati on of the vein) is 6 greater. The femoral veins are suitable for short-term use and may be the safest and quickest option in an emergency (see b p. 180). \200 The subclavian route shou ld be avoided: there is a higher risk of subsequent central venous stenoses, potentially re ndering the ipsilateral arm unsuitable for permanent access in the form of an AVF. Pre-insertion \200 Is the line really needed? Check that dialysis is required and whether the patient is suitable for a tunneled line ( b p. 296) or a PD catheter insertion. Check FBC and clotting. \200 The operator should obtain consent from the patient (verbal and/or a family member if too unwell). Warn about potential complications: \200 Failure t o cannulate vein. \200 Haematoma 9 arterial puncture. \200 Pneumothorax, haemothorax (a nd potential need for a chest drain). \200 Air embolus. \200 Catheter malfunction. \200 Infection and its complications. Equipment: every renal unit should have a dedicated room for line insertion. It should be a sterile environment, containing a 2D ultrasound probe to fac ilitate vein cannulation. You will need: \200 An assistant. \200 Sterile gown, gloves, drapes, and towels. Iodine or chlorhexidine to clean the skin. An ultrasound device to guide line insertion. \200 Dressing pack, with a supply of syring es, swabs, and needles. \200 Sterile saline for injection. \200 Lidocaine (1 or 2%): 10 mL. \200 Line pack (if pre-packed, will contain introducer needle, guidewire, and the lin e itself). Choose a shorter line (e.g. 15cm) for the RIJ route, longer line (e.g. 20cm) for the left side or the larger patient. Tri-sodium citrate lock (Citralock ) (46.7%) or Duralock have largely replaced heparin (5,000 units/mL) to \200\230lock \200\231 the line once it is in situ . INSERTION OF TEMPORARY HAEMODIALYSIS CATHETERS 937 Wash and dry hands, as for a surgical procedure; wear sterile gown, mask, and sterile gloves. Position the patient slightly head down (for right-sided insertion). \200 Clean a wide ar ea of skin around the insertion site. Lay out sterile drapes on all sides. Prime both lumens of the line with sterile normal saline solution. Tips for problematic line insertions \200 Stop insertion if the patient is in pain . Seek senior assistance. Pain probably indicates that the guidewire or line is in the wrong location. Venous blood looks darker than arterial blood and does not \201 ll a syringe with its own pressure: if in doubt, put a 5mL syringe on either hub, and see if it \201 lls in a pulsatile fashion. If still in doubt, perform an ABG. 1 Do not take the lin e out yet (see later in this list). Inadvertent arterial puncture with an 18-gauge needle can usually be controlled by pressure for 5 \200\22315min. If the larger introducer needle punctures the artery, it is often saf est to abandon the procedure. Do not attempt the other side of the neck after signi \201 c ant arterial puncture and haematoma (risk of airway obstruction, if bilateral). \200 1 If an artery is accidentally cannulated (i.e. the line is inserted), or if the vein is traver sed such that the line tip is found to lie outside the vein, then leave the line in situ an d seek expert help . \200 Check FBC, clotting, and cross-match blood. \200 Con \201 rm your concern by aspirating blood (if possible) for ABG from both lumens. If unable to aspirate, you are in the wrong place. \200 Perform a C XR. \200 If there is a rapid deterioration in the patient s condition, a haemothorax or pneumothorax is likely, and an urgent chest drain may be required prior to C XR. Monitor closely: pulse BP, and O 2 saturation. \200 Find out where the line is: either a CT or a contrast examination through the line to determine its location. The line must be removed under controlled conditions, with vascular/thoracic surgical input. Insertion technique \200 Establish anatomical landmarks and the planned point of cannulat ion (see Figs 14.1 to 14.3). Ultrasound-guided insertion is now the technique of choice. \200 Insert local anesthetic under the skin. Always attempt to aspirate before injecting lidocaine to avoid inadvertent IV injection. Once the needle is in the vein, use a Seldinger technique. Pass the guidewire through the n eedle ( there should be no resistance \200\224if resistance is felt, do not force it, but remove the guidewire, and check the needle position. Do not advance the guidewire beyond th e line length, i.e. 15/20cm, to avoid arrhythmias. Remove the needle, leaving the guidewire in situ . CHAPTER 14 Appendices 938 Use a scalpel blade to make a small nick in the skin. Pass the dilator over the guidewire u ntil it is in the vein. Remove the dilator, keeping the guidewire in position. Without dela y, pass the line over the guidewire until it is in position. Remove the guidewire. Vein localization Ultrasound-guided cannulation Most probes have a sterile disposable cover . Follow the manufac turer \200\231s instructions. After in \201 ltration of local anesthe tic, identify the vein with the probe. The introducer needle can be inserted with real-time guidance. Blind insertion (no longer recommended) The approach can be medial to SCM muscle, between t he two heads, or lateral to the muscle. Local practice and expertise will dictate the optim al route. \200 Palpate the carotid artery with one hand (the left for a right-sided insertion). Insert the needle just lateral to the carotid pulse, aiming at 45 * to the skin, in the direction of the ipsilateral nipple. Aspirate gently on the syringe, as yo u go, until the vein is reached. If unsuccessful, withdraw the needle slowly (continuing with attempted aspiration \200\224you may have traversed the vein). Reinsert, varying the angle of insertion slightly to the medial side. If still unsuccessful, invite a colleague or seni or to have a go before you (and the patient) lose con \201 dence. Once the vein is identi \201 ed with a green needle, pass the introducer needle down the s ame track, in the same manner, into the vein, palpating the carotid pulse at the same time. Check the \202 ows in both lumens. A 20mL syringe should \201 ll easily with minimal as piration (general rule of thumb: 20mL should aspirate in <5s (= 240mL/min). Flush both lume ns with saline (which should also be easy), and then lock with Citralock , Duralock , or heparin. The priming volume should be written on the line (typically 1.3 \200\2232.0mL/ lumen). Put caps on the lumens, and suture the line in position. Arrange a CXR and review it yourself: look for line position (tip should be in SVC), and do uble-check there is no pneumothorax. INSERTION OF TEMPORARY HAEMODIALYSIS CATHETERS 939 Sternal head Sternal head Clavicular head Sternocleidomastoid muscle External jugular vein Fig. 14.1 Anatomical landmarks for internal jugular vein cannulation. Surface anatomy of ex ternal and internal jugular veins. Reproduced with permission from Ramrakha, P, and Hill, J , Oxford Handbook of Acute Medicine 2e (2004), with permission from Oxford University Press .