Renal Physiology

KIDNEYS Major excretory organ in the body: kidneys (since they have maximum capacity of excretory function, they play a major role in homeostasis). Other excretory organs: Skin: excretes water, salts & removes heat from the body. Lungs: removes CO2 & water vapour Alimentary canal: excretes food residues in the form of feces. Liver: excretes bile pigments, heavy metals, drugs, toxins etc. thru bile

3 layers of structures are seen in the kidney: Outer cortex: dark & granular in appearance. Contains renal corpuscles & convoluted tubules. At intervals, the cortical tissue penetrates into medulla as Renal columns (Columns of Bertini). Inner medulla: contains tubular & vascular structures. It is divided into medullary or malphigian pyramids with the base in contact with cortex & apex (papilla) projecting into the minor calyx. Renal sinus: Consists of renal pelvis, major calyces, minor calyces, nerves, vessels & fat. Parenchyma of kidney: consists of uriniferous tubules. The tubules are of two types: a) Terminal or secretory tubules (nephrons): concerned with formation of urine. b) Collecting ducts or tubules: carriage of urine from nephrons to pelvis of ureter. The collecting ducts unite to form ducts of Belini, which open into minor calyces. Kidney Structure and Function

3 fundamental mechanisms characterizing kidney function are: (1) Filtration: of water & solutes from the blood. (2) Reabsortion: primary urine enters the tubule & most of it passes back into the blood. (3) Secretion: Certain substances (e.g., toxins) are actively secreted into the tubule lumen. Also synthesis: for eg: synthesis of NH3 by the tubular epithelium. Excretion: Non-reabsorbed residual filtrate + secreted substances  excreted together in the final urine. Excretion of metabolic end products of: Amino acid metabolism  Urea Nucleic acid metabolism  Uric acid Muscle metabolism  Creatinine Hemoglobin degradation  Bilirubin

Functions of kidneys: (primary function: homeostasis) (1) adjust salt & water excretion to maintain a constant ECF volume (hence ABP) & osmolality*); (2) acid-base homeostasis (ph = 7.4); (3) eliminate end-products of metabolism & foreign substances while (4) preserving useful compounds (e.g., glucose) by reabsorption; (5) Endocrine function: Secrete erythropoietin, thrombopoietin, rennin, calcitrol**, PGs. (6) have metabolic functions (protein & peptide catabolism, gluconeogenesis, etc.). (7) Detoxication [*Kidneys retain Na if osmolarity of body water decreases & eliminate Na, if it increases] [**Kidneys play a role in regulation of blood Ca2+ level by activating calcitrol into Vit D  necessary for Ca2+ reabsorption from intestine] Nephron Structure Each kidney contains about 106 nephrons. Each nephrons consists of: malpighian body & the tubule. Malpighian body/ Malphigian / Renal corpuscle: (_A) It is located in the renal cortex & its primary function is filtration.

On the basis of situation of the corpuscle, the nephrons are divided into:

Cortical nephrons (superficial)

Juxtamedullary nephrons

1) Corpuscle in the outer cortex of the kidney 2) Short loop of Henle penetrating only the outer zone of medulla 3) (70-80%) of nephrons

1) Corpuscle in the inner cortex (near the corticomedullary junction) 2) Long loop of Henle which runs up to the inner zone of medulla 3) (20-30%) of nephrons

4) Formation of urine

4) Mainly concentration of urine

5) Supplied by peritubular capillaries

5) Supplied by vasa recta

Any change in blood distribution to these two types of nephrons affects NaCl excretion. ADH increases the GFR of the juxtamedullary nephrons.

Structure of Renal corpuscle: Consists of glomerulus (tuft of capillaries) & Bowman’s capsule (a double-walled capsule). Glomerulus: • Consists of many capillaries called glomerular capillaries which connect an afferent arteriole with an efferent arteriole. • Thus, the vascular system in the glomerulus is purely arterial. • The capillaries in the glomerulus arise from the afferent arteriole, divide & finally reunite to form the efferent arteriole. • The diameter of the efferent arteriole is smaller than that of afferent arteriole. • The capillary endothelium contains many pores called slit pores for filtration function. Bowman’s capsule: It is formed of two layers: visceral layer (covering the glomerulus) & parietal layer.

Podocytes: epithelial cells with pedicles. The visceral layer is formed by podocytes with numerous interdigitating foot like processes (pedicels). Fenestra: Small cleft like spaces in between the pedicles. The fenestra are covered by the slit membrane, the pores of which are about 5nm in diameter. They are shaped by the protein nephrine, which is anchored to the cytoskeleton of the podocytes. The basement membrane forms the separation between the glomerular capillary endothelium & the epithelium of the visceral layer (as they are fused). The epithelial cells of the visceral layer do not fuse continuously with basement membrane but are connected by a series of processes called pedicles or feet (cytoplasmic extensions of epithelial cells). Visceral layer  continues as the parietal layer at the visceral pole

Parietal layer  continues with the wall of the tubular portion of nephron. Cleft btwn the two layers  continues as the lumen of tubular portion. (primary urine accumulates in this space.) (_B). Blood enters glomerulus  by an afferent arteriole (vas afferens)  exits via an efferent arteriole (vas efferens)  from which the peritubular capillary network arises. The glomerular filter (_B): Separates the blood side from the Bowman’s capsular space. It comprises the perforated endothelium of glomerular capillaries + basement membrane + visceral membrane of Bowman’s capsule on the urine side.

◆ Proximal tubule: It is the longest part of a nephron (ca. 10 mm). Its twisted initial segment (proximal convoluted tubule, PCT) merges into a straight part, PST (pars recta). ◆ Loop of Henle: consists of: a thick descending limb that extends into the renal medulla (_A4 = PST), a thin descending limb, a thin ascending limb (only in juxtamedullary nephrons which have long loops), and a thick ascending limb, TAL. It contains the macula densa: a group of specialized cells that closely communicate with the glomerulus of the nephron. ◆ The distal tubule: has an initially straight part (= TAL of Henle’s loop) that merges with a convoluted part (distal convoluted tubule, DCT; _A7). The DCT merges with a connecting tubule (_A8). Many of them lead into a collecting duct, CD(_A9) which extends through the renal cortex (cortical CD) and medulla (medullary CD). At the renal papilla the collecting ducts opens in the minor calyx  major calyx  renal pelvis.

From there, the urine (by peristaltic contractions):  ureter  urinary bladder  urethra  exits the body. The collecting duct is formed by two types of epithelial cells: • The Principal or P cells • Intercalated or I cells Micturition • Voiding (emptying) of the bladder is controlled by reflexes. • Filling of the bladder activates the smooth detrusor muscle of the bladder wall via stretch sensors & parasympathetic neurons (S2–S4). • At low filling volumes: the wall relaxes via sympathetic neurons (L1–L2) controlled by supraspinal centers (pons). • At higher filling volumes (>0.3 L): the threshold pressure (about 1 kPa) is reached, this triggers the micturition reflex via a +ve feedback loop: • The detrusor muscle contracts  pressure  contraction  and so on until the internal (smooth m.) and external sphincter (striated m.) open so the urine can exit the body. Renal Circulation Kidneys are the second organs to receive maximum blood flow (1200ml/min), after Liver being the first organ with maximum blood flow (1500ml/min). The glomerular capillaries form a high pressure bed (60-70mmHg)  important for glomerular filtration. [capillary pressure elsewhere in the body (25mmHg)] The peritubular capillaries form a low pressure bed (8-10mmHg)  important for tubular reabsorption. Blood is supplied to the kidney by the renal artery (direct branch of abdominal aorta). The pressure in the aorta is very high & it facilitates a high blood flow to the kidneys. Unlike other organs, the renal circulation has a portal system (double network of capillaries)  two successive capillary networks connected with each other by an efferent arteriole (or vas efferens)(_A, B).

In the renal sinus, renal artery  segmental arteries  interlobar arteries (pass in btwn the medullary pyramids) arcuate arteries (run in btwn the cortex & medulla)  interlobular arteries (run thru the renal cortex ⊥ to arcuate artery)  afferent arterioles  glomerular capillaries (bowman’s capsulue)  efferent arteriole. Efferent arteriole  renal portal system (i.e a second capillary network surrounding the tubular portions of the nephrons)  peritubular capillaries/vasa recta Peritubular capillaries  peritubular veins  interlobular veins  arcuate veins  interlobar veins  segmental veins  renal vein (which leaves the kidney thru the hilus). Straight vessel or vasa recta: the efferent arteriole of the juxtramedullary nephrons give rise to a vessel that runs parallel to the renal tubule into the medulla & ascends up towards the cortex. The vasa recta supply the renal medulla. Their hairpin shape is important for the concentration of urine. Pressure in the first network of glomerular capillaries: It is relatively high (_B) & is regulated by adjusting the width of interlobular artery, the afferent or efferent arterioles (_A 3,4). The second network of peritubular capillaries: (_A) winds around the cortical tubules & supplies the tubule cells with blood & is also involved reabsorption & secretion. Renal blood flow (RBF): Blood volume passing thru both kidneys/min (mainly to renal cortex). It is relatively high (1200 ml/min = 1.2 L/min = 25% of the CO) RBF is required to maintain a high GFR & results in a very low arteriovenous O2 difference (ca. 15 mL/L of blood).

RBF is mainly to produce a high GFR than to provide O2 for the metabolism of the kidney cells. Renal plasma flow: 650ml/min = 55% of blood volume In the renal cortex: O2 is consumed (ca. 18 mL/min) for oxidative metabolism of fatty acids, etc. Most of the ATP produced in the process is used to fuel active transport. In the renal medulla: metabolism is mainly anaerobic. So, around 90% of the renal blood supply goes to the cortex. Per gram of tissue, approx. the amount of blood that passes thru is 5, 1.75 – cortex and outer medulla 0.5 mL/min - inner medulla. (This value is still higher than in most organs (_p. 215 A). Autoregulation of renal blood flow Autoregulation: The intrinsic ability of an organ to regualate its own blood flow. (present in brain, heart & kidneys). Due to autoregulation, only slight changes in RPF & GFR occur - even in a denervated kidney i.e regulation of blood flow to the kidneys is independent of the nerves innervating the renal blood vessels. Range of autoregulation: 80 - 180mmHg (_C). If BP < 80 or > 180mmHg, renal circulation & filtration fail (_C). Resistance in the interlobular arteries & afferent arterioles located upstream to the cortical glomeruli is automatically adjusted when the MBP changes (_B, C). RBF & GFR can also be regulated independently by making isolated changes in the (serial) resistances of the afferent & efferent arterioles (_p. 152). Two mechanisms are involved in renal autoregulation: Myogenic response: ↑RBF  stretching of the elastic wall of afferent arteriole  ↑ flow of Ca2+ ions from ECF fluid into cells  contraction of smooth muscles in afferent arteriole  constriction  ↑resistance of afferent arteriole  RBF normal Tubuloglomerular feedback: ↑GFR  ↑NaCl conc. in renal tubule  Macula densa releases adenosine from ATP  constriction of afferent arteriole  ↓ RBF  ↓ GFR EXAMINATION OF URINE Plasma Clearance / Renal clearance: It is the amount of plasma flowing thru the kidney that is cleared off a substance (indicator) in a given unit of time. Clearance, C = U. V`U / P

Where U = conc. of substance in urine V = vol. of urine flow (urine output / time) P = conc. of substance in plasma Glomerular filtration rate (GFR): Total volume of fluid filtered by the glomeruli of both kidneys per min. (120 mL/min per 1.73m2 of body surface area or 180 L/day)



So, the volume of exchangeable ECF of the whole body (≅ 17 L) enters the renal tubules about 10 times a day. About 99% of the GFR returns to the extracellular compartment by tubular reabsorption.



Mean fractional excretion of H2O ≅ 1% of GFR



Absolute H2O excretion (= urine output / time = V`U) = 1 to 2 L per day.



GFR ≅ 20% of RPF.



Measurement of GFR: Indicators present in the plasma are used to measure GFR. They must have the following properties: — They must be freely filterable (no resorption or secretion in the tubule) — They must not be metabolized in the kidney — They must not alter renal function Inulin: which must be infused intravenously, fulfills these requirements. Hence, it is an ideal substance to measure GFR (completely secreted). GFR = V`U . UIn / PIn [L/min] Endogenous creatinine (normally present in blood) can also be used with certain limitations. [bcoz the plasma conc. of creatinine, Pcr, rises as the GFR falls] Measurement of Renal Plasma Flow: (by Fick’s principle) To measure RPF, we need to use a substance that is completely filtered & secreted (no reabsorbtion) i.e it is completely eliminated in the urine during one renal pass. For eg: para-aminohippurate = (PAH is filtered & highly secreted). Renal plasma flow = UPAH. V`U / PPAH This equation is only valid when the PaPAH is not too high. Otherwise, PAH secretion will be saturated and PAH clearance will be much smaller than RPF Measurement of Renal Blood Flow (RBF) Non-invasive determination of RBF is possible if the RPF is known (0.6 L/min). RBF is derived by inserting the known hematocrit (Hct) value into the following equation: RBF = RPF/(1–Hct) Or

RBF = RPF / % of plasma in blood Fractional excretion (FE): [ratio of Cx to CIn] It is the ratio of clearance of a given substance X to inulin clearance (CX/CIn). It defines what fraction of the filtered quantity of X was excreted. FE < 1, if the substance is removed from the tubule by reabsorption (e.g. Na+, Cl–, amino acids, glucose, etc.), FE > 1, if the substance is subject to filtration plus tubular secretion. For PAH, tubular secretion is so effective that FEPAH > 5 (500%). Absolute rate of reabsorption or secretion by the kidneys of a freely filterable substance = filtered amount/time – excreted amount/time, X (mol/min) = (GFR · PX) - (V`U · UX) a +ve result means net reabsorption a –ve result means net secretion. (For inulin, the result would be zero.) Filtration fraction (FF) = ratio of GFR/RPF = 20% Amount of plasma filtered, while the remaining returns to the efferent arteriole. Ra = afferent arteriolar resistance and Re = efferent arteriolar resistance Atriopeptin = It is a peptide hormone that increases the filtration fraction by ↑Re & ↓Ra. This ↑Peff in the glomerular capillaries without significantly changing the overall resistance in the renal circulation. PRESSURES DETERMINING FILTRATION: Pressure in the glomerular capillaries: Capillary pressure in the glomerulus is the highest capillary pressure in the body. It is the only pressure that favors filtration. Colloidal osmotic pressure: Pressure exerted by the plasma proteins in the glomeruli. The plasma proteins are not filtered thru the capillaries & their conc. increases & GFR↓ (in case of dehydration & inc. protein conc). Whereas in hypoproteinemia, GFR↑. Hydrostatic pressure in Bowman’s capsule: It is the pressure exerted by the filtrate in Bowman’s capsule during filtration. It increases in conditions like obstruction of urethra & edema of kidney beneath renal capsule. So GFR↓. Effective filtration pressure (Peff): It is the balance btwn pressures favoring filtration & pressures opposing filtration. It is the driving “force” for filtration. Peff = Pcap – (PBow + πcap) Where, Pcap = Glomerular capillary pressure (= 48mmHg) [favors filtration]

PBow = Pressure in Bowman’s capsule (= 13mmHg) [opposes filtration] πcap = oncotic pressure in plasma (= 25 to 35 mmHg) [opposes filtration] Peff at the arterial end of the capillaries equals 48–13–25 = 10mmHg.

Because of the high filtration fraction, the plasma protein conc. & therefore, πcap values along the glomerular capillaries increase and Peff decreases. (The mean Peff, is therefore used in Eq. 7.7.) Thus, filtration ceases (near distal end of capillary) when πcap rises to about 35mmHg, decreasing Peff to zero (filtration equilibrium). GFR = Peff x Kf. [7.7] where Kf = ultrafiltration coefficient = A · k Peff (mean for all glomeruli), A = the glomerular filtration area (dependent on the number of intact glomeruli), k = water permeability of the glomerular filter. Transport Processes at the Nephron Filtration of solutes: Ultrafiltrate: Small dissolved molecules of plasma present in the GF. Glomerular sieving coefficient (GSC) of a substance = conc. in filtrate/conc. in plasma water) It is a measure of the permeability of glomerular filter for this substance.

Radius Freely filtered Not filtered

r < 1.8nm

Molecular GSC mass < ca. 10 000 Da 1

Example

r > 4.4nm

> 80 000Da

0

Globulin

Partially filtered

1.8nm < r < 4.4nm

between 1&0

Albumin

Na+

Negative charged particles (e.g., albumin: r = 3.4 nm; GSC ≅ 0.0003) are less permeable than neutral substances of equal radius bcoz -ve charges on the wall of the glomerular filter repel the ions. When small molecules are bound to plasma proteins (protein binding), the bound fraction is practically non-filterable. Molecules entrapped in the glomerular filter are believed to be eliminated by phagocytic mesangial macrophages and glomerular podocytes. Tubular epithelium The epithelial cells lining the renal tubule & CD are polar cells - their luminal (or apical) membrane on the urine side differs from that of the basolateral membrane on the blood side. Luminal membrane of PT: has a high brush border consisting of microvilli that greatly increase the surface area (especially in the PCT). Basolateral membrane of PT: segment has deep folds (basal labyrinth) that are in close contact with the intracellular mitochondria, which produce the ATP needed for Na+-K+-ATPase located in the basolateral membrane (of all epithelial cells). The large surface areas (about 100m2) of the PT cells of both kidneys are needed to reabsorb the lion’s share (large amount) of filtered solutes within the contact time of a couple of seconds. Post PT cells do not need a brush border since the amount of substances reabsorbed decreases sharply from the proximal to the distal segments of the tubules. Routes of reabsorption: a) Transcellular transport:  In this route, the substances move thru the cell.





It includes: Transport of substance from tubular lumen  tubular cell thru luminal (apical) surface of cell membrane  interstitial fluid  capillary Permeability of the two membranes in series is decisive for transcellular transport.

b) Paracellular transport:  In this route, the substances move thru the intercellular space.  It includes: transport from tubular lumen  interstitial fluid present in *lateral intercellular space thru tight junctions btwn cells  capillary.  The tightness of tight junctions determines the paracellular permeability of epithelium for water & solutes. *Lateral intercellular space: The tubular epithelial cells are connected with their neighbouring cells by tight junctions at their luminal (apical) edges. But, beyond the tight junction, a small space is left between the adjoining cells along their lateral borders called the lateral intercellular space. The interstitium extends into this space. The tight junctions in the PT are relatively permeable to water & small ions which, together with the large surface area of the cell membranes, makes the epithelium well equipped for para & transcellular mass transport. The thin limbs of Henle’s loop are relatively “leaky”, while the TAL & the rest of the tubule and CD are “moderately tight” epithelia. The tighter epithelia can develop much higher transepithelial chemical and electrical gradients than “leaky” epithelia. Measurement of reabsorption, secretion and excretion Whether and to which degree a substance filtered by the glomerulus is reabsorbed or secreted at the tubule and CD cannot be determined based on its urinary conc. alone as conc. rises due to the reabsorption of water. Urinary / plasma inulin (or creatinine) concentration ratio, Uin/Pin: It is a measure of the degree of water reabsorption. These substances can be used as indicators because they are neither reabsorbed nor secreted. Thus, changes in indicator concentration along the length of the tubule occur due to the H2O reabsorption alone. If Uin/Pin = 200, the inulin concentration in the final urine is 200 times higher than in the original filtrate. This implies that fractional excretion of H2O (FEH2O) is 1/200 = 0.005 = 0.5% of GFR Determination of the concentration of a (freely filterable & secreted) substance X in the same plasma and urine samples for which Uin/Pin was measured will yield Ux/Px. Fractional excretion of X, FEX = (UX/PX)/(UIn/PIn) [7.9]

Eq. 7.9 can also be derived from Cx/Cin when simplified for V`U. Fractional reabsorption of X (FRX) = 1 – FEX Reabsorption in different segments of the tubule. The conc. of a substance X (TFX) and inulin (TFIn) in tubular fluid can be measured via micropuncture (_A). Non-reabsorbed fraction (fractional delivery, FD) of a freely filtered substance X: FD = (TFX/PX) / (TFin/Pin) where PX and Pin are the respective conc. in plasma. Fractional reabsorption (FR): 1 – FD Reabsorption and secretion of various substances Substances undergoing tubular reabsorption:  H2O  Inorganic ions (e.g., Na+, Cl–, K+, Ca2+, and Mg2+) and  Organic substances (e.g., HCO3–, D-glucose, L-amino acids, urate, lactate, vitamin C, peptides & proteins).  Endogenous products of metabolism (e.g., urate, glucuronides, hippurates & sulfates) and  Foreign substances (e.g., penicillin, diuretics, & PAH) enter the tubular urine by way of transcellular secretion.   



  

NH3 & H+ are first produced by tubule cells before they enter the tubule by cellular secretion. NH3 enters the tubule lumen by passive transport, while H+ ions are secreted by active transport Na+/K+ transport by Na+-K+-ATPase in the basolateral membrane of the tubule & CD serves as the “motor” for most of these transport processes. By primary active transport (fueled directly by ATP consumption): Na+-K+-ATPase pumps Na+ out of the cell into the blood while pumping K+ in the opposite direction (subscript “i” = intracellular & “o” = extracellular). This creates two driving “forces” essential for the transport of numerous substances (including Na+ and K+): first, a chemical Na+ gradient ([Na+]o > [Na+]i and (because [K+]i > [K+]o, second, a membrane potential inside the cell is -ve relative to the outside which represents an electrical gradient and can drive ion transport,

Transcellular transport: implies that two membranes must be crossed, usually by two different mechanisms. If a given substance (Dglucose,PAH, etc.) is actively transported across an epithelial barrier (i.e., against an electrochemical gradient) at least one of the two serial membrane transport steps must also be active.

Interaction of transporters.  The active absorption of a solute (Na+ or D-glucose)  devp. of an osmotic gradient  passive absorption of water  absorption of solutes by solvent drag  conc. of the remaining substrates within the tubule  Absorption of the latter solutes (e.g., Cl–, urea) along their concentration gradients (passively)     

Electrogenic ion transport & ion-coupled transport can depolarize or hyperpolarize only the luminal or the basolateral membrane of the tubule cells. This causes a transepithelial potential which serves as the driving “force” for paracellular ion transport . Non-ionized forms of weak electrolytes are more lipid-soluble than ionized forms, so they are better able to penetrate the membrane (non-ionic diffusion). Thus, the pH of the urine has a greater influence on passive reabsorption by non-ionic diffusion. Molecular size also influences diffusion: smaller a molecule  larger its diffusion coefficient.

Reabsorption of Organic Substances Filtered load of a substance: plasma conc. x GFR Since the GFR is high (ca.180 L/day), enormous quantities of substances enter the primary urine each day (e.g., 160g/day of D-glucose). Fractional excretion of D-glucose is very low (FE ≅ 0.4%). So the complete reabsorption is achieved by secondary active transport (Na+-glucose symport) at the luminal cell membrane. About 95% of this activity occurs in the proximal tubule.

Renal threshold of blood glucose: It is the blood glucose concentration - Below it, the filtered glucose will be completely reabsorbed (no glucosuria). - Above it, the filtered glucose is more than the capacity of glucose transporters (glucosuria)

In normal person, the blood glucose shud be less than renal threshold. To diagnose a diabetic patient, the blood glucose level shud be greater than renal threshold (13mmol/L)

Tubular transport maximum (TmG ): Maximal amount of glucose transported (reabsorbed) across the renal tubules /min. If the filtered glucose is more than TmG, the subject will have glucosuria. At renal threshold, glucose conc. = TmG Normal plasma glucose conc. = 5 mmol/L Glucosuria: abnormal presence of glucose in urine Types: Metabolic (prerenal): blood glucose > normal Renal: blood glucose is normal Normal glucose + Glucosuria  kidney disease Hyperglycemia + glucosuria  diabetes mellitus. Prerenal glucosuria If the plasma glucose conc. > 10–15 mmol/L, as in diabetes mellitus  glucosuria  urinary glucose conc. rises. Glucose reabsorption therefore exhibits saturation kinetics (Michaelis-Menten kinetics; _p. 28). Renal glucosuria: Can occur when one of the tubular glucose carriers is defective.

Carriers for D-glucose reabsorption: SGLT - Na+-glucose transporter, in the luminal cell membrane Low-affinity carriers SGLT2  in the PCT [co-transport takes place in the ratio Na+/G – 1:1]; High-affinity carriers (SGLT1) in the PT  [Na+ / Glucose – 1:2] The energy required for this form of secondary active glucose transport is supplied by the electrochemical Na+ gradient directed towards the cell interior. Because of the co-transport of two Na+ ions, the gradient for SGLT1 = 2 times that for SGLT2 A uniporter (GLUT2 = glucose transporter type 2) on the blood side facilitates the passive transport of accumulated intracellular glucose out of the cell (facilitated diffusion). D-galactose  reabsorbed by SGLT1 [in the ration Na+ / Galactose – 1:2] D-fructose  reabsorbed by tubule cells by GLUT5 [passively]      

The plasma contains over 25 amino acids 70 g of amino acids are filtered each day. Like D-glucose, most L-amino acids are reabsorbed at the PT by Na+-coupled secondary active transport. At least 7 different amino acid transporters are in the PT (specificities of some overlap). Jmax and KM (_p. 28) and, therefore, saturability & reabsorption capacities vary acc. to the type of amino acid & carrier involved. Fractional excretion of most amino acids ≅ to 1% (ranging from 0.1% for L-valine to 6% for L-histidine).

Hyperaminoaciduria: Increased urinary excretion of amino acids. Causes: • Pre-renal (increased plasma a.a. concentration > normal ) • Renal (abnormal a.a transport carriers in renal tubule).

Prerenal hyperaminoaciduria: occurs when plasma amino acid conc. is elevated (and reabsorption becomes saturated) Renal hyperaminoaciduria: occurs due to deficient transport. Such a dysfunction may be: Specific (e.g., in cystinuria, where only L-cystine, L-arginine & L-lysine are hyperexcreted) or Unspecific (e.g., in Fanconi’s syndrome, where not only amino acids but also glucose, P-, HCO3- etc. are hyperexcreted). Certain substances (lactate, sulfate, P-, dicarboxylates, etc.) are also reabsorbed at the PT by way of Na+ symport. Urea  is subject to passive back diffusion. Urate and oxalate are both reabsorbed & secreted, with the predominant process being reabsorption for urate and secretion for oxalate. (FE of urate ≅ 0.1) and (FE of oxalate ≅ 1) If the urinary conc. of these poorly soluble substances rises above normal, they will start to precipitate (increasing the risk of urinary calculus formation). Excessive urinary excretion of cystine  cystine calculi. Oligopeptides: (eg: glutathione) & angiotensin II Broken down by luminal peptidases (in the brush border)  so they can be reabsorbed as free amino acids. Dipeptides: (e.g., carnosine). They are resistant to luminal hydrolysis. So, they must be reabsorbed as intact molecules by symport carrier (pepT2) with H+ (tertiary active transport). PepT2: a symport carrier, driven by the inwardly directed H+ gradient transports the dipeptides into the cells (tertiary active H+ symport). The dipeptides are then hydrolyzed within the cell. PepT2 carrier: also used by certain drugs & toxins. Proteins (albumin, lysozyme, α1-microglobulin, α2-microglobulin etc) In the PT: Reabsorbed by receptor mediated endocytosis and are “digested” by lysosomes & the free amino acids are released into the interstitium. Although albumin has a low sieving coefficient (0.0003), 2400 mg/day are filtered at a plasma conc. of 45 g/L. FE of albumin ≅ 1% (Only 2 - 35mg of albumin excreted eacy day).

Since this type of reabsorption is nearly saturated at normal filtered loads of proteins, an elevated plasma protein conc. or increased protein sieving coefficient will lead to proteinuria. 25-OH-cholecalciferol, which is bound to DBP (vitamin D-binding protein) in plasma & GF, is reabsorbed in combination with DBP by receptor- mediated endocytosis (_p. 294).

Excretion of Organic Substances The body can sort out the harmful/inert substances in food from the nutrients present in it at the time of intake (based on their smell or taste or, if already eaten, with the help of specific digestive enzymes & intestinal absorptive mechanisms (e.g., D-glucose & L-amino acids are absorbed, but L-glucose & D-amino acids are not). Similar distinctions are made in hepatic excretion ( bile  stools): useful bile salts are completely reabsorbed from the gut by way of specific carriers, while waste products (eg: bilirubin) are mainly eliminated in the feces. 

The liver & kidney are able to modify endogenous waste products & foreign compounds (xenobiotics) so that they are “detoxified” if toxic & made ready for rapid elimination.



In unchanged form or after the enzymatic addition of an OH or COOH group, the substances then combine with glucuronic acid, sulfate, acetate or glutathione to form conjugates.



The conjugated substances are then secreted into the bile & lumen of PT (with or w/o further metabolic processing).

Tubular secretion The PT utilizes active transport mechanisms to secrete waste products & xenobiotics. This is done thru carriers for organic anions (OA–) & organic cations (OC+). The secretion of these substances makes it possible to raise their clearance level above that of inulin & therefore, to raise their FE above 1.0 = 100% (to eliminate them more effectively) Secretion is carrier-mediated & is therefore subject to saturation kinetics. Unlike reabsorbed substances such as D-glucose, FE of organic anions & cations decreases when their plasma conc. rise. (_A; PAH secretion curve reaches plateau, and slope of PAH excretion curve decreases).

Some organic anions (e.g., urate & oxalate) & cations (e.g., choline) are both secreted & reabsorbed (bidirectional transport)  net reabsorption (urate, choline) or net secretion (oxalate). Secreted organic anions (OA–) include:  Indicators such as PAH and phenol red;  Endogenous substances such as oxalate, urate, hippurate;  Drugs such as penicillin G, barbiturates, and numerous diuretics; a  Conjugated substances containing glucuronate, sulfate or glutathione. Probenecid: is a potent inhibitor of OA– secretion due to its high affinity for the transport system.

Tubular Secretion Summary of secretion of organic anions: 1- Active step occurs across the basolateral membrane of PT by - hNaDC1 (human sodium dicarboxylate transporter ) by secondary active transport - OAT1 (organic anion transporter 1) by tertiary active transport. 2- Passive transport (efflux) at the luminal border -by facilitated diffusion -by MRP2 (multi drug resistance protein type 2) - ATP dependent conjugate pump *Active step occurs across the basolateral membrane of PT that accumulates organic anions in the cell. We need to actively transport –ve charged ions from the interstitium into the lumen against their electric gradient. (as the membrane potential inside the cell is negative). [the –ve membrane potential is due to 3Na+/2K+ATPase] OAT1 = organic anion transporter type 1: transports OA– from the blood into the tubule cells in exchange for a dicarboxylate, such as succinate2–or α-ketoglutarate2–;). The latter substance arises from the glutamine metabolism of the cell. hNaDC-1: conveys dicarboxylates (in combination with 3 Na+) into the cell by secondary active transport (_B2). The transport of OA– is therefore called tertiary active transport. MRP2 = used for secretion of amphiphilic conjugates, such as glutathione-linked lipophilic toxins (_B4).

The organic cations (OC+) secreted include: Endogenous substances (epinephrine, choline, histamine, serotonin, etc.) and Drugs (atropine, quinidine, morphine, etc.)

In contrast to OA– secretion, the active step of OC+ secretion occurs across the luminal membrane of PT cells (luminal accumulation occurs after overcoming the negative membrane potential inside the cell). Summary of secretion of organic cations: At the basolateral membrane: passive transport (from the blood into the cell via polyspecific organic cation transporter (OCT). At the luminal border: active secretion by Primary active (mdr1 - membrane direct ATP-driven carrier) Tertiary active (multi specific OC+/H+ antiporter)

Reabsorption of Na+ and Cl– About 99% of the filtered load of Na+  reabsorbed (≅ 27,000 mmol/day) i.e. Fractional excretion of Na+ (FENa) ≅ 1% FENa range = 0.5 to 5% The value of FENa needed is regulated by aldosterone, ANF, ADH etc. Sites of Na+ reabsorption: All parts of the renal tubule & CD. Nephron is divided into 2 main parts: Proximal nephron: PCT & loop of henle Distal nephron: DCT & CD 99.5% of filtered Na is reabsorbed in proximal nephron. Absorption from PCT, DCT and CD  peritubular capillaries Absorption from ascending, descending loop of Henle  vasa recta

Uphill transport: active, downhill transport: passive Electrogenic Symport: Na+ is transported with some other non-charged substances such as Dglucose. The filtered Na+ is reabsorbed: 65% = in the PCT, while the luminal Na+ conc. remains constant 25% = in the loop of Henle, where luminal Na+ conc. drops sharply. The DCT & CD also reabsorb Na+. Collecting duct: serves as the site of hormonal fine adjustment of Na+ excretion. Na+ reabsorption in the loop of henle: occurs only in the ascending limb No reabsorption in the descending limb

Summary of reabsorption in proximal tubule : 1-electro-neutral Na+/H+ exchanger type 3(NHE3) ↓ 2-electrogenic Na+ symport carrier for D-glucose ↓ LNTP (Lumen negative trans-epithelial potential) ↓ Passive Cl- influx ↓ H2O (passive transport by osmosis) ↓ Cl- transport (passively) ↓ LPTP (Lumen positive trans-epithelial potential) ↓ Passive Na+,K+,Ca2+, Mg2+ paracellular transport Mechanisms of Na+ reabsorption

•

Na+-K+-ATPase  pumps Na+ ions out of the cell (actively)  & conveys K+ ions into the cell  decreased conc. of Na+ in the cell  development of a chemical Na+ gradient  downhill flow of Na+ ions from the lumen into the cell.

•

Back diffusion of K+  associated with trapping of non–diffusible –ve charged ions into the cell  development of electrical gradient (membrane potential)  -ve charged ions in the cell attract the +ve charged Na+ ions into the cell.

This electrochemical Na+ gradient provides the driving “force” for passive Na+ influx, the features of which vary in the individual nephron segments (_B):

◆ In PCT: Na+ ions diffuse passively from the tubule lumen into the cells: (a) In the early PCT: 1/3 of 65% of Na+ reabsorption in PT takes place via the electroneutral Na+/H+ exchanger type 3 (NHE3): an Na+/H+-antiport carrier for electroneutral exchange of Na+ for H+ and (b) Mid/Late PCT: 2/3 of Na+ reabsorption takes place It occurs passively thru paracellular transport via various Na+ symport carriers for reabsorption of D-glucose etc. Since most of these symport carriers are electrogenic, the luminal cell membrane is depolarized, & an early proximal lumen-negative transepithelial potential (LNTP) develops. (The inflow of Na+ into the cell causes excess of +ve charges in the cell & –ve charges in the lumen, creating LNTP.) Due to LNTP, Cl- reabsorption will occur passively with the help of paraluminal transport mechanism. ◆ In the thick ascending limb (TAL of the loop of Henle (_B6), Na+ is reabsorbed via: The bumetanide-sensitive co-transporter BSC: a Na+-K+-2 Cl– symport carrier (has 4 binding sites). 1) Cl- influx against its conc. gradient (active influx) as most of Cl- is reabsorbed in PCT, so remaining Cl- is small in amount. 2) Na+ inflow into the cell. Then outflow into the blood by Na+/K+ ATPase pump actively. 3) Although BSC is primarily electroneutral, absorbed K+ recirculates back to the lumen thru K+ channels  hyperpolarization of the luminal membrane  devp of a lumen-positive transepithelial potential (LPTP). (This causes paracellular transport of K+, Na+, Ca2+ & Mg+ ions into the interstitium.) ◆ In the distal convoluted tubule, DCT (_B8), Na+ is reabsorbed via the thiazide-sensitive cotransporter TSC, an electroneutral Na+ Clsymport carrier. ◆ In principal cells of the connecting tubule & CD: Na+ exits the lumen via Na+ channels (ENaC) activated by  aldosterone & ADH and inhibited by  prostaglandins & ANF Since these four passive Na+ transport steps in the luminal membrane are serially connected to active Na+ transport in the basolateral membrane ( by Na+-K+-ATPase), the associated transepithelial Na+ reabsorption is also active.

On the basolateral side Na+ exits PT cell via: -3Na+-2K+ ATPase Pump (Primary active) 1 ATP molecule is consumed for each 3 Na+ ions absorbed -Na+-3Hco3 symport carrier (tertiary active) NB: Na+ exits the cell via tertiary active transport as secondary active secretion of H+ (on the opposite cell side) results in intracellular accumulation of HCO3. The 2/3 of sodium reabsorption in PT is passive and paracellular. Two driving “forces” responsible for this are: (1) the LPTP in the mid & late proximal tubule and in the loop of Henle drives Na+ & other cations onto the blood side of the epithelium. (2) Solvent drag: When water is reabsorbed, solutes for which the reflection coefficient