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Renal Physiology (4th^ Sem CC4- 9 - TH) Structural Organization of Kidney : The kidneys are a pair of bean-shaped structures that are located just below and posterior to the liver in the peritoneal cavity. The adrenal glands sit on top of each kidney and are also called the suprarenal glands. Kidneys filter blood and purify it. All the blood in the human body is filtered many times a day by the kidneys; these organs use up almost 25 percent of the oxygen absorbed through the lungs to perform this function. Oxygen allows the kidney cells to efficiently manufacture chemical energy in the form of ATP through aerobic respiration. The filtrate coming out of the kidneys is called urine. Externally, the kidneys are surrounded by three layers. The outermost layer is a tough connective tissue layer called the renal fascia. The second layer is called the perirenal fat capsule , which helps anchor the kidneys in place. The third and innermost layer is the renal capsule. Internally, the kidney has three regions—an outer cortex , a medulla in the middle, and the renal pelvis in the region called the hilum of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters. The renal cortex is granular due to the presence of nephrons —the functional unit of the kidney. The medulla consists of multiple pyramidal tissue masses, called the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. There are, on average, eight renal pyramids in each kidney. The renal pyramids along with the adjoining cortical region are called the lobes of the kidney. The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces , which further branch into the minor calyces. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder. The kidney tissue (parenchyma) contains about one million little filter units, the so-called Nephrons. These nephrons filter the blood and produce the urine. Each nephron consists of a so-called renal corpuscule followed by a tubule. Together, corpuscule and tubule form a functional unit: the corpuscules filter the blood, thereby producing about 100 litres of urine every day. To reduce this large volume, the tubules regain most of the filtered electrolytes and fluids (about 99 %) and return it to the blood circulation. As a result, only about a tenth of the initially produced urine (called primary urine), for example 1 - 1. liters, is finally released. The volume of the primary and final urine produced by
the corpuscules per day mainly depends on the fluid intake as well as on the total blood volume, which is higher in adults than in children. While the renal corpuscules are found in the outer renal cortex, the tubules are located within the inner renal medulla and drain – via special collecting ducts – into the so-called renal calyces. This is where the urine is collected and transported to the renal pelvis, ureter and urinary bladder. Figure1. Kidneys filter the blood, producing urine that is stored in the bladder prior to elimination through the urethra. Microstructural organization of kidney : Because the kidney filters blood, its network of blood vessels is an important component of its structure and function. The arteries, veins, and nerves that supply the kidney enter and exit at the renal hilum. Renal blood supply starts with the branching of the aorta into the renal arteries (which are each named based on the region of the kidney they pass through) and ends with the exiting of the renal veins to join the inferior vena cava. The renal arteries split into several segmental arteries upon entering the kidneys. Each segmental artery splits further into several interlobar arteries and enters the renal columns, which supply the renal lobes. The interlobar arteries split at the junction of the renal cortex and medulla to form the arcuate arteries. The arcuate “bow shaped” arteries form arcs along the base of the medullary pyramids. Cortical radiate arteries , as the name suggests, radiate out from the arcuate arteries. The cortical radiate arteries branch into numerous afferent arterioles, and then
The third part of the renal tubule is called the distal convoluted tubule (DCT) and this part is also restricted to the renal cortex. The DCT, which is the last part of the nephron, connects and empties its contents into collecting ducts that line the medullary pyramids. The collecting ducts amass contents from multiple nephrons and fuse together as they enter the papillae of the renal medulla. Capillary Network within the Nephron The capillary network that originates from the renal arteries supplies the nephron with blood that needs to be filtered. The branch that enters the glomerulus is called the afferent arteriole. The branch that exits the glomerulus is called the efferent arteriole. Within the glomerulus, the network of capillaries is called the glomerular capillary bed. Once the efferent arteriole exits the glomerulus, it forms the peritubular capillary network , which surrounds and interacts with parts of the renal tubule. In cortical nephrons, the peritubular capillary network surrounds the PCT and DCT. In juxtamedullary nephrons, the peritubular capillary network forms a network around the loop of Henle and is called the vasa recta. Figure 3. The nephron is the functional unit of the kidney. The glomerulus and convoluted tubules are located in the kidney cortex, while collecting ducts are located in the pyramids of the medulla.
Mechanism of Urine formation in Kidney : Every one of us, including plants and animals, depend on the excretion process for the removal of certain waste products from our body. During the process of excretion, both the kidneys play an important role in filtering the blood cells. Excretion is a biological process, which plays a vital role by eliminating toxins and other waste products from the body. In plants and animals including humans, as the part of metabolism, lot of waste products are produced. Plants usually excrete through the process of transpiration and animals excrete the wastes in different forms such as by urine, sweat, faeces, and tears. Among all these, the usual and the main form of excretion is the urine. Urine formation begins with the delivery of blood to the glomerulus followed by its filtration past the glomerular barrier. The filtered portion of plasma continues through the nephron whereas the unfiltered portion passes into the peritubular capillaries. As the filtered portion travels through the nephron, water and certain solutes are resorbed back into the peritubular capillaries whilst other solutes are secreted from the peritubular capillaries into the nephron. Whatever fluid is remains at the end of the nephron is discarded as urine. Urine Formation - Formation of urine is a process important for the whole organism. Not only acid-base balance is modulated by it, but also blood osmolarity, plasma composition and fluid volume, and thus it influences all cells in our body. A healthy adult person produces 1.5-2 liters of urine per day and this process involves three basic mechanisms:
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion Functional histology - Glomerulus consists of fenestrated capillaries without diaphragm that form important part of a renal filtration barrier. Blood flow and blood pressure in afferent and efferent arteriole is strictly regulated, which allows glomerular filtration into Bowman’s capsule. Visceral layer of Bowman’s capsule consists of podocytes and their pedicels that tightly fit to the basement membrane of capillaries. Parietal layer is formed by a single layer of simple squamous epithelium. The renal filtration barrier is composed of the fenestrated capillary endothelium, the basement membrane and the pedicles of podocytes. Pedicels interdigitate with one another forming filtration slits that are spanned by slit diaphragms (formed by protein nephrin). Due to its negative charge it prevents the filtration of plasma protei
- Filtration barrier (see above) has a unique structure and properties which do not allow passage of proteins into the filtrate (primary urine) GFR is therefore dependent on the renal blood flow, the filtration pressure, the plasma oncotic pressure, and the size of the filtration area. The principles governing the GFR are in reality a specialized application of the principles discussed in microcirculatory physiology and are thus ultimately governed by Starling Forces. However, because the glomerular capillaries are surrounded by the fluid in the Bowman's Capsule, the hydrostatic and oncotic pressure of Bowman's Space is used instead of those of the 'Interstitial Fluid'. Importantly, because no plasma proteins can cross the glomerular barrier during glomerular filtration, the oncotic pressure of fluid in Bowman's Space is essentially zero and is thus removed from the starling equation. Formally: GFR = Kf [(Pc-Pb)-(Πc)] GFR = Glomerular Filtration Rate (ml/min). Equivalent of Jv in Starling Forces Kf = Permeability Constant of glomerular capillaries Pc = Glomerular Capillary Hydrostatic Pressure Pb = Bowman's Space Hydrostatic Pressure Πc = Glomerular Capillary Oncotic Pressure Glomerular Capillary Hydrostatic Pressure Modulation Overview The GFR can be modulated by changing any of the variables mentioned above. However, the primary physiological mechanism by which GFR is modulated is through modification of the glomerular capillary hydrostatic pressure. The glomerular capillary pressure is itself determined by three variables as discussed below. Systemic Arterial Pressure Naturally, increased or decreased incoming systemic arterial blood pressures will increase or decrease the hydrostatic pressures within the glomerular capillaries. However, due to autoregulatory mechanisms discussed in Autoregulation of GFR and RBF incoming arterial pressures into the glomerulus are largely constant over a wide range of systemic arterial pressures and thus are not a major source of regulation.
Renal Afferent Arteriolar Resistance Whatever the incoming arterial pressure, the resistance offered by the renal afferent arteriole is a major determinant of the ultimate hydrostatic pressure within the glomerulus. If afferent arteriolar resistance is increased, the hydrostatic pressure within the glomerulus declines and so too does the glomerular filtration rate. Conversely, if the afferent arteriolar resistance is decreased, the hydrostatic pressure within the glomerulus increases and so too does the glomerular filtration rate. Consequently, regulation of renal afferent arteriolar resistance is a major source of modulation of the GFR. Renal Efferent Arteriolar Resistance The relationship between the resistance of the renal efferent arteriole with GFR is more complex and not well-understood. A hand-waving explanation is that increased renal efferent arteriolar resistance causes backup of blood into the glomerulus and consequently increases glomerular hydrostatic pressures and thus GFR. However, as efferent arteriolar resistance increases, the renal blood flow also declines, resulting in reduced GFR. Empirically, these two antagonistic phenomenon result in a biphasic relationship to increased efferent arteriolar resistance and GFR. Initial increments in efferent arteriolar resistance increase GFR; however, larger increments in efferent resistance result in decreased GFR. Modulation of other variables Overview Although modulation of any of the other variables mentioned under "Physical Determinants" can change the GFR, these are not major sources of physiological GFR regulation. However, as mentioned below, some of these variables are affected in certain disease states and thus can result in pathological changes in GFR. Permeability Coefficient The permeability of the glomerular barrier can decline due to thickening of the glomerular basement membrane as occurs in diabetic nephropathy. Additionally, total glomerular permeability can decline due to reductions in the total number of functional glomeruli, thus reducing the total glomerular surface area as occurs in hypertension. Bowman's Space Hydrostatic Pressure
oxide or kinins,whereas antidiuretichormone (ADH), ATP and endothelin cause a reduction in the renal blood flow. Assessment of the glomerular filtration rate - If we want to determine GFR, which is one of the basic function of our kidneys , we have to use a substance that is excreted from the body only by glomerular filtration (inulin, creatinine) and is not affected by tubular processes. As an example we can mention the calculation of the clearance ( plasma volume that is per unit time completely cleaned of marker substances ) of endogenous creatinine , whose formula has the following form: U – urine creatinine concentration in mmol/l V – volume of urine (diuresis) in ml/s P – plasma creatinine concentration in mmol/l In clinical practice, we use more complex calculations, corrected for body surface area (and other physical parameters) – e.g. equation by Cockroft and Gault , equation MDRD etc. Tubular reabsorption and secretion - As it is mentioned above, about 99 % of the filtrate gets reabsorbed by the tubular resorption to the extracellular fluid (back into the body), leaving only 1.5-2 l of urine per day. The main task for renal tubules is therefore an isosmotic tubular reabsorption of primary urine. They absorb water, ions (sodium, chlorides, potassium, calcium, magnesium, bicarbonate or phosphate), urea, glucose and amino acids. All of this is independent on the extracellular fluid volume in the body – we speak about the obligatory resorption. Its primary role is to maintain fluid volume in the body under normal conditions. Transport can be carried by passive diffusion (in the direction of the concentration or electrical gradient), primary active transport against gradient (needs energy – ATP) or secondary active transport (transport protein uses the concentration gradient created by a primary active transport realized by other transport protein). Substances can be transported by paracellular or transcellular routes. Transport of water is always passive. Na+/K+-ATPase located on the basolateral membrane plays important role in the secondary active transport. It creates a concentration gradient for Na+. Transport proteins act as symporters (transport of compound is coupled to the transport of Na+^ in the same direction) or antiporters (transport of compound is coupled to the transport of Na+^ in the opposite direction). To understand the
processes in the tubular system, we must imagine tubular epithelial cells, their apical membrane facing the tubular fluid (primary urine), basolateral membrane, on the other hand, is in contact with the peritubular fluid (here is located the Na+/K+-ATPase). The proximal tubule - Reabsorption of sodium ions is in the first half of the proximal tubule coupled with the reabsorption of bicarbonate, glucose, amino acids, lactate, urea and phosphate. Absorbed compounds are osmotically active, thereby draining water from tubules. This leads to an increased concentration of chloride ions in the tubular fluid that is very important for a resorption in other parts of the proximal tubule. Reabsorption of bicarbonate ions in the proximal tubule Movement of bicarbonate and hydrogen ions depends on the transport sodium ions. This process is catalyzed by enzyme carbonic anhydrase (located in the apical membrane and in the intracellular part of the epithelial cells). The first step is the secretion of H+^ into the tubular fluid through the Na+/H+^ antiport , located at the luminal (apical) membrane of proximal tubule cells. Transferred H
may in the tubular fluid react with filtered bicarbonate ions to form carbonic acid. Carbonic anhydrase facilitates the decomposition of carbonic acid in the tubular fluid to water and carbon dioxide. Both compounds can freely diffuse into the tubule epithelial cells, where carbonic acid is restored by the carbonic anhydrase. Molecules of carbonic acid dissociates into hydrogen and bicarbonate ions. Bicarbonate ions then pass through the basolateral membrane into the interstitial fluid through Na+/3HCO 3 – - cotransporter or anion exchanger (Cl–/HCO 3 – ). H+^ returns via antiport with Na+^ into the tubular fluid. For each secreted H+, Na+^ and HCO 3 –^ is absorbed
Clinical correlation: Substances that block the symport (e.g. furosemide) are used as very effective diuretic drugs – loop diuretics. Distal convoluted tubule and collecting duct Distal convoluted tubule and collecting duct resorbe about 7 % of solutes (mainly Na+^ and Cl–) and approximately 17 % water. Their resorption is affected by hormones (e.g. ADH) – facultative resorption. Hydrogen and potassium ions are secreted here. The distal convoluted tubule and the collecting duct thus play an important role in the formation of the final urine and in the regulation of osmolarity and pH. Sodium and chloride ions are absorbed in the first part of the distal convoluted tubule. The distal part of the distal convoluted tubule and the collecting duct consist of two cell types: 1) Principal cells responsible for the resorption of sodium ions and water (dependent on ADH) and secretion of K+^ ions 2) Intercalated cells containing carbonic anhydrase. They are involved in acid- base balance, because they can secrete both hydrogen and bicarbonate ions Calcium and phosphate reabsorption and secretion Plasma concentration of total calcium is 2.25-2.75 mmol/l and for ionized calcium 1.1-1.4 mmol/l. Only ionized calcium (about 48 % of total) is filterable by kidneys. Resorption takes place by both active (15- 20 %) and passive paracellular (80 %) mechanisms. It is localized in the proximal tubule , the ascending part of Henle’s loop and partially in the distal convoluted tubule. Parathyroid hormone stimulates the reabsorption by transcellular
route in this segment. Calcitriol acts the same way, just mostly in the distal convoluted tubule. In contrast, calcitonin increases the excretion of calcium ions by inhibition of tubular reabsorption. Serum phosphate concentration is 0.7-1.5 mmol/l , urine concentration is 15 - 90 mmol/l. Phosphates are also influenced by the parathyroid hormone ( inhibits the resorption of phosphates ) and by the calcitonin (also reduces the resorption of phosphates). Control of tubular processes We can distinguish local and central regulatory mechanisms. Local mechanisms Local mechanisms are represented mainly by Starling´s forces (increased plasma oncotic pressure leads to an increased reabsorption of water and solutes from the interstitium into the capillaries, thereby supporting the tubular resorption) and glomerulotubular balance (increased GFR leads to an increase in glucose, amino acids and sodium ions resorption, these are followed by water
- volume of resorbed fluid increases proportionally with increased GFR). Central mechanisms Central mechanisms are represented by many hormones – such as ADH , aldosterone , angiotensin II , epinephrine , natriuretic peptides ( ANP and BNP ) or parathyroid hormone. Sympathetic nervous system has a role also. ADH (antidiuretic hormone, vasopressin) is produced in the hypothalamus and secreted by the posterior pituitary gland in a response to an increased osmolarity of extracellular fluid (to a lesser extent as an answer to a decrease of extracellular fluid volume). ADH binds to the V 2 - receptor located on collecting duct cells (partly on distal tubule cells). Its effect increases the number of aquaporins in cell membranes and water molecules can pass along the osmotic gradient into peritubular fluid (ECF). ADH acts also on a transport of urea in the collecting duct and on a transport of Na+^ and Cl–^ in the thick segment of the ascending limb of the loop of Henle. Aldosterone is secreted by the zona glomerulosa of the adrenal cortex in response to increasing plasma concentrations of angiotensin II and potassium ions. It plays therefore an important role in maintaining of constant level of potassium ions ( accelerates secretion of potassium ions in the thick segment of the loop of Henle and in the distal tubule) and in regulation of volume of ECF. As the part of the renin-angiotensin-aldosterone system , it stimulates reabsorption of sodium ions , accompanied by passive water
b) Production of ADH is increased c) Urea circulates in the renal medulla – increased hypertonicity of the medulla Final urine - Final urine is characteristically malodorous, clear, golden yellow liquid. Its specific gravity varies between 1 003-1 038 kg/m^3 and its pH between 4.4-8.. It contains Na+^ (100-250 mmol/l), K+^ (25-100 mmol/l), Cl–^ (about 135 mmol/l), Ca2+, creatinine, vanillylmandelic acid (degradation product of catecholamines), uric acid, urea, etc. Healthy kidneys do not allow a significant amount of proteins and glucose to reach the final urine (they are almost completely reabsorbed). Presence of high amount of proteins and glucose in the final urine is a pathological finding. Normal diuresis is 1.5-2 l/day. Polyuria is diuresis higher than 2 l/day , oliguria lower than 0.5 l/day and anuria lower than 0.1 l/day Acid Base balance by kidney – Acid-base homeostasis and pH regulation are critical for both normal physiology and cell metabolism and function. The importance of this regulation is evidenced by a variety of physiologic derangements that occur when plasma pH is either high or low. The kidneys have the predominant role in regulating the systemic bicarbonate concentration and hence, the metabolic component of acid-base balance. This function of the kidneys has two components: reabsorption of virtually all of the filtered HCO 3 −^ and production of new bicarbonate to replace that consumed by normal or pathologic acids. This production or generation of new HCO 3 −^ is done by net acid excretion. Under normal conditions, approximately one-third to one-half of net acid excretion by the kidneys is in the form of titratable acid. The other one-half to two-thirds is the excretion of ammonium. The capacity to excrete ammonium under conditions of acid loads is quantitatively much greater than the capacity to increase titratable acid. Multiple, often redundant pathways and processes exist to regulate these renal functions. Derangements in acid-base homeostasis, however, are common in clinical medicine and can often be related to the systems involved in acid-base transport in the kidneys. Acid-base homeostasis and pH regulation are critical for both normal physiology and cell metabolism and function. The importance of this regulation is evidenced by a variety of physiologic derangements that occur when plasma pH is either high or low. The kidneys have the predominant role in regulating the systemic bicarbonate
concentration and hence, the metabolic component of acid-base balance. This function of the kidneys has two components: reabsorption of virtually all of the filtered HCO 3 −^ and production of new bicarbonate to replace that consumed by normal or pathologic acids. This production or generation of new HCO 3 −^ is done by net acid excretion. Under normal conditions, approximately one-third to one-half of net acid excretion by the kidneys is in the form of titratable acid. The other one-half to two-thirds is the excretion of ammonium. The capacity to excrete ammonium under conditions of acid loads is quantitatively much greater than the capacity to increase titratable acid. Multiple, often redundant pathways and processes exist to regulate these renal functions. Derangements in acid-base homeostasis, however, are common in clinical medicine and can often be related to the systems involved in acid-base transport in the kidneys Renal Control of Plasma HCO 3 − The kidneys have the predominant role of regulating the systemic HCO 3 −^ concentration and hence, the metabolic component of acid-base balance. This function of the kidneys has two components: reabsorption of virtually all of the filtered HCO 3 −^ and production of new HCO 3 −^ to replace that consumed by normal or pathologic acids. This production or generation of new HCO 3 −^ is done by net acid excretion. In other words, the kidneys make new HCO 3 −^ by excreting acid. Because HCO 3 −^ is freely filtered at the glomerulus, approximately 4.5 mol HCO 3 −^ is normally filtered per day (HCO 3 −^ concentration of 25 mM/L ×GFR of 0.120 L/min ×1440 min/d). Virtually all of this filtered HCO 3 −^ is reabsorbed, with the urine normally essentially free of HCO 3 −. Seventy to eighty percent of this filtered HCO 3 −^ is reabsorbed in the proximal tubule; the rest is reabsorbed along more distal segments of the nephron. In addition to reabsorption of filtered HCO 3 −, the kidneys also produce additional HCO 3 −^ beyond that which has been filtered at the glomerulus. This process occurs by the excretion of acid into urine. (As indicated above, the excretion of acid is equivalent to the production of alkali.) The net acid excretion of the kidneys is quantitatively equivalent to the amount of HCO 3 −^ generation by the kidneys. Generation of new HCO 3 −^ by the kidneys is usually approximately 1 mEq/kg body wt per day (or about 70 mEq/d) and replaces that HCO 3 −^ that has been consumed by usual endogenous acid production (also about 70 mEq/d) as discussed above. During additional acid loads and in certain pathologic conditions, the kidneys increase the amount of acid excretion and the resulting new HCO 3 −^ generation. Net acid excretion by the kidneys occurs by two processes: the excretion of titratable acid and the excretion of ammonium (NH 4 +). Titratable acid refers to the excretion of protons with urinary buffers. The capacity of the nephron to excrete acid as free protons is limited as illustrated by the fact that the concentration of protons
CO 3 −2^ Eq are transported with each Na+; this electrogenic stoichiometry results in Na+^ and HCO 3 −^ being driven out of the cell by the cell–negative membrane voltage. Mutations in this (NBC-e1) protein cause proximal renal tubular acidosis ( 24 ). Chloride-bicarbonate exchange may also be present on the basolateral membrane of the proximal tubule but is not the main mechanism of HCO 3 −^ reabsorption. The proximal tubule is a leaky epithelium on the basis of its particular tight junction proteins and therefore, unable to generate large transepithelial solute or electrical gradients; the minimal luminal pH and HCO 3 −^ obtained at the end of the proximal tubule are approximately pH 6.5–6.8 and 5 mM, respectively. The inability of the proximal tubule to lower pH further may also result from the dependence on the Na+^ gradient driving the H+^ (pH) gradient. In the late proximal tubule, ongoing reabsorption of the low concentrations of luminal HCO 3 −^ may be countered and balanced by passive backleak of plasma or peritubular HCO 3 −^ into the lumen; however, continued Na+/H+^ exchange activity will result in continued Na+^ reabsorption. Distal Tubule Acidification Several segments after the proximal tubule contribute substantially to acid-base homeostasis. First, the thick ascending limb (TAL) reabsorbs a significant amount of HCO 3 −, approximately 15% of the filtered load, predominantly through an apical Na+/H+^ exchanger. TAL acid-base transport is regulated by a variety of factors ( e.g. , Toll-like receptors, dietary salt, aldosterone, angiotensin II, etc. , but the integrated role of the thick limb acid-base transport in various acid-base disorders has not been well clarified. Beyond the TAL, the distal tubule compared with the proximal tubule has a limited capacity to secrete H+^ and thereby, reabsorb HCO 3 −. During inhibition of proximal tubule HCO 3 −^ reabsorption, increased distal delivery of HCO 3 −^ can overwhelm this limited reabsorptive capacity. However, the collecting tubule is able to generate a large transepithelial pH gradient (urine pH <5 with blood pH
approximately 7.4). This large pH gradient is achievable because of the primary active pumps responsible for distal nephron H+^ secretion (discussed below) and because of the relative impermeability of the distal tubule to ions ( i.e. , a tight epithelial membrane). The large pH gradient ensures the generation of titratable acid and the entrapment of NH 4 +. The limitation in HCO 3 −^ reabsorption is likely, in part, caused by the absence of luminal CA discussed below, but there may be other factors as well, such as a limited number of H+^ pumps. The distal tubule beyond the TAL consists of several distinct morphologic and functional segments, including the distal convoluted tubule, the connecting segment, and several distinct collecting duct segments, each with several cell types. Although several of these cell types can secrete H+, the CA–containing (and mitochondrial–rich) intercalated cells (ICs) are chiefly responsible for acid-base transport. There are at least three types of ICs: type A or α ICs, which secrete H+; type B or β ICs, which secrete HCO 3 −; and non–A, non–B ICs, with a range of function that remains under investigation. Conceptually, H+^ secretion in the type A ICs will result in HCO 3 −^ reabsorption in the presence of luminal HCO 3 −^ but will acidify the urine and generate new HCO 3 −^ in the absence of luminal HCO 3 −^ ( i.e. , if virtually all of the filtered HCO 3 −^ has been reabsorbed upstream). The mechanism of this H+^ secretion involves primarily an apical H+- ATPase as illustrated in. The apical H+-ATPase is an electrogenic active pump (vacuolar ATPase) able to secrete H+^ down to a urine pH of approximately 4.5. This is a multisubunit ATPase resembling that in intracellular organelles. The subunits are in two domains: a V 0 transmembrane domain and a V 1 cytosolic domain. Mutations in certain subunits are a known cause of distal renal tubular acidosis. Regulation of this pump is primarily by recycling between subapical vesicles and the plasma membrane involving the actin cytoskeleton and microtubules. Regulation may also be accomplished, in certain situations by assembly or disassembly of the two domains and phosphorylation of subunits. H+^ secretion also occurs through another set of pump(s): apical H+/K+-ATPases. H+/K+-ATPases in the collecting duct include both the gastric form of H+/K+- ATPase and the colonic form of H+/K+-ATPase. On the basis of inhibitor studies, there may also be a third type of H+/K+-ATPase in the collecting duct. These transporters likely contribute to urine acidification under normal conditions but particularly during states of potassium depletion. Conceptually, HCO 3 −^ reabsorption (or new HCO 3 −^ generation) as in the proximal tubule is a two-part process: secretion of H+^ into the lumen and HCO 3 −^ exit from the cell across the basolateral membrane. Therefore, to complete the process of HCO 3 −^ reabsorption or new HCO 3 −^ generation, HCO 3 −^ produced in the cell from CO 2 and H 2 O exits across the basolateral membrane. In other words, the HCO 3 −^ generated intracellularly by any of these pumps exits the basolateral membrane. This exit into the blood occurs through a chloride-bicarbonate exchanger, a truncated version of the anion exchanger 1 (AE1; band 3 protein), which is the Cl−/ HCO 3 −^ exchanger in red blood cells that facilitates
