The proximal tubule is contiguous with Bowman's capsule and takes a tortuous path until finally forming a straight portion that dives into the renal medulla. Based on the morphology of the epithelial cells lining the tubule, the proximal tubule has been subdivided into S1, S2, and S3 segments. Normally, ~65% of filtered Na+ is reabsorbed in the proximal tubule, and since this part of the tubule is highly permeable to water, reabsorption is essentially isotonic.
Between the outer and inner strips of the outer medulla, the tubule abruptly changes morphology to become the descending thin limb (DTL), which penetrates the inner medulla, makes a hairpin turn, and then forms the ascending thin limb (ATL). At the juncture between the inner and outer medulla, the tubule once again changes morphology and becomes the thick ascending limb, which is made up of three segments: a medullary portion (MTAL), a cortical portion (CTAL), and a postmacular segment. Together the proximal straight tubule, DTL, ATL, MTAL, CTAL, and postmacular segment are known as the loop of Henle. The DTL is highly permeable to water, yet its permeability to NaCl and urea is low. In contrast, the ATL is permeable to NaCl and urea but is impermeable to water. The thick ascending limb actively reabsorbs NaCl but is impermeable to water and urea. Approximately 25% of filtered Na+ is reabsorbed in the loop of Henle, mostly in the thick ascending limb, which has a large reabsorptive capacity.
The thick ascending limb passes between the afferent and efferent arterioles and makes contact with the afferent arteriole by means of a cluster of specialized columnar epithelial cells known as the macula densa. The macula densa is strategically located to sense concentrations of NaCl leaving the loop of Henle. If the concentration of NaCl is too high, the macula densa sends a chemical signal (perhaps adenosine or ATP) to the afferent arteriole of the same nephron, causing it to constrict. This, in turn, causes a reduction in PGC and QA and decreases SNGFR. This homeostatic mechanism, known as tubuloglomerular feedback (TGF), serves to protect the organism from salt and volume wasting. Besides mediating the TGF response, the macula densa also regulates renin release from the adjacent juxtaglomerular cells in the wall of the afferent arteriole.
Approximately 0.2 mm past the macula densa, the tubule changes morphology once again to become the distal convoluted tubule (DCT). The postmacular segment of the thick ascending limb and the distal convoluted tubule often are referred to as the early distal tubule. Like the thick ascending limb, the DCT actively transports NaCl and is impermeable to water. Since these characteristics impart the ability to produce a dilute urine, the thick ascending limb and the DCT are collectively called the diluting segment of the nephron, and the tubular fluid in the DCT is hypotonic regardless of hydration status. However, unlike the thick ascending limb, the DCT does not contribute to the countercurrent-induced hypertonicity of the medullary interstitium (described later in this section). There are marked species differences in transporter expression in the DCT. The DCT has been subdivided into DCT1 and DCT2. DCT1 expresses the thiazide-sensitive NaCl co-transporter but does not express genes involved in transepithelial Ca2+ transport such as the Ca2+ entry channel, TRPV5, and the Na+/Ca2+ exchanger. DCT2 expresses Ca2+ transport proteins and amiloride-sensitive epithelial Na+ channels.
The collecting duct system (connecting tubule + initial collecting tubule + cortical collecting duct + outer and inner medullary collecting ducts—segments 10–14 in Figure 25–1) is an area of fine control of ultrafiltrate composition and volume. It is here that final adjustments in electrolyte composition are made, a process modulated by the adrenal steroid aldosterone. In addition, antidiuretic hormone modulates permeability of this part of the nephron to water.
The more distal portions of the collecting duct pass through the renal medulla, where the interstitial fluid is markedly hypertonic. In the absence of ADH, the collecting duct system is impermeable to water, and a dilute urine is excreted. In the presence of ADH, the collecting duct system is permeable to water, so water is reabsorbed. The movement of water out of the tubule is driven by the steep concentration gradient that exists between tubular fluid and medullary interstitium.
The hypertonicity of the medullary interstitium plays a vital role in the ability of mammals and birds to concentrate urine, and therefore is a key adaptation necessary for living in a terrestrial environment. This is accomplished by a combination of the unique topography of the loop of Henle and the specialized permeability features of the loop's subsegments. Although the precise mechanisms giving rise to the medullary hypertonicity have remained elusive, the "passive countercurrent multiplier hypothesis" is an intuitively attractive model that is qualitatively accurate. According to this hypothesis, the process begins with active transport in the thick ascending limb, which concentrates NaCl in the interstitium of the outer medulla. Since this segment of the nephron is impermeable to water, active transport in the ascending limb dilutes the tubular fluid. As the dilute fluid passes into the collecting-duct system, water is extracted if, and only if, ADH is present. Since the cortical and outer medullary collecting ducts have a low permeability to urea, urea is concentrated in the tubular fluid. The inner medullary collecting duct, however, is permeable to urea, so urea diffuses into the inner medulla, where it is trapped by countercurrent exchange in the vasa recta. Since the DTL is impermeable to salt and urea, the high urea concentration in the inner medulla extracts water from the DTL and concentrates NaCl in the tubular fluid of the DTL. As the tubular fluid enters the ATL, NaCl diffuses out of the salt-permeable ATL, thus contributing to the hypertonicity of the medullary interstitium.
General Mechanism of Renal Epithelial Transport. Figure 25–2 illustrates seven mechanisms by which solutes may cross renal epithelial cell membranes (see also Figure 5–4). If bulk water flow occurs across a membrane, solute molecules will be transferred by convection across the membrane, a process known as solvent drag. Solutes with sufficient lipid solubility also may dissolve in the membrane and diffuse across the membrane down their electrochemical gradients (simple diffusion). Many solutes, however, have limited lipid solubility, and transport must rely on integral proteins embedded in the cell membrane. In some cases the integral protein merely provides a conductive pathway (pore) through which the solute may diffuse passively (channel-mediated diffusion). In other cases the solute may bind to the integral protein and, owing to a conformational change in the protein, be transferred across the cell membrane down an electrochemical gradient (carrier-mediated or facilitated diffusion, also called uniport). However, this process will not result in net movement of solute against an electrochemical gradient. If solute must be moved "uphill" against an electrochemical gradient, then either primary active transport or secondary active transport is required. With primary active transport, ATP hydrolysis is coupled directly to conformational changes in the integral protein, thus providing the necessary free energy (ATP-mediated transport). Often, ATP-mediated transport is used to create an electrochemical gradient for a given solute, and the free energy of that solute gradient is then released to drive the "uphill" transport of other solutes. This process requires symport (co-transport of solute species in the same direction) or antiport (countertransport of solute species in opposite directions) and is known as secondary active transport.
The kinds of transport achieved in a particular nephron segment depend mainly on which transporters are present and whether they are embedded in the luminal or basolateral membrane. A general model of renal tubular transport is shown in Figure 25–3 and can be summarized as follows:
Na+, K+-ATPase (sodium pump) in the basolateral membrane hydrolyzes ATP, which results in the transport of Na+ into the intercellular and interstitial spaces, the movement of K+ into the cell, and the establishment and maintenance of an electrochemical gradient for Na+ across the cell membrane directed inward. Although other ATPases exist in selected renal epithelial cells and participate in the transport of specific solutes (e.g., Ca2+-ATPase and H+-ATPase), the bulk of all transport in the kidney is due to the abundant supply of Na+, K+-ATPase in the basolateral membranes of renal epithelial cells and the separation of Na+ and K+ across the cell membrane.
Na+ may diffuse across the luminal membrane by means of Na+ channels into the epithelial cell down the electrochemical gradient for Na+ that is established by the basolateral Na+, K+-ATPase. In addition, free energy available in the electrochemical gradient for Na+ is tapped by integral proteins in the luminal membrane, resulting in co-transport of various solutes against their electrochemical gradients by symporters (e.g., Na+-glucose, Na+-H2PO4−, and Na+-amino acid). This process results in movement of Na+ and co-transported solutes out of the tubular lumen into the cell. Also, antiporters (e.g., Na+-H+) move Na+ out of and some solutes into the tubular lumen.
Na+ exits the basolateral membrane into the intercellular and interstitial spaces by means of the Na+ pump or symporters or antiporters in the basolateral membrane.
The action of Na+-linked symporters in the luminal membrane causes the concentration of substrates for these symporters to rise in the epithelial cell. These electrochemical gradients then permit simple diffusion or mediated transport (e.g., symporters, antiporters, uniporters, and channels) of solutes into the intercellular and interstitial spaces.
Accumulation of Na+ and other solutes in the intercellular space creates a small osmotic pressure differential across the epithelial cell. In water-permeable epithelium, water moves into the intercellular spaces driven by the osmotic pressure differential. Water moves through aqueous pores in both the luminal and the basolateral cell membranes, as well as through tight junctions (paracellular pathway). Bulk water flow carries some solutes into the intercellular space by solvent drag.
Movement of water into the intercellular space concentrates other solutes in the tubular fluid, resulting in an electrochemical gradient for these substances across the epithelium. Membrane-permeable solutes then move down their electrochemical gradients into the intercellular space by both the transcellular (e.g., simple diffusion, symporters, antiporters, uniporters, and channels) and paracellular pathways. Membrane-impermeable solutes remain in the tubular lumen and are excreted in the urine with an obligatory amount of water.
As water and solutes accumulate in the intercellular space, hydrostatic pressure increases, thus providing a driving force for bulk water flow. Bulk water flow carries solute (solute convection) out of the intercellular space into the interstitial space and, finally, into the peritubular capillaries. The movement of fluid into peritubular capillaries is governed by the same Starling forces that determine transcapillary fluid movement for any capillary bed.
Mechanism of Organic Acid and Organic Base Secretion. The kidney is a major organ involved in the elimination of organic chemicals from the body. Organic molecules may enter the renal tubules by glomerular filtration of molecules not bound to plasma proteins or may be actively secreted directly into tubules. The proximal tubule has a highly efficient transport system for organic acids and an equally efficient but separate transport system for organic bases. Current models for these secretory systems are illustrated in Figure 25–4. Both systems are powered by the sodium pump in the basolateral membrane, involve secondary and tertiary active transport, and use a facilitated-diffusion step. There are at least nine different organic acid and five different organic base transporters; the precise roles that these transporters play in organic acid and base transport remain ill defined (Dresser et al., 2001). A family of organic anion transporters (OATs) countertransport organic anions with dicarboxylates (Figure 25–4A). OATs most likely exist as α-helical dodecaspans connected by short segments of ~10 or fewer amino acids, except for large interconnecting stretches of amino acids between helices 1 and 2 and helices 6 and 7 (Eraly et al., 2004); see Chapter 5.
Renal Handling of Specific Anions and Cations. Reabsorption of Cl− generally follows reabsorption of Na+. In segments of the tubule with low-resistance tight junctions (i.e., "leaky" epithelium), such as the proximal tubule and thick ascending limb, Cl− movement can occur paracellularly. With regard to transcellular Cl− flux, Cl−crosses the luminal membrane by antiport with formate and oxalate (proximal tubule), symport with Na+/K+ (thick ascending limb), symport with Na+ (DCT), and antiport with HCO3− (collecting-duct system). Cl− crosses the basolateral membrane by symport with K+ (proximal tubule and thick ascending limb), antiport with Na+/HCO3− (proximal tubule), and Cl− channels (thick ascending limb, DCT, collecting-duct system).
Eighty to ninety percent of filtered K+ is reabsorbed in the proximal tubule (diffusion and solvent drag) and thick ascending limb (diffusion) largely through the paracellular pathway. In contrast, the DCT and collecting-duct system secrete variable amounts of K+ by a conductive (channel-mediated) pathway. Modulation of the rate of K+ secretion in the collecting-duct system, particularly by aldosterone, allows urinary K+ excretion to be matched with dietary intake. The transepithelial potential difference (VT), lumen-positive in the thick ascending limb and lumen-negative in the collecting-duct system, provides an important driving force for K+ reabsorption and secretion, respectively.
Most of the filtered Ca2+ (~70%) is reabsorbed by the proximal tubule by passive diffusion through a paracellular route. Another 25% of filtered Ca2+ is reabsorbed by the thick ascending limb in part by a paracellular route driven by the lumen-positive VT and in part by active transcellular Ca2+ reabsorption modulated by parathyroid hormone (PTH; see Chapter 44). Most of the remaining Ca2+ is reabsorbed in DCT by a transcellular pathway. The transcellular pathway in the thick ascending limb and DCT involves passive Ca2+ influx across the luminal membrane through Ca2+ channels (TRPV5), followed by Ca2+ extrusion across the basolateral membrane by a Ca2+-ATPase. Also, in DCT and CNT, Ca2+ crosses the basolateral membrane by Na+-Ca2+ exchanger (antiport).
Inorganic phosphate (Pi) is largely reabsorbed (80% of filtered load) by the proximal tubule. The Na+-Pi symporter uses the free energy of the Na+ electrochemical gradient to transport Pi into the cell. The Na+-Pi symporter is inhibited by PTH. Pi exits the basolateral membrane down its electrochemical gradient by a poorly understood transport system.
Only 20-25% of Mg2+ is reabsorbed in the proximal tubule, and only 5% is reabsorbed by the DCT and collecting-duct system. The bulk of Mg2+ is reabsorbed in the thick ascending limb by a paracellular pathway driven by the lumen-positive VT. However, transcellular movement of Mg2+ also may occur with basolateral exit by Na+-Mg2+ antiport or an Mg2+-ATPase.
The renal tubules play an extremely important role in the reabsorption of HCO3− and secretion of protons (tubular acidification) and thus participate critically in the maintenance of acid-base balance. These processes are described in the section on carbonic anhydrase inhibitors.