Glomerulus

The glomerulus is formed from a specialized capillary network. Each capillary develops into approximately 40 glomerular loops around 200 µm in size and consisting of a variety of different cell types supported on a specialized basement membrane (Figure 48-3, top). Some endothelial and epithelial cells act in concert with the specialized glomerular basement membrane (GBM) to form the glomerular filtration barrier, in addition to mesangial cells.

The capillary endothelial cells are about 40 nm thick and are in contact with each other. However, in contrast to the continuous endothelial linings seen elsewhere in the body, circular fenestrations (pores) with diameters of approximately 60 nm collectively constitute 20 to 50% of the glomerular endothelial surface.¹⁷⁰ The endothelium permits virtually free access of plasma and small solutes to the basement membrane. However, although the fenestrations are far larger than the diameter of albumin (3.5 nm), it is thought that perm selectivity to such larger molecules begins at the endothelium because of the endothelial surface lining—a glycocalyx coating of negatively charged glycoproteins, glycosaminoglycans, proteoglycans, and absorbed plasma proteins, including orosomucoid and albumin. Estimates of the thickness of this layer vary depending on the visualization and preparation techniques used, but it may be between 200 and 400 nm thick.¹⁷⁰

The basement membrane (Figure 48-3, bottom) of the glomerular capillaries is much thicker (approximately 300 nm) than that of other vascular beds and consists of three distinct electron-dense layers: the lamina rara interna, the lamina densa, and the lamina rara externa. The lamina densa consists of a close feltwork of fine, mainly type IV, collagen fibrils (each 3 to 5 nm thick) embedded in a gel-like matrix of laminin, nidogen/entactin, glycoproteins, and proteoglycans such as agrin and perlecan. The lamina densa forms the main size discriminant barrier to protein passage into the tubular lumen. The other two layers of the basement membrane are rich in negatively charged polyanionic glycoproteins, such as heparan sulfate; these may form a charge discriminant barrier to the passage of proteins, although the importance of the GBM in charge discrimination is still uncertain.¹⁷⁰

The epithelial cells of the glomerulus line the outside of the glomerular capillaries, thus facing Bowman’s capsule and the primary urine (see Figure 48-3, top). These cells are called podocytes and have an unusual octopus-like structure in that they have a large number of cytoplasmic extensions or foot processes that are embedded in the basement membrane. Foot processes are anchored to the GBM via integrin molecules and dystroglycans and are divided into primary and secondary. The secondary processes between adjacent cells interdigitate to form filtration slits, which are 25 to 60 nm wide.¹⁷⁰ The podocytes are covered by a complex diaphragm (“slit diaphragm”), some of the molecular components of which (e.g., nephrin) appear crucial for the maintenance of larger proteins within the circulation. The resulting structure is relatively impermeable to most proteins above 60 kDa, but passage of proteins is modulated by their charge and shape. Podocytes are also covered by a glycocalyx of sulfated molecules, including glycosaminoglycans and glycoconjugates (e.g., podocalyxin). The surface anionic charge helps to maintain the foot process structure and the distance between the parietal and visceral epithelial cells constituting Bowman’s space.¹⁷⁰

The final cellular components of the glomerulus are the mesangial cells, which are found in the central part (“stalk”) of the glomerulus between and within the capillary loops suspended in a matrix that they synthesize. They are in direct contact with glomerular endothelial cells and the inner layer of the GBM (lamina rara interna), and also with the extraglomerular mesangium and the JGA (see Figure 48-3, top). Mesangial cells have the characteristics of smooth muscle cells (pericytes), in that they are rich in microfilaments and respond to and produce a variety of stimuli [e.g., angiotensin II (AII) and antidiuretic hormone (ADH) or arginine vasopressin].³⁹¹ Mesangial matrix is rich in collagens and proteoglycans but is different in composition from the matrix of the GBM. Its composition and volume are tightly regulated in health but can be markedly altered during certain diseases [e.g., diabetic nephropathy, immunoglobulin (Ig)A nephropathy]. Mesangial cells have both structural and housekeeping functions. They have anchoring filaments to GBM opposite the podocytes, and their contractile properties enable them to alter intraglomerular capillary flow and glomerular ultrafiltration surface area, and thereby single nephron glomerular filtration rate (GFR). The cells appear to respond to capillary stretch by generating soluble factors such as vascular endothelial growth factor and transforming growth factor-β (TGF-β), and by activating intracellular signaling pathways. Mesangial cells also have specific and nonspecific mechanisms for removing macromolecules that reach the mesangial and subendothelial space, preventing their accumulation. These mechanisms include phagocytosis and degradation by the cells and trafficking along the mesangial stalk to the juxtaglomerular region, followed by elimination via the renal lymphatics, or by regurgitation into the glomerular capillary.

Proximal Tubule

Bowman’s capsule forms the beginning of the tightly coiled, proximal convoluted tubule (pars convoluta), which on its progress toward the renal medulla becomes straightened and is then called the pars recta. The proximal tubule is about 15 mm long. Epithelial cells lining the convoluted section are cuboidal/columnar cells with a luminal brush border consisting of millions of microvilli, which expand the surface area for absorption of tubular fluid. The proximal tubule is the most metabolically active part of the nephron (Table 48-1).

Loop of Henle

The pars recta drains into the descending thin loop of Henle, which, after passing through a hairpin loop, becomes first the thin ascending limb, and then the thick ascending limb. The cells of the thin ascending limb are very similar to those in the descending (with little brush border, flattened and interdigitated), but important differences are evident in their permeability to water and in their capability for active transport. The thick ascending limb is lined with cuboidal/columnar cells similar in size to those in the proximal tubule, but they do not possess a brush border. At the end of the thick ascending limb, near where it re-enters the cortex and is closely associated with the glomerulus and the efferent arteriole, a cluster of cells known as the macula densa is present (Figure 48-4; see later). The main role of the loop of Henle is to assist in generating concentrated urine, hypertonic with respect to plasma; it also has several other functions (see Table 48-1).

Distal Convoluted Tubule

The distal convoluted tubule begins at a variable distance beyond the macula densa and extends to the first fusion with other tubules to form the collecting ducts. The cells of the distal convoluted are tall and cuboidal and contain numerous mitochondria. Na⁺,K⁺-ATPase activity is higher than in any other segment of the nephron, being located in the basolateral membrane and providing the main driving force for ion transport. Reabsorption of sodium and chloride, with passive reabsorption of water, is the main function of the distal convoluted tubule (see later).

Collecting Duct

The collecting ducts are formed from approximately six distal tubules. These are successively joined by other tubules to form ducts of Bellini, which ultimately drain into a renal calyx. Two main cell types are found in the collecting duct: principal (light) cells and intercalated (dark) cells. Intercalated cells have a dark granular cytoplasm with high carbonic anhydrase activity but no Na⁺,K⁺-ATPase activity.

Definitions

Urine is defined as a fluid excreted by the kidneys, passed through the ureters, stored in the bladder, and discharged through the urethra. In health, it is sterile and clear and has an amber color, a slightly acid pH (≈5.0 to 6.0), and a characteristic odor. In addition to dissolved compounds, it contains a number of cellular fragments and complete cells, derived from normal turnover of tubular cells, casts, and crystals (formed elements). Urinary casts are cylindrical proteinaceous structures formed in the distal convoluted tubule and collecting ducts, which dislodge and pass into the urine, where they can be detected by microscopy.

Urination, also termed micturition, is the discharge of urine. In normal adults, adequate homeostasis is maintained with a urine output of 400 to 2000 mL/d. Alterations in urinary output are described as anuria (<100 mL/d), oliguria (<400 mL/d), and polyuria (>3 L/d or 50 mL/kg body weight/d). The most common disorder of urination is altered frequency, which may be associated with increased urinary volume or with partial urinary tract obstruction (e.g., in prostatic hypertrophy).

Formation of Urine—An Overview

The first step in urine formation is filtration of plasma water at the glomeruli. A net filtration pressure of about 17 mm Hg in the capillary bed of the tuft drives the filtrate through the glomerular membrane. The filtrate is called an ultrafiltrate because its composition is essentially the same as that of plasma, but with a notable reduction in molecules of molecular weight exceeding 15 kDa. Each nephron produces about 100 µL of ultrafiltrate per day. Overall, approximately 170 to 200 L of ultrafiltrate passes through the glomeruli daily. In the passage of ultrafiltrate through the tubules, reabsorption of solutes and water in various regions of the tubules reduces the total urine volume.

Transport of solutes and water occurs both across and between the epithelial cells that line the renal tubules. Transport is both active requiring energy and passive, but many of the so-called passive transport processes are dependent upon or secondary to active transport processes, particularly those involving sodium transport. All known transport processes involve receptor or mediator molecules, the activity of many of which is regulated by phosphorylation facilitated by protein kinase C or A. Their renal distribution has been shown to correlate with known regional functional activities, but the same transporters, or isoforms of them, can be found in other tissues, particularly the digestive tract. For instance, at least five independent proximal tubular transport processes may be noted for amino acids, including those for (1) basic amino acids plus cystine, (2) glutamic and aspartic acid, (3) neutral amino acids, (4) imino amino acids, and (5) glycine.⁴⁸⁹ Inherited disorders of tubular transporters, discussed later in this chapter, may occur, as well as a well-known generalized disorder affecting all of the transport processes, causing Fanconi syndrome and resulting in decreased reabsorption of electrolytes and nutrients (e.g., glucose, amino acids).

Direct coupling of adenosine triphosphate (ATP) hydrolysis is an example of an active transport process. The most important enzymatic transporter in the nephron is Na⁺ K⁺-ATPase, which is located on the basolateral membranes of the tubuloepithelial cells, accounts for much of the renal oxygen consumption, and drives more than 99% of renal sodium reabsorption (Figure 48-5). Other examples of primary active transport mechanisms include a calcium-ATPase, an H⁺-ATPase, and an H⁺,K⁺-ATPase. These enzymes establish ionic gradients, polarizing cell membranes and thus driving secondary transport processes.

Many renal epithelial cell membranes also contain proteins that act as ion channels. For example, there is one for sodium that is closed by amiloride and modulated by hormones such as atrial natriuretic peptide (ANP). Ion channels enable much faster rates of transport than ATPases but are relatively fewer in number (e.g., ≈100 sodium and chloride channels vs. 10⁷ Na⁺,K⁺-ATPase molecules per cell).

Different regions of the tubule have been shown to specialize in certain functions. The proximal tubule facilitates the reabsorption of 60 to 80% of the glomerular filtrate volume, including 70% of the filtered load of sodium and chloride, and most of the potassium, glucose, bicarbonate, calcium, phosphate, sulfate, and other ions—secreting 90% of the hydrogen ion excreted by the kidney (see Table 48-1). Glucose is virtually completely reabsorbed, predominantly in the proximal tubule, by a passive but sodium-dependent process that is saturated at a blood glucose concentration of about 180 mg/dL (10 mmol/L). Uric acid is also reabsorbed in the proximal tubule by a passive sodium-dependent mechanism, but an active secretory mechanism is present. Creatinine is secreted but only to a small extent—approximately 2.5 µmol/min.

Certain nonbiological compounds, such as phenolsulfonphthalein and p-aminohippurate, are secreted by the proximal tubule and have been used for the evaluation of renal tubular secretory capacity. When blood concentrations of creatinine increase to above normal, creatinine is secreted in this region of the nephron. In the loop of Henle, chloride and more sodium without water are reabsorbed, generating dilute urine. Water reabsorption in the more distal tubules and collecting ducts is then regulated by ADH. In the distal tubule, secretion is the prominent activity; organic ions, potassium ions, and hydrogen ions are transported from the blood in the efferent arteriole into the tubular fluid.

Tubular epithelial cells synthesize a vast range of growth factors and cytokines in response to a variety of stimuli that can have both autocrine and paracrine effects. All cells secrete a variety of cell adhesion molecules that are essential for cellular attachment to the tubular basement membrane.

Electrolyte Homeostasis

A complex interplay has been noted between the tubular transport systems regulating individual electrolytes. For simplicity, we have considered each electrolyte individually and have restricted our discussion to the systems of major physiological, pharmacologic, and pathologic significance.

Water Homeostasis

Approximately 180 L glomerular filtrate is formed each day. The unique physiology of the kidney enables approximately 99% of this to be reabsorbed in the production of urine with variable osmolality (between 50 and 1400 mOsmol/kg H₂O at extremes of water intake). Plasma membranes of all mammalian cells are water permeable but to variable degrees. In the kidney, different segments of the nephron show differing permeability to water, enabling the body to both retain water and produce urine of variable concentration. Water reabsorption occurs both isosmotically, in association with electrolyte reabsorption in the proximal tubule, and differentially, in the loop of Henle, distal tubule, and collecting duct in response to the action of ADH. Absorption of water depends on the driving force for water reabsorption (predominantly active sodium transport) and the osmotic equilibration of water across the tubular epithelium. The generation of concentrated urine depends upon medullary hyperosmolality; this in turn requires low water permeability in some kidney segments (ascending limb of the loop of Henle), whereas in other kidney segments (e.g., proximal tubule), there is a requirement for high water permeability. Differing permeability and enaction of hormonal control appear to be largely caused by differential expression along the nephron of a family of proteins known as the aquaporins (AQPs), which act as water channels.

At least 11 different mammalian AQPs have been identified, of which seven (AQP1, -2, -3, -4, -6, -7, -8) are expressed in the kidney.320,321 Many of these have extrarenal expression sites as well (e.g., AQP1 may be important in fluid removal across the peritoneal membrane). Two asparagine-proline-alanine sequences in the molecule are thought to interact in the membrane to form a pathway for water translocation. AQP1, which is found in the proximal tubule and the descending thin limb of the loop of Henle, constitutes almost 3% of total membrane protein in the kidney. It appears to be constitutively expressed and is present in both the apical and basolateral plasma membranes, representing entry and exit ports for water transport across the cell, respectively. Approximately 70% of water reabsorption occurs at this site, predominantly via a transcellular (e.g., AQP1) rather than a paracellular route. Water reabsorption in the proximal tubule passively follows sodium reabsorption, so that fluid entering the loop of Henle is almost isosmotic with plasma.

Urinary concentration is predominantly achieved by countercurrent multiplication in the loop of Henle (Figure 48-8).^364,383 The descending thin limb is very permeable to water, but the ascending limb and the collecting duct are not (the collecting ducts are also poorly permeable to urea). Fluid entering the loop of Henle is isotonic to plasma but is hypotonic on leaving it. The ascending limb has active sodium reabsorption driven by Na⁺,K⁺-ATPase with electroneutralizing transport of chloride—a combined process that can be inhibited by the so-called loop diuretics (e.g., furosemide; see discussion later in this chapter). In this section of the nephron, sodium reabsorption is not accompanied by water, creating a hypertonic medullary interstitium and facilitating water reabsorption from the anatomically adjacent descending limb. The descending limb cells are permeable to sodium chloride, which is cycled from the descending limb back to the ascending limb. Continuous flow along the loop generates an osmotic gradient at the tip of the loop that can reach 1400 mOsmol/kg H₂O. Approximately 5% of water is reabsorbed in the loop of Henle.

A further 10% of water reabsorption occurs in the distal tubule, with the remainder (>20 L/d) reabsorbed in the collecting ducts. Entry of water into the collecting duct cells occurs via apical AQP2 channels, with exit probably occurring via basolateral AQP3 (cortical and outer medullary collecting ducts) and AQP4 (inner medullary collecting ducts). AQP2 appears to be the primary target for ADH regulation of water reabsorption. AQP2 is stored in subapical vesicles in the collecting duct cells. In response to ADH stimulation, these vesicles are cycled through, and inserted into, the plasma membrane by a cytoskeletal, dynein-mediated transport process. Stimulation occurs following binding of ADH to a V₂ receptor in the basolateral plasma membrane of the principal cells of the collecting duct, which promotes a cyclic adenosine monophosphate (cAMP)/protein kinase A cascade, resulting in phosphorylation and activation of AQP2. ADH regulates the acute cellular water-retaining response (AQP2 trafficking) and its longer-term regulation via a conditioning effect on AQP2 gene transcription. The AQP2 gene has a cAMP response element that is involved in the long-term upregulation of AQP2 expression by ADH. It is likely that there are also ADH-independent regulatory pathways of AQP2 expression. Membrane insertion of AQP2 allows water to pass into the collecting duct cells under the influence of medullary hyperosmolality. Maintenance of medullary hyperosmolality depends upon efficient fluid removal, which is the function of the ascending vasa recta, a specialized medullary vasculature, and the close anatomic relations of all medullary constituents (see Figure 48-2). AQP2 expression is decreased in a variety of polyuric conditions (e.g., diabetes insipidus, lithium treatment, hypokalemia, hypercalcemia, urinary obstruction) and is increased in some water-retaining states (e.g., heart failure, cirrhosis, pregnancy).^321,396 A variety of V₂ receptor antagonists have been designed that block the actions of ADH. In contrast to diuretics, these agents promote the excretion of electrolyte-free water and have exciting therapeutic potential in water-retaining states.³⁹⁶

ADH also increases the permeability of collecting duct cells to urea, which is the major osmotically active component of luminal fluid in the distal tubule. Fluid of high urea concentration therefore enters the deepest layers of the medullary interstitium, passing down its concentration gradient, contributing to medullary hyperosmolality. Regulation of ADH excretion is of vital importance to fluid homeostasis. Normal plasma osmolality is maintained very tightly between 280 and 290 mOsmol/kg H₂O and is regulated by means of specific osmoreceptors found in the anterior hypothalamus. These receptors modulate the release of ADH and also affect thirst. ADH release may be stimulated by hypotension, hypovolemia, and vomiting independently of osmoregulation.

Erythropoietin

EPO is a large glycoprotein hormone (MW 34 kDa) containing 165 amino acids and is responsible for stimulating erythroid progenitor cells within the bone marrow to produce red blood cells. It is secreted chiefly by renal peritubular capillary endothelial cells in the adult and by the liver in the fetus. Physiologically, the kidneys sense a reduction in oxygen delivery to tissues by blood and release EPO, thereby stimulating erythropoiesis. Conversely, with a surplus of oxygen in blood traversing the kidneys, as in some forms of polycythemia, the release of EPO into blood is diminished. The use of recombinant human erythropoietin (rhEPO, epoetin) in the management of anemia of kidney disease is discussed later.

Prostaglandins and Thromboxanes

Prostaglandins and thromboxanes are synthesized from arachidonic acid by the cyclooxygenase (COX) enzyme system (see Chapter 27 for further details). The COX system is present in many parts of the kidney and has an important role in regulating the physiologic action of other hormones on renal vascular tone, mesangial contractility, and tubular processing of salt and water. Prostaglandins have a critical role in renal hemodynamics, control of tubular function, and renin release. The major renal vasodilatory prostaglandin is PGE₂, which is synthesized predominantly in the medulla. The major vasoconstrictor prostaglandin is thromboxane A₂, which is produced primarily within the renal cortex.¹⁰¹ PGE₂ increases renal blood flow rate, inhibits sodium reabsorption in the distal nephron and collecting duct, and stimulates renin release.¹⁰¹ These actions promote natriuresis and diuresis. In patients with chronic kidney disease (CKD), renal PGE₂ production is increased, representing a compensatory response to loss of nephron mass.⁴³⁸ Vasodilatory prostaglandins are synthesized following stimulation with renal sympathetic adrenergic and AII-dependent mechanisms to offset or modulate vasoconstriction.²⁹⁵ In the tubule, prostaglandins act as autocoids, exerting their effects locally, near the site of synthesis.

In pathophysiologic circumstances, including various forms of acute kidney injury, thromboxane A₂ and various prostaglandins may have a significant role in inflammation and alteration of vascular tone. The effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on renal prostaglandin metabolism are considered later in this chapter. The lipoxygenase pathway, which leads to formation of leukotrienes, is also present within the kidneys, although the major source of leukotrienes in inflammatory disease of the kidneys is infiltrating white cells and macrophages.

1,25(OH₂)D₃

The kidneys are primarily responsible for producing 1,25(OH₂)D₃ from 25-hydroxycholecalciferol as a result of the action of the enzyme 25-hydroxycholecalciferol 1α-hydroxylase found in proximal tubular epithelial cells. Regulation of this system is discussed in Chapter 52. The management of renal osteodystrophy is considered later in this chapter.

K_f = (hydraulic permeability × surface area)

P_GCap = glomerular-capillary hydrostatic pressure

Π_BC = oncotic pressure in Bowman’s capsule

P_BC = hydrostatic pressure in Bowman’s capsule

Π_GCap = oncotic pressure in the glomerular capillary

Regulation of GFR

The factors involved in regulation of GFR are listed in Table 48-3. Autoregulation of renal blood flow and GFR is widely thought to be explained by the myogenic theory. This theory is based on the principle that an increase in wall tension of the afferent arterioles, brought about by an increase in perfusion pressure, causes automatic contraction of arteriolar smooth muscle, thus increasing resistance and keeping the flow constant despite the increase in perfusion pressure.

The tubuloglomerular feedback mechanism, involving the macula densa and release of the vasodilator adenosine, must also be considered. Although not fully understood, this mechanism appears to regulate GFR, with changes in renal blood flow as a secondary consequence. For individual nephrons, evidence indicates that each single nephron GFR (SNGFR) is influenced by the composition of the tubular fluid in the distal tubule, which in turn is influenced by the filtration rate. The macula densa is thought to sense the distal tubular sodium chloride content, its osmolality, or the rate at which sodium chloride is transported. The macula densa then signals the JGA via an uncertain mechanism to cause the release of adenosine and possibly AII and prostaglandins, which in turn affects vascular resistance.*

The result of the combination of myogenic mechanisms and tubuloglomerular feedback is that the net filtration pressure or P_GCap is kept reasonably constant over a wide variety of systemic arterial pressures. It should be noted that renal blood flow and GFR change across this variety of systemic pressures but to a significantly smaller extent than would be predicted if these autoregulatory mechanisms were not in place.

Other factors influencing renal blood flow are indicated in Table 48-4. The afferent and efferent arterioles are richly supplied with renal sympathetic nerves. Epinephrine acts via α-adrenergic receptors, leading to constriction of both arterioles and causing a decrease in renal blood flow.

Nitric oxide (NO) has been identified as an important vasodilator produced by vascular endothelial cells. NO is synthesized from L-arginine and oxygen by nitric oxide synthetase (NOS), of which three isoenzymes are differentially located and regulated. Within the kidney are eNOS (endothelial) and iNOS (inducible) isoenzymes. Activation of NOS has been shown to occur as a result of shear stress (e.g., increased arteriolar tone). A variety of physiologic vasoconstrictors are present, including acetylcholine, bradykinin, endothelin, and serotonin; a rise in intracellular ionized calcium is required for the vasoconstrictors. NO synthesis is now known to play an important role in the regulation of human vascular tone and has a crucial role in control of blood pressure and kidney function.102,160,359 It has also been found in the macula densa and has been implicated in the regulation of renin release.

Age and the Kidney

Kidney function is not constant throughout life. In utero, urine is produced by the developing fetus from about the ninth week of gestation. Nephrogenesis is complete by approximately 35 weeks’ gestation, although kidney function remains immature during the first 2 years of life. The kidney of the term infant receives approximately 6% of the cardiac output, compared with 25% in adults. Renal vascular resistance is relatively high, and the low renal blood flow is particularly directed to the medulla and inner cortex. The gradual increase in renal blood flow that occurs with increasing age is directed mainly to the outer cortex and is mediated by local neurohormonal mechanisms.²¹⁶ The GFR at birth is approximately 30 mL/min/1.73 m².⁷² It increases rapidly during the first weeks of life to reach approximately 70 mL/min/1.73 m² by age 16 days.⁷² Normal adult values are achieved by age 14 years. Tubular functions, including salt and water conservation, are also immature at birth. Birth is associated with rapid changes in kidney function, with a switch to salt and water conservation mediated by catecholamines, the renin-angiotensin system, ADH, glucocorticoids, and thyroid hormone.⁷² The immaturity of the neonatal kidney contributes to the relatively common problems of water and electrolyte disturbances in infants. These disturbances are more likely to occur in premature infants, particularly those born before 35 weeks’ gestation.

Aging is associated with a variety of structural changes in the kidney, which begin in early middle age, including decreasing kidney weight and number of glomeruli, with the cortical glomeruli being particularly affected.110,222 Changes in the afferent and efferent arteriolar systems are evident, with formation of direct channels (shunts) between afferent and efferent arterioles in the medulla. Aging is also associated with the development of tubulointerstitial fibrosis, loss of tubular mass, and decreasing length of the proximal tubule.

Structural change is accompanied by functional changes, which in many respects are the reverse of those seen in early life. On average, GFR declines with age by approximately 1 mL/min/1.73 m²/y over the age of 40 years.11,92,375,459 Renal blood flow, particularly to the cortical area, also decreases with age, while the filtration fraction (i.e., GFR/renal plasma flow)¹¹ and renal vascular resistance^123,124 increase. Tubular function, such as the ability to concentrate urine and excrete a water and salt load, is decreased, and nocturnal polyuria is common. Renal salt conservation is impaired,¹¹⁰ and the prevalence of albuminuria rises over the age of approximately 40 years.^95,213

It is not known whether these changes are the result of a normal aging process (i.e., involutional) or whether they are caused by the interplay of pathology and age. Cumulative exposure to common causes of CKD such as (1) atherosclerosis, (2) hypertension, (3) heart failure, (4) diabetes,⁶⁵ (5) obstructive nephropathy, (6) infection, (7) immune insult, (8) nephrotoxins such as lead,²⁶² and (9) dietary protein^5,47 increases with age, and it is difficult to separate these effects from those of “healthy” aging. The decline in GFR with increasing age may be largely attributable to hypertension,^265,266 atherosclerosis,²²² or heart failure.¹²² In the absence of these and other identifiable causes of kidney disease, many individuals have stable GFR as they age.

Loss of kidney function with aging appears to be heterogeneous and is not inevitable.123,264 Kidney function may be well preserved in healthy older people, and assumptions with respect to GFR based solely on age could be erroneous. Conversely, attention to the common causes of CKD could preserve function in older people.³⁹⁵ Kidney disease is more common among older people. Studies from England,¹¹⁶ France,²¹⁷ and Iceland²⁸¹ have demonstrated a near exponential rise in CKD with age. Data from the United States on CKD prevalence show the prevalence of a GFR between 30 and 60 mL/min/1.73 m² to be 4.3% of the total noninstitutionalized population overall, but this rises to 25% among those over 70 years.⁸³ The prevalence may be even higher among institutionalized older people (e.g., 82% of a residential home population were identified as having a GFR <60 mL/min/1.73 m²).⁶¹ The incidence of acute kidney injury also increases with age.²³⁰

Glomerular Sieving

The glomerular permeability of a molecule is expressed in terms of its glomerular sieving coefficient (GSC). Molecules smaller than approximately the molecular weight of inulin (5 kDa) are freely filtered. Therefore, inulin, urea, creatinine, glucose, and electrolytes all have a GSC = 1.0. Classic experiments in the 1970s used linear dextran chains of varying molecular weight and charge to study glomerular filtration characteristics. However, linear carbohydrate chains do not necessarily behave in the same manner as a globular protein of equal molecular weight or charge. For example, neutral dextran chains of 15 kDa (diameter 2.4 nm) have GSC = 1.0, whereas the smaller β₂-microglobulin (11.8 kDa, diameter 1.6 nm) has GSC = 0.7.²⁴ Linear molecules have higher GSC than globular proteins, hence theoretical glomerular pore dimensions based on dextran studies were overestimated. More recently, Ficoll polymers have been used. These are neutral, heavily cross-linked, sucrose-epichlorohydrin copolymers that behave as rigid hydrated spheres and are thought to behave more like globular proteins in their sieving behavior.¹⁷⁰

As a result of such studies, some general conclusions can be drawn with respect to glomerular protein handling. The glomerulus acts as a selective filter of the blood passing through its capillaries, restricting the passage of macromolecules in a size-, charge-, and shape/configuration-dependent manner. Sieving coefficients (1) decrease as molecular size increases, (2) are lower for anionic proteins than for neutral proteins of equivalent size, and (3) are lower for globular rather than elongated proteins. Examples of the GSC for major urinary proteins are listed in Table 25-2.

The protein concentration in the glomerular filtrate has been measured in several animal models by direct glomerular puncture. The concentration of total protein found is in the range of several hundred mg/L (≈1% of plasma), with albumin concentrations varying from less than 40 to a few hundred mg/L. The filtered load of protein depends on the product of the GSC and the free plasma concentration; therefore the albumin load per nephron is much greater than that of the other filtered proteins.24,275 In general, proteins larger than albumin (66 kDa, diameter 3.5 nm, charge −23) are retained by the healthy glomerulus and are termed high molecular weight proteins. However, lower molecular weight proteins are also retained to a significant extent.

Tubular Reabsorption

The final urinary concentration of proteins depends on the filtered load, but also on the efficiency of the proximal tubular reabsorptive process, in addition to any contribution of tubular secretion. Proteins are reabsorbed by receptor-mediated, low-affinity, high-capacity processes. Megalin (MW 600 kDa) and cubulin (MW 460 kDa) are endocytic, multiligand receptors that are important in protein reabsorption.⁴⁵⁵ Megalin belongs to the low-density lipoprotein (LDL) receptor family, whereas cubulin is identical to the intestinal intrinsic factor–vitamin B₁₂ receptor. In the kidney, both are localized in clathrin-coated pits in the apical brush border of renal proximal tubular cells and bind filtered proteins in a calcium-dependent process. Megalin appears capable of both binding and internalizing its ligands, whereas the cubulin-ligand complex requires megalin to be internalized. Some proteins such as albumin will bind to either receptor, whereas others are specific [e.g., transferrin binds to cubulin only, retinol-binding protein (RBP) and α₁-microglobulin to megalin only]. Once proteins have been internalized, they are transported by the endocytic vesicle and fuse with lysosomes. Proteolysis occurs, and the resultant amino acids are released into the tubulointerstitial space across the basolateral surface of the tubular epithelial cell. The membrane vesicles are then recycled to the brush border to complete the reabsorption cycle. In health, the reabsorptive mechanism removes 99% of the filtered protein, thus retaining most of the essential amino acid constituents for reuse.^24,162,275 Capture of filtered transport proteins is also important in conserving vitamin status (e.g., vitamin A associated with RBP).

The tubular reabsorptive process is saturable. Any increase in the filtered load [caused by glomerular damage, increased glomerular vascular permeability (e.g., inflammatory response), or increased circulating concentration of low molecular weight proteins] or decrease in reabsorptive capacity (caused by tubular damage) can result in increased urinary protein loss (proteinuria).

Tubular secretion of proteins also contributes to urinary total protein concentration; in particular, in health, Tamm Horsfall glycoprotein (THG) accounts for ≈50% of urinary total protein. THG (MW 200 kDa), a highly glycosylated acidic protein, is secreted into the tubular fluid only by the thick ascending limb and the early distal convoluted tubule and is thought to play a role in inhibiting kidney stone formation.190,373 It is a major constituent of renal tubular casts, along with albumin and traces of other proteins. Investigation for increased urinary protein loss is mandatory in any patient with suspected kidney disease and was considered in Chapter 25.

Consequences of Proteinuria

It is increasingly accepted that proteinuria is not just a marker of, but contributes directly to, progression of kidney disease.52,54 The accumulation of proteins in abnormal amounts in the tubular lumen may trigger an inflammatory reaction, which in turn may contribute to interstitial structural damage and expansion, and progression of kidney disease.⁵¹ Increasing evidence suggests that megalin may not just be a scavenger receptor for albumin, but that it may have signaling functions that regulate cell survival. Excessive quantities of albumin in the tubular lumen may downregulate proximal tubular megalin expression, increasing cell sensitivity to apoptosis.¹³ Evidence gathered from in vitro studies suggests that glomerular filtration of abnormal amounts or types of protein induces mesangial cell injury, leading to glomerulosclerosis, and that these same proteins can have adverse effects on proximal tubular cell function.¹⁰⁵ Numerous studies have demonstrated that proteinuria is a potent risk marker for progression of renal disease in both nondiabetic^202,346,377 and diabetic^48,189 kidney disease. Furthermore, reducing proteinuria slows the rate of progression of proteinuric kidney disease. This effect has been observed in clinical trials in patients treated with ACE inhibitors and angiotensin II receptor blockers (ARBs), given alone or in combination.^337,376 These drugs reduce protein excretion by reducing intraglomerular filtration pressure and possibly by stabilizing the glomerular epithelial cell slit diaphragm proteins.^6,276 Consequently, reduction of proteinuria is an important therapeutic target.^185,203,225