Kidney physiology 1

Kidney anatomy

• The human kidneys are paired, bean-shaped structures that lie behind the peritoneum on each side of the vertebral column.

• They extend from the 12th thoracic vertebra to the 3rd lumbar vertebra.

• The two kidneys together comprise less than 0.5% of the total body weight; each kidney weighs 125-170 g in men, 115-155 g in women.

• A fibrous, almost nondistensible capsule covers each kidney.• In the middle of the concave surface, a slit in the capsule—the hilus—

serves as the port of entry for the renal artery and nerves and as the site of exit for the renal vein, the lymphatics, and the ureter.

• The hilus opens into a shallow space = renal sinus, which is completely surrounded by renal parenchyma, except where it connects with the upper end of the ureter.

• The renal sinus includes the urine-filled spaces: the renal pelvis proper and its extensions, the major and the minor calyces. Blood vessels and nerves also pass through the sinus.

• The renal capsule reflects into the sinus at the hilus so that its inner layers line the sinus, and the outer layers anchor to the blood vessels and renal pelvis.

Kidney anatomy

Kidneys functions

The kidneys serve three essential functions:

• First, they act as filters, removing metabolic products and toxins from the blood and excreting them through the urine.

• Second, they regulate the body's fluid status, electrolyte balance, and acid-base balance.

• Third, the kidneys produce or activate hormones that are involved in erythrogenesis, Ca2+ metabolism, and the regulation of blood pressure and blood flow.

Renal circulation

• Although the kidneys comprise <0.5% of total body weight, they receive ~20% of the cardiac output. This high blood flow provides the blood plasma necessary for forming an ultrafiltrate in the glomeruli.

• The renal circulation has a unique sequence of vascular elements: a high-resistance arteriole (the efferent arteriole), followed by a high-pressure glomerular capillary network for filtration, followed by a second high-resistance arteriole (the afferent arteriole), followed by a low-pressure capillary network that surrounds the renal tubules (peritubular capillaries) and takes up the fluid absorbed by these tubules.

• A single renal artery enters the hilus and divides into anterior and posterior branches, which give rise to interlobar and then arcuate arteries. The latter arteries skirt the corticomedullaryjunction, where they branch into ascending interlobular arteries that enter the cortex and give rise to numerous afferent arterioles. These give rise to glomerular capillaries that rejointo form efferent arterioles. For nephrons in the superficial portion of the cortex, the efferent arterioles are the origin of a dense peritubular capillary network that supplies oxygen and nutrients to the tubules in the cortex.

• The afferent and efferent arterioles determine the hydrostatic pressure in the interposed glomerular capillaries. The tone of both arterioles is under the control of a rich sympathetic innervation, as well as a wide variety of chemical mediators. Very small branches of the arcuate artery, or the proximal portion of the interlobular artery, supply a subpopulation of “juxtamedullary” glomeruli that are located at or near the junction of cortex and medulla. The efferent arterioles of these nephrons descend into the renal papillae to form hairpin-shaped vessels called the vasa recta, which provide capillary networks for tubules in the medulla. Some 90% of the blood entering the kidney perfuses superficial glomeruli and cortex; only ~10% perfuses juxtamedullary glomeruli and medulla.

Renal circulation

Kidneys lymph vessels

• Lymph vessels, which drain the interstitial fluid of the cortex and may contain high concentrations of renal hormones such as erythropoietin, leave the kidney by following arteries toward the hilus.

• Lymphatics are absent from the renal medulla, where they would otherwise tend to drain the high-osmolality interstitial fluid, which is necessary for producing a concentrated urine.

• Each kidney consists of 800,000 to 1,200,000 nephrons. Each nephron is an independent entity until the point at which its initial collecting tubule merges with another tubule.

• Superficial nephronshave short loops extending to the boundary between outer and inner medulla.

• Juxtamedullarynephrons, which play a special role in the production of a concentrated urine, have long loops that extend as far as the tip of the medulla.

The renal corpuscle has three components: vascular elements, the mesangium, and Bowman's capsule and space

• The renal corpuscle, the site of formation of the glomerular filtrate, comprises a glomerulus, Bowman's space, and Bowman's capsule. During the development of the kidney, the interaction between the ureteric bud—which gives rise to the urinary system from the collecting duct to the ureters—and the surrounding loose mesenchyme leads to the branching of the ureteric bud and condensation of the mesenchyme .

• These condensed cells differentiate into an epithelium that forms a hollow, S-shaped tubular structure that gives rise to the nephron's tubular elements between Bowman's capsule and the connecting segment.

• The distal portion of the S-shaped tubular structure elongates and connects with branches of the developing ureteric bud .At the same time, the blind proximal end of this S-shaped tubule closely attaches to the arterial vascular bundle that develops into the glomerular capillary tuft.

• Thinning of the epithelium on one circumference of the blind end of the S-shaped tubule leads to the emergence of the future parietal layer of Bowman's capsule. In contrast, the opposite visceral layer thickens and attaches to the glomerular capillaries

• In the mature kidney, foot processes of the podocytes cover the glomerular capillaries. These podocytes, modified epithelial cells, thus represent the visceral layer of Bowman's capsule. Beginning at the vascular pole, the podocytes are continuous with the parietal layer of Bowman's capsule. Glomerular filtrate drains into the space between these two layers (Bowman's space) and flows into the proximal tubule at the urinary pole of the renal corpuscle.

The glomerular filtration barrier• between the glomerular capillary lumen and Bowman's space

• comprises four elements with different functional properties : (1) a glycocalyx covering the luminal surface of endothelial cells, (2) the endothelial cells, (3) the glomerular basement membrane, and (4) epithelial podocytes.• The glycocalyx consists of negatively charged glycosaminoglycans (GAGs) that may play a

role in preventing leakage of large negatively charged macromolecules.• Endothelial cells of the glomerular capillaries are almost completely surrounded by the

glomerular basement membrane and a layer of podocyte foot processes . The exception is a small region toward the center of the glomerulus, where the endothelial cells have neither basement membrane nor podocytes and come into direct contact with mesangial cells, which resemble smooth muscle.

• Filtration occurs away from the mesangial cells, at the peripheral portion of the capillary wall, which is covered with basement membrane and podocytes. The endothelial cells contain large fenestrations, 70-nm holes that provide no restriction to the movement of water and small solutes—including proteins or other large molecules—out of the lumen of the capillary . Thus, the endothelial cells probably serve only to limit the filtration of cellular elements (e.g., erythrocytes).

• Glomerular capillary covered by the foot processes of podocytes. This scanning electron micrograph shows a view of glomerular capillary from the vantage point of Bowman's space. The outer surfaces of the capillary endothelial cells are covered by a layer of interdigitating foot processes of the podocytes. The podocyte cell body links to the foot processes by leg-like primary and secondary processes.

Inner aspect of glomerular capillary, showing fenestrations of endothelial cells. This scanning electron micrograph shows a view of the glomerular capillary wall from the vantage point of the capillary lumen. Multiple fenestrations, each ~70 nm in diameter, perforate the endothelial cells

• The basement membrane, between endothelial cells and podocyte foot processes), separates the endothelial layer from the epithelial layer in all parts of the glomerular tuft. The basement membrane itself has three layers : (1) an inner thin layer (lamina rara interna); (2) a thick layer (lamina densa), and (3) an outer thin layer (lamina raraexterna). The basement membrane makes an important contribution to the permeability characteristics of the filtration barrier by restricting intermediate-sized to large solutes (molecular weight > 1 kDa). Because the basement membrane contains heparan sulfate proteoglycans (HSPGs), it especially restricts large, negatively charged solutes .

• Podocytes have foot interdigitating processes that cover the basement membrane . Between these interdigitations are filtration slits ; the interdigitations are connected by thin diaphragmatic structures—the slit diaphragms—with pores of 4 - 14 nm. Glycoproteins with negative charges cover the podocyte bodies, the interdigitations, and the slit diaphragms. These negative charges contribute to the restriction of filtration of large anions. The extracellular domains of the integral membrane proteins nephrin and NEPH1 from adjacent podocytes appear to zip together to help form the slit diaphragm.

• Podocin and other proteins also contribute to the slit diaphragm. Phosphotyrosinemotifs on the intracellular domains of some of these proteins may recruit other molecules involved in signaling events that control slit permeability.

The glomerular filtration barrier

A. Proximal convoluted tubule (PCT). The first portion of the PCT consists of S1 cells, and thelatter portion of S2 cells. Both cells have abundant apical microvilli and a deeply infoldedbasolateral membrane. They also have a rich supply of mitochondria, which lie between theinfoldings. These complexities diminish from the S1 to the S2 segments.

B. Proximal straight tubule (PST). The first portion of the PST consists of S2 cells, and thelatter portion of S3 cells. The ultrastructural complexity diminishes from the S2 to the S3segments.

C. Thin descending limb (tDLH). The cells are less complex and flatter than those of the S3segment of the proximal tubule.

D. Thin ascending limb (tALH). Away from the nucleus, the cells are even thinner than thoseof the descending limb.

E. Thick ascending limb (TAL). The cells, which lack microvilli, are substantially taller andmore complex than those of the thin limbs.

F. Distal convoluted tubule (DCT). The cells are very similar to those of the TAL.

G. Connecting tubule (CNT). This segment consists of both connecting-tubule cells, whichsecrete kallikrein, and intercalated cells, which are rich in mitochondria.

H. Initial collecting tubule (ICT). The ICT is defined as the segment just before the firstconfluence of tubules. About one third of the cells in this segment are intercalated cells, andthe rest are principal cells.

I. Cortical collecting tubule (CCT). The CCT is defined as the segment after the firstconfluence of tubules. The cells in this segment are very similar to those in the ICT.

J. Outer medullary collecting duct (OMCD). The principal cells in this nephron segment havea modest cell height. The number of intercalated cells progressively decreases along thelength of this segment.

K. Inner medullary collecting duct (IMCD). This segment consists only of principal cells. Evenat the beginning of the IMCD, the principal cells are taller than in the OMCD. At the end ofthe IMCD, the “papillary” collecting-duct cells are extremely tall.

The tightness of tubule epithelia increases from the proximal to the medullary collecting tubule

• Epithelia may be either “tight” or “leaky,” depending on the permeability of their tight junctions .

• In general, the tightness of the tubule epithelium increases from the proximal tubule to the collecting duct.

• In the leaky proximal tubule, junctional complexes are shallow and, in freeze-fracture studies, show only a few strands of membrane proteins (see pp. 43–44). In contrast, in the relatively tight collecting tubule, tight junctions extend deep into the intercellular space and consist of multiple strands of membrane proteins. Tubule segments with tight junctions consisting of only one strand have low electrical resistance and high solute permeability, whereas tubules with several strands tend to have high electrical resistance and low permeability.

• Gap junctions provide low-resistance pathways between some, but not all, adjacent tubule cells. These gap junctions are located at various sites along the lateral cell membranes. Electrical coupling exists among proximal-tubule cells, but not among heterogenous cell types, such as those found in the connecting and collecting tubules.

The nephron forms an ultrafiltrate of the blood plasma and then selectively reabsorbs the tubule fluid or secretes solutes into it

• Starling forces govern the flow of fluid across the capillary walls in the glomerulus and result in net filtration. In the glomerular capillaries, the filtrate flows not into the interstitium, but into Bowman's space, which is contiguous with the lumen of the proximal tubule.

• The main function of renal tubules is to recover most of the fluid and solutes filtered at the glomerulus. If the fluid were not recovered, the kidney would excrete the volume of the entire blood plasma in less than half an hour.

• The retrieval of the largest fraction of glomerular filtrate occurs in the proximal tubule, which reabsorbs NaCl, NaHCO3, filtered nutrients (e.g., glucose and amino acids), divalent ions (e.g., Ca2+) and water. Finally, the proximal tubule secretes a variety of endogenous and exogenous solutes into the lumen.

• The main function of the loop of Henle—tDLH, tALH, and TAL—is to participate in forming concentrated or dilute urine. The loop does this by pumping NaCl into the interstitium of the medulla without appreciable water flow, thus making the interstitium hypertonic.

• The medullary collecting duct exploits this hypertonicity by either permitting or not permitting water to flow by osmosis into the hypertonic interstitium.

• In humans, only ~15% of the nephrons, the juxtamedullary nephrons, have long loops that descend to the tip of the papilla. Nevertheless, this subpopulation of nephrons is extremely important for creating the osmotic gradients within the papilla that allow water movement out of the lumen of the entire population of medullary collecting ducts. As a result of this water movement, urine osmolality in the collecting ducts can far exceed that in the plasma.

• TAL cells secrete the Tamm-Horsfall glycoprotein (THP), also known as uromodulin. Normal subjects excrete 30 to 50 mg/day into the urine, which—along with albumin (<20 mg/day)—accounts for most of the protein normally present in urine. THP adheres to certain strains of Escherichia coli and may be part of the innate defense against urinary tract infections. This protein may also have a role in reducing aggregation of calcium crystals and thereby preventing formation of kidney stones. THP also constitutes the matrix of urinary casts. A cast is cylindrical debris in the urine that has taken the shape of the tubule lumen in which it was formed.

The kidneys, as endocrine organs, produce renin, 1,25-dihydroxy-vitamin D, erythropoietin, prostaglandins, and bradykinin

• Besides renin production by the JGA granular cells, the kidneys play several other endocrine roles.

• Proximal-tubule cells convert circulating 25-hydroxyvitamin D to the active metabolite, 1,25-dihydroxyvitamin D. This hormone controls Ca2+ and phosphorus metabolism by acting on the intestines, kidneys, and bone, and is important for developing and maintaining bone structure.

• Fibroblast-like cells in the interstitium of the cortex and outer medulla secrete erythropoietin (EPO) in response to a fall in the local tissue . EPO stimulates the development of red blood cells by action on hematopoietic stem cells in bone marrow. In chronic renal failure, the deficiency of EPO leads to severe anemiathat can be treated with recombinant EPO.

• The kidney releases prostaglandins and several kinins, paracrine agents that control circulation within the kidney. These substances are generally vasodilators and may play a protective role when renal blood flow is compromised. Tubule cells also secrete angiotensin, bradykinin, cAMP, and ATP into the lumen, which can modulate downstream nephron function.

• All solutes excreted into the urine ultimately come from the blood plasma perfusing the kidneys. Thus, the rate at which the kidney excretes a solute into the urine equals the rate at which the solute disappears from the plasma, provided the kidney does not produce, consume, or store the solute.

• In 1 minute, 700 mL of plasma flow through the kidneys, containing 0.7 L × 142 mM or ~100 mmol of Na+. Of this Na+, the kidneys remove and excrete into the urine only a tiny amount, ~0.14 mmol. In principle, these 0.14 mmol of Na+ could have come from only 1 mL of plasma, had all Na+ ions been removed (i.e., cleared) from this volume.

• The clearance of a solute is defined as the virtual volume of blood plasma (per unit time) needed to supply the amount of solute that appears in the urine.

• In our example, Na+ clearance was 1 mL/min, even though 700 mL of plasma flowed through the kidneys. Renal clearance methods are based on the principle of mass balance and the special anatomy of the kidney . For any solute (X) that the kidney does not synthesize, degrade, or accumulate, the only route of entry to the kidney is the renal artery, and the only two routes of exit are the renal vein and the ureter. Because the input of X equals the output of X, PX,a and PX,v are plasma concentrations of X in the renal artery and renal vein, respectively.

• RPFa and RPFv are rates of renal plasma flow (RPF) in the renal artery and vein, respectively. UX is the concentration of X in urine. is urine flow (the overdot represents the time derivative of volume). The product UX ⋅is the urinary excretion rate, the amount of X excreted in urine per unit time.

• In developing the concept of renal clearance, we transform Equation 33-1 in two ways, both based on the assumption that the kidneys clear all X from an incoming volume of arterial plasma. First, we replace RPFa with the inflow of the virtual volume—the clearance of X (CX)—that provides just that amount of X that appears in the urine. Second, we assign the virtual venous output a value of zero. Thus, the clearance equation becomes

• Solving for clearance yields

• This is the classic clearance equation that describes the virtual volume of plasma that would be totally cleared of a solute in a given time. We need to know only three parameters to compute the clearance of a solute X:

1. The concentration of X in the urine (UX)

2. The volume of urine formed in a given time

3. The concentration of X in systemic blood plasma (PX)

• If a solute is only reabsorbed, but not secreted,we can rearrange the equation to obtain the rate of reabsorption:

• Conversely, if a solute is only secreted, but not reabsorbed, the rate of secretion is

When the kidney both reabsorbs and secretes a substance, clearance data are inadequate to describe renal handling.

Other ways of investigating renal function

The Ureters and Bladder

• The ureters serve as conduits for the passage of urine from the renal pelvis into the urinary bladder. Located in the retroperitoneum, each ureter loops over the top of the common iliac artery and vein on the same side of the body and courses through the pelvis.

• The ureters enter the lower posterior portion of the bladder (ureterovesicaljunction), pass obliquely through its muscular wall, and open into the bladder lumen 1 to 2 cm above, and lateral to, the orifice of the urethra

• The two ureteral orifices, connected by a ridge of tissue, and the urethral orifice form the corners of a triangle (bladder trigone). A flap-like valve of mucous membrane covers each ureteral orifice. This anatomical valve, in conjunction with the physiological valve-like effect created by the ureter's oblique pathway through the bladder wall, prevents reflux of urine back into the ureters during contraction of the bladder.

A, Anatomy of the ureters and bladder. B, Ureteral smooth-muscle cells generally have a resting membrane potential of about −60 mV, mainly determined by a high K+ membrane permeability. Na+ channels speed the upstroke of the action potential, although Ca2+ channels are mainly responsible for the action potential.

• Ureteral peristaltic waves originate from electrical pacemakers in the proximal portion of the renal pelvis. These waves propel urine along the ureters and into the bladder in a series of spurts at frequencies of 2 to 6 per minute. The intraureteral hydrostatic pressure is 0 to 5 cm H2O at baseline but increases to 20 to 80 cm H2O during peristaltic waves. Blockade of ureteral outflow to the bladder, as by a kidney stone, causes the ureter to dilate and increases the baseline hydrostatic pressure to 70 to 80 cm H2O over a period of 1 to 3 hours. This pressure is transmitted in retrograde fashion to the nephrons, creating a stopped-flow condition in which glomerular filtration nearly comes to a halt.

• Although ureteral peristalsis can occur without innervation, the autonomic nervous system can modulate peristalsis. As in other syncytial smooth muscle, autonomic control of the ureters occurs by diffuse transmitter release from multiple varicosities formed as the postganglionic axon courses over the smooth-muscle cell. Sympathetic input (via aortic, hypogastric, and ovarian or spermatic plexuses) modulates ureteral contractility as norepinephrine acts by excitatory α-adrenergic receptors and inhibitory β-adrenergic receptors. Parasympathetic input enhances ureteral contractility via acetylcholine, either by directly stimulating muscarinic cholinergic receptors or by causing postganglionic sympathetic fibers to release norepinephrine, which then can stimulate α adrenoceptors. Some autonomic fibers innervating the ureters are afferent pain fibers. In fact, the pain of renal colic associated with violent peristaltic contractions proximal to an obstruction is one of the most severe encountered in clinical practice.

• The bladder and sphincters receive sympathetic and parasympathetic (autonomic) as well as somatic (voluntary) innervation .

• The sympathetic innervation to the bladder and internal sphincter arises from neurons in the intermediolateral cell column of the tenth thoracic to the second lumbar spinal cord segment .The preganglionic fibers then pass via lumbar splanchnic nerves to the superior hypogastric plexus, where they give rise to the left and right hypogastric nerves. These nerves lead to the inferior hypogastric/pelvic plexus, where preganglionic sympathetic fiberssynapse with postganglionic fibers. The postganglionic fibers continue to the bladder wall via the distal portion of the hypogastric nerve. This distal portion also contains the preganglionic parasympathetic axons.

• The parasympathetic innervation of the bladder originates from the intermediolateral cell column in segments S2 through S4 of the sacral spinal cord. The parasympathetic fibers approaching the bladder via the pelvic splanchnic nerve are still preganglionic. They synapse with postganglionic neurons in the body and neck of the urinary bladder.The somatic innervation originates from motor neurons arising from segments S2 to S4. Via the pudendal nerve, these motor neurons innervate and control the voluntary skeletal muscle of the external sphincter.

Bladder tone is defined by the relationship between bladder volume and internal (intravesical) pressure. One can measure the volume-pressure relationship by first inserting a catheter through the urethra and emptying the bladder, and then recording the pressure while filling the bladder with 50-mL increments of water. The record of the relationship between volume and pressure is a cystometrogram. Increasing bladder volume from 0 to ~50 mL produces a moderately steep increase in pressure. Additional volume increases up to ~300 mL produce almost no pressure increase; this high compliance reflects relaxation of bladder smooth muscle. At volumes >400 mL, additional increases in volume produce steep increments in passive pressure. Bladder tone, up to the point of triggering the micturition reflex, is independent of extrinsic bladder innervation.

• Cortical and suprapontine centers in the brain normally inhibit the micturition reflex, which thepontine micturition center (PMC) coordinates. The PMC controls both the bladder detrusor muscle andthe urinary sphincters. During the storage phase, stretch receptors in the bladder send afferent signalsto the brain via the pelvic splanchnic nerves. One first senses the urge for voluntary bladder emptying ata volume of ~150 mL and senses fullness at 400 to 500 mL. Nevertheless, until a socially acceptableopportunity to void presents itself, efferent impulses from the brain, in a learned reflex, inhibitpresynaptic parasympathetic neurons in the sacral spinal cord that would otherwise stimulate thedetrusor muscle.

• Voluntary contraction of the external urinary sphincter probably also contributes to storage.

• The voiding phase begins with a voluntary relaxation of the external urinary sphincter, followed byrelaxation of the internal sphincter. When a small amount of urine reaches the proximal (posterior)urethra, afferents signal the cortex that voiding is imminent. The micturition reflex now continues aspontine centers no longer inhibit the parasympathetic preganglionic neurons that innervate thedetrusor muscle. As a result, the bladder contracts, expelling urine. Once this micturition reflex hasstarted, the initial bladder contractions lead to further trains of sensory impulses from stretchreceptors, thus establishing a self-regenerating process

• At the same time, the cortical centers inhibit the external sphincter muscles. Voluntary urination alsoinvolves the voluntary contraction of abdominal muscles, which further raises bladder pressure andthus contributes to voiding and complete bladder emptying.The basic bladder reflex that we have justdiscussed, although inherently an autonomic spinal cord reflex, may be either facilitated or inhibited byhigher centers in the central nervous system that set the level at which the threshold for voiding occurs.

• Because of the continuous flow of urine from the kidneys to the bladder, the function of the varioussphincters, and the nearly complete emptying of the bladder during micturition, the entire urinarysystem is normally sterile

Glomerular filtration rate determination

• Qualitatively, the filtration of blood plasma by the renal glomeruli is the same as the filtration of blood plasma across capillaries in other vascular beds

• Glomerular ultrafiltration results in the formation of a fluid, the glomerular filtrate, with solute concentrations that are similar to those in plasma water. However, proteins, other high-molecular-weight compounds, and protein-bound solutes are present at reduced concentration. The glomerular filtrate, like filtrates formed across other body capillaries, is free of formed blood elements, such as red and white blood cells.

• Quantitatively, the rate of filtration that occurs in the glomeruli greatly exceeds that in all the other capillaries of the circulation combined because of greater Starling forces and higher capillary permeability. Compared with other organs, the kidneys receive an extraordinarily large amount of blood flow—normalized to the mass of the organ—and filter an unusually high fraction of this blood flow. Under normal conditions, the glomerular filtration rate of the two kidneys is 125 mL/min or 180 L/day. Such a large rate of filtrate formation is required to expose the entire extracellular fluid (ECF) frequently (>10 times a day) to the scrutiny of the renal-tubule epithelium.

• If it were not for such a high turnover of the ECF, only small volumes of blood would be “cleared” per unit time of certain solutes and water. Such a low clearance would have two harmful consequences for the renal excretion of solutes that renal tubules cannot adequately secrete.

• First, in the face of a sudden increase in the plasma level of a toxic material—originating either from metabolism or from food or fluid intake—the excretion of the material would be delayed. A high blood flow and a high GFR allow the kidneys to eliminate harmful materials rapidly by filtration.

• A second consequence of low clearance would be that steady-state plasma levels would be very high for waste materials that depend on filtration for excretion.

• The ideal glomerular marker for measuring GFR would be a substance X that has the same concentration in the glomerular filtrate as in plasma and that also is not reabsorbed, secreted, synthesized, broken down, or accumulated by the tubules

becomes

For a substance to be adequate for using in determining the clearance:1. Substance must be freely filterable in the glomeruli.2. Substance must be neither reabsorbed nor secreted by the renal tubules.3. Substance must not be synthesized, broken down, or accumulated by the kidney.4. Substance must be physiologically inert (not toxic and without effect on renal

function).

Inulin is an exogenous starch-like fructose polymer that is extracted from the Jerusalem artichoke and has a molecular weight of 5000 Da. Inulin is freely filtered at the glomerulus, but neither reabsorbed nor secreted by the renal tubules

Because inulin is not a convenient marker for routine clinical testing, nephrologists use other compounds that have clearances similar to those of inulin. The most commonly used compound in human studies is 125I-iothalamate. However, even 125I-iothalamate must be infused intravenously and is generally used only in clinical research studies rather than in routine patient care.The problems of intravenous infusion of a GFR marker can be completely avoided by using an endogenous substance with inulin-like properties. Creatinine is such a substance, and creatinine clearance (CCr) is commonly used to estimate GFR in humans. Tubules, to a variable degree, secrete creatinine, which, by itself, would lead to a ~20% overestimation of GFR in humans. Moreover, when GFR falls to low levels with chronic kidney disease, the overestimation of GFR by CCr becomes more appreciable. In clinical practice, determining CCr is an easy and reliable means of assessing the GFR, and such determination avoids the need to inject anything into the patient. One merely obtains samples of venous blood and urine, analyzes them for creatinine concentration, and makes a simple calculation

Cockroft-Gauld

MDMR

The glomerular filtration barrier consists of four elements

• (1) the glycocalyx overlying the endothelial cells,

• (2) endothelial cells,

• (3) the glomerular basement membrane, and

• (4) epithelial podocytes.

Layers 1, 3, and 4 are covered with negative charges from anionic proteoglycans. The gene mutations that cause excessive urinary excretion of albumin generally affect slit diaphragm proteins, which suggests that the junctions between adjacent podocytes are the predominant barrier to filtration of macromolecules.

Substances of low molecular weight (<5500 Da) and small effective molecular radius (e.g., water, urea, glucose, and inulin) appear in the filtrate in the same concentration as in plasma (UFX/PX ≈ 1). In these instances, no sieving of the contents of the fluid moving through the glomerular “pores” occurs, so that the water moving through the filtration slits by convection carries the solutes with it. As a result, the concentration of the solute in the filtrate is the same as that in bulk plasma. The situation is different for substances with a molecular weight that is greater than ~14 kDa, such as lysozyme. Larger and larger macromolecules are increasingly restricted from passage.

• The net driving force favoring ultrafiltration (PUF) at any point along the glomerular capillaries is the difference between the hydrostatic pressure difference and the oncotic pressure difference between the capillary and Bowman's space. Thus, GFR is proportional to the net hydrostatic force (PGC − PBS) minus the net oncotic force (πGC − πBS).As far as the hydrostatic pressure difference is concerned, the unique arrangement in which afferent and efferent arterioles flank the glomerular capillary keeps the first term, PGC, at ~50 mm Hg, a value that is twice as high as that in most other capillaries . Moreover, direct measurements of pressure in rodents show that PGC decays little between the afferent and efferent ends of glomerular capillaries. The second term of the hydrostatic pressure difference, PBS, is ~10 mm Hg and does not vary along the capillary.

• Renal blood flow (RBF) is ~1 L/min out of the total cardiac output of 5 L/min. Normalized for weight, this blood flow amounts to ~350 mL/min for each 100 g of tissue, which is 7-fold higher than the normalized blood flow to the brain. Renal plasma flow (RPF) is(34-5)Given a hematocrit (Hct) of 0.40, the “normal” RPF is ~600 mL/min.

The relationship between GFR and RPF also defines a parameter known as the filtration fraction (FF), which is the volume of filtrate that forms from a given volume of plasma entering the glomeruli:

• The renal microvasculature has two unique features. First, this vascular bed has two major sites of resistance control, the afferent and the efferent arterioles. Second, it has two capillary beds in series, the glomerular and the peritubular capillaries.

• As a consequence of this unique architecture, significant pressure drops occur along both arterioles , glomerular capillary pressure is relatively high throughout, and peritubular capillary pressure is relatively low. Selective constriction or relaxation of the afferent and efferent arterioles allows for highly sensitive control of the hydrostatic pressure in the intervening glomerular capillary, and thus of glomerular filtration.

• Peritubular capillaries originate from the efferent arterioles of the superficial and juxtamedullaryglomeruli . The capillaries from the superficial glomeruli form a dense network in the cortex, and those from the juxtamedullary glomeruli follow the tubules down into the medulla, where the capillaries are known as the vasa recta . The peritubular capillaries have two main functions. First, these vessels deliver oxygen and nutrients to the epithelial cells. Second, they are responsible for taking up from the interstitial space the fluid and solutes that the renal tubules reabsorb.

• The Starling forces that govern filtration in other capillary beds apply here as well . However, in peritubular capillaries, the pattern is unique. In “standard” systemic capillaries, Starling forces favorfiltration at the arteriolar end and absorption at the venular end. Glomerular capillaries resemble the early part of these standard capillaries: the Starling forces always favor filtration. The peritubular capillaries are like the late part of standard capillaries: the Starling forces always favor absorption

An important feature of the renal circulation is its remarkable ability to maintain RBF and GFR within narrow limits, although mean arterial pressure may vary between ~80 and 170 mm Hg.

Stability of blood flow—known as autoregulation—is also a property of the vascular beds serving two other vital organs, the brain and the heart. Perfusion to all three of these organs must be preserved in emergency situations, such as hypotensive shock. Autoregulation of the renal blood supply is independent of the influence of renal nerves and circulating hormones, and persists even when one perfuses isolated kidneys with erythrocyte-free solutions.

Autoregulation of RBF—and, consequently, autoregulation of GFR, which depends on RBF —stabilizes the filtered load of solutes that reaches the tubules over a wide range of arterial pressures. Autoregulation of RBF also protects the fragile glomerular capillaries against increases in perfusion pressure that could lead to structural damage.

• The kidney autoregulates RBF by responding to a rise in renal arterial pressure with a proportional increase in the resistance of the afferent arterioles. Autoregulation comes into play during alterations in arterial pressure that occur, for example, during changes in posture, light to moderate exercise, and sleep. It is the afferent arteriole where the autoregulatory response occurs, and where the resistance to flow rises with increasing perfusion pressure

• In contrast, efferent arteriolar resistance, glomerular and peritubular capillary resistances, as well as venous resistance all change very little over the range of normal to high renal arterial pressures. However, in the range of low renal perfusion pressures—as in congestive heart failure—efferent arteriolar resistance increases, and thereby minimizes decreases in GFR.

• Two basic mechanisms—equally important—underlie renal autoregulation: a myogenic response of the smooth muscle of the afferent arterioles and a tubuloglomerular feedback mechanism.

Myogenic Response

The afferent arterioles have the inherent ability to respond to changes in vessel circumference by contracting or relaxing—a myogenic response .The mechanism of contraction is the opening of stretch-activated nonselective cation channels in vascular smooth muscle. The resultant depolarizing leads to an influx of Ca2+ that stimulates contraction .

Tubuloglomerular Feedback

The juxtaglomerular apparatus (JGA) mediates tubuloglomerular feedback (TGF). The macula densa cells in the thick ascending limb sense an increase in GFR and, in classic feedback fashion, translate this to a contraction of the afferent arteriole, a fall in PGC and RPF, and hence a decrease in GFR.

The mechanism of TGF is thought to be the following:1. An increase in arterial pressure leads to increases in glomerular capillary pressure, RPF,

and GFR.

2. Increased GFR leads to an increased delivery of Na+, Cl−, and fluid into the proximal tubule and, ultimately, to the macula densa cells of the JGA.

3. Macula densa cells do not sense flow per se, but the higher luminal [Na+] or [Cl−] resulting from high flow. Increases in luminal [Na+] and [Cl−]—via the apical Na/K/Cl cotransporter of the macula densa cell—translate to parallel increases in intracellular [Na+] and [Cl−]. Indeed, blocking the Na/K/Cl cotransporter with furosemide (see p. 757) not only blocks the uptake of Na+ and Cl− into the macula densa cells, but also interrupts TGF.

4. The rise in [Cl−]i, in conjunction with a basolateral Cl− channel, leads to a depolarization.

5. The depolarization activates a basolateral nonselective cation channel, which allows Ca2+ to enter the macula densa cell.

6. Increased [Ca2+]i causes the macula densa cell to release paracrine agents, particularly adenosine and ATP, which breaks down to adenosine.

7. Adenosine, binding to A1 adenosine receptors on the smooth-muscle cells, triggers contraction of nearby vascular smooth-muscle cells (see Table 20-8).

8. Increased afferent arteriolar resistance decreases GFR, counteracting the initial increase in GFR.

Factors that Increase Sensitivity of TGFVolume contractionAdenosinePGE2ThromboxaneHydroxyeicosatetraenoic acid (HETE)ANG IIFactors that Decrease Sensitivity of TGFVolume expansionANPNOcAMPPGI2High-protein diet

Renin-Angiotensin-Aldosterone Axis• In terms of renal hemodynamic effects, the most important part of the renin-angiotensin-aldosterone axis is its

middle member, the peptide hormone ANG II.

• ANG II has multiple actions on renal hemodynamics. The net effect of ANG II on blood flow and GFR depends on multiple factors. Under normal conditions, the effect of ANG II is primarily to mediate efferent arteriolar constriction, an effect that tends to maintain GFR when renal perfusion is reduced. The reason is that prostaglandins counteract any tendency of ANG II to constrict the afferent arteriole. Indeed, inhibition of prostaglandin production by NSAIDs unmasks the constriction of the afferent arteriole, resulting in a decline in GFR.

Sympathetic Nerves

• Sympathetic tone to the kidney may increase either as part of a general response—as occurs with pain, stress, trauma, hemorrhage or exercise—or as part of a more selective renal response to a decrease in effective circulating volume. In either case, sympathetic nerve terminals release norepinephrine into the interstitial space. At relatively high levels of nerve stimulation, both afferent and efferent arteriolar resistances rise, thus generally decreasing RBF and GFR.

• The observation that the RBF may fall more than the GFR is consistent with a preferential efferent arteriolar constriction. With maximal nerve stimulation, however, afferent vasoconstriction predominates and leads to drastic reductions in both RBF and GFR.

• In addition, sympathetic stimulation triggers granular cells to increase their release of renin, raising levels of ANG II, which acts as described above. Finally, sympathetic activation—even at levels too low to reduce RBF and GFR—causes increased reabsorption of Na+ by proximal tubules.

Arginine Vasopressin

In response to increases in the osmotic pressure of the ECF, the posterior pituitary releases AVP—also known as antidiuretic hormone . Although the principal effect of this small polypeptide is to increase water absorption in the collecting duct, AVP also increases vascular resistance. Despite physiological fluctuations of circulating AVP levels, total RBF and GFR remain nearly constant. Nevertheless, AVP may decrease blood flow to the renal medulla, thereby minimizing the washout of the hypertonic medulla; this hypertonicity is essential for forming a concentrated urine.

Atrial Natriuretic Peptide

Atrial myocytes release ANP in response to increased atrial pressure and thus effective circulating volume . ANP markedly vasodilates afferent and efferent arterioles, thereby increasing cortical and medullary blood flow, and lowers thesensitivity of the TGF mechanism .

The net effect is an increase in RPF and GFR. ANP also affects renal hemodynamics indirectly by inhibiting secretion of renin (thus lowering ANG II levels). Even without affecting GFR, low levels of ANP can be natriuretic by inhibiting Na+ reabsorption by tubules. First, ANP inhibits aldosterone secretion by the adrenal gland (and thereby reduces Na+ reabsorption;

Second, ANP acts directly to inhibit Na+ reabsorption by the inner medullary collecting duct . At higher levels, ANP decreases systemic arterial pressure and increases capillary permeability. ANP plays a role in the diuretic response to the redistribution of ECF and plasma volume into the thorax that occurs during space flight and water immersion

Other agents that influence GFR

Transport of Sodium and Chloride

• The kidneys help to maintain the body's extracellular fluid (ECF) volume by regulating the amount of Na+ in the urine. Sodium salts (predominantly NaCl) are the most important contributor to the osmolality of the ECF; hence, where Na+ goes, water follows. This chapter focuses on how the kidneys maintain the ECF volume by regulating excretion of Na+ and its most prevalent anion, Cl−.

• The normal daily urinary excretion of Na+ is only a tiny fraction of the total Na+ filtered by the kidneys .

• The filtered load of Na+ is the product of the glomerular filtration rate (GFR, ~180 L/day) and the plasma Na+ concentration of ~142 mM(neglecting the small difference from [Na+] in protein-free plasma water;), or ~25,500 mmol/day. This amount is equivalent to the Na+ in ~1.5 kg of table salt, more than nine times the total quantity of Na+ present in the body fluids. For subjects on a typical Western diet containing ~120 mmol of Na+, the kidneys reabsorb ~99.6% of the filtered Na+ by the time the tubule fluid reaches the renal pelvis. Therefore, even minute variations in the fractional reabsorptive rate can lead to changes in total-body Na+ that markedly alter ECF volume and, hence, body weight and blood pressure. Thus, it is not surprising that each nephron segment makes its own unique contribution to Na+ homeostasis.

• Distribution and balance of Na+ throughout the body. The values in the boxes are approximations. ICF, intracellular fluid.

Estimates of renal handling of Na+ along the nephron. The numbered yellow boxes indicate the absolute amount of Na+—as well as the fraction of the filtered load—that various nephron segments reabsorb. The green boxes indicate the fraction of the filtered load that remains in the lumen at these sites. The values in the boxes are approximations. PNa, plasma sodium concentration; UNa, urine sodium concentration;V , urine flow.

• The loop of Henle reabsorbs a smaller but significant fraction of filtered Na+ (~25%). Because of the low water permeability of the thick ascending limb (TAL), this nephron segment reabsorbs Na+ faster than it reabsorbs water, so that [Na+] in the tubule fluid entering the distal convoluted tubule has decreased substantially (TFNa/PNa ≅ 0.45).

• The classic distal tubule and collecting ducts reabsorb smaller fractions of filtered Na+ and water than do more proximal segments. The segments between the distal convoluted tubule (DCT) and the cortical collecting tubule (CCT), inclusive, reabsorb ~5% of the filtered Na+ load under normal conditions. Finally, the medullary collecting duct reabsorbs ~3% of the filtered Na+ load. Although the distal nephron reabsorbs only small amounts of Na+, it can establish a steep transepithelial concentration gradient and can respond to several hormones, including mineralocorticoids and arginine vasopressin (AVP).

• The tubule can reabsorb Na+ and Cl− via both transcellular and paracellular pathways. In the transcellular pathway, Na+ and Cl− sequentially traverse the apical and basolateral membranes before entering the blood. In the paracellular pathway, these ions move entirely by an extracellular route, through the tight junctions between cells. In the transcellular pathway, transport rates depend on the electrochemical gradients, ion channels, and transporters at the apical and basolateral membranes. However, in the paracellular pathway, transepithelial electrochemical driving forces and permeability properties of the tight junctions govern ion movements.

Transcellular and paracellular mechanisms of Na+ and Cl− reabsorption. The example in B illustrates the electrochemical driving forces for Na+ in the early proximal tubule. The equivalent circuit demonstrates that the flow of positive charge across the apical membrane slightly depolarizes the apical membrane (−67 mV) relative to the basolateral membrane (−70 mV).

The basic mechanism of transcellular Na+ reabsorption is similar in all nephron segments and is a variation on the classic two-membrane model of epithelial transport. The first step is the passive entry of Na+ into the cell across the apical membrane. Because the intracellular Na+ concentration ([Na+]i) is low and the cell voltage is negative with respect to the lumen, the electrochemical gradient is favorable for passive Na+ entry across the apical membrane . However, different tubule segments use different mechanisms of passive Na+ entry across the apical membrane. The proximal tubule, the TAL, and the DCT all use a combination of Na+-coupled cotransporters and exchangers to move Na+ across the apical membrane; however, in the cortical and medullary collecting ducts, Na enters the cell through epithelial Na+ channels (ENaCs).The second step of transcellular Na+ reabsorption is the active extrusion of Na+ out of the cell across the basolateral membrane .This Na+ extrusion is mediated by the Na-K pump which keeps [Na+]ilow (~15 mM) and [K+]i high (~120 mM). Because the basolateral membrane is primarily permeable to K+, it develops a voltage of ~70 mV, with the cell interior negative with respect to the interstitial space. Across the apical membrane, the cell is negative with respect to the lumen. The magnitude of the apical membrane voltage may be either lower or higher than that of the basolateral membrane, depending on the nephron segment and its transport activity.

The basic mechanism of paracellular Na+ transport is similar among nephron segments: the transepithelial electrochemical gradient for Na+ drives transport. However, both the transepithelial voltage (Vte) and luminal [Na+] vary along the nephron.

As a result, the net driving force for Na+ is positive—favoring passive Na+ reabsorption—only in the S2 and S3 segments of the proximal tubule and in the TAL. In the other segments, the net driving force is negative—favoring passive Na+ diffusion from blood to lumen (“backleak”). In addition to undergoing purely passive, paracellular reabsorption in the S2 and S3 segments and TAL, Na+ can move uphill from lumen to blood via solvent drag across the tight junctions. In this case, the movement of H2O from the lumen to the lateral intercellular space—energized by the active transport of Na+ into the lateral intercellular space—also sweeps Na+ and Cl− in the same direction.