RENAL BLOOD FLOW-GLOMERULAR FILTRATION RATE
George N. Coritsidis, MD

Objectives

-To refresh the knowledge of anatomy and physiology of renal circulation.

-To understand mechamisms regulating renal blood flow and glomerular filtration.

-To learn the physiologic and pathophysiologic mechanisms triggered by changes in renal blood flow and glomerular ultrafiltration.

-To apply simple clearance tests to calculate renal blood flow and glomerular filtration rate; learn about their dynamics with aging.

-To learn about the sieving properties of glomerular ultrafiltration barrier and its disturbances in disease processes.

INTRODUCTION

Figure 1: Kidney with X-Section Figure 2: Functions of the Kidney

The principal function of the mammalian kidney is to maintain homeostasis or equilibrium between our internal volume and electrolyte status and that of the environment’s influences, diet and intake. It functions to maintain our intra and extracellular fluid status at a constant despite the wide variety of daily fluid and electrolyte intake. In man, this is accomplished by the kidneys, which consist of 2-million nephrons and weigh only 250 grams. The kidneys’ extraordinary excretory and regulatory objectives are achieved through the processes of glomerular ultrafiltration, tubular reabsorption and tubular secretion.

RENAL BLOOD FLOW (RBF)

To a large extent, these excretory and regulatory processes depend on the blood supply to the kidney. It is not surprising therefore that it receives the highest blood flow per gram of organ weight in the body at 1 liter/min. As blood flows through these vascular organs, its composition is appropriately altered according to homeostatic requirements.

One manner of understanding the magnitude of the renal blood flow is to consider the renal fraction. This is the fraction of the total cardiac output that flows through the kidneys. A 70 Kg man with a cardiac output of 6 L/min. has a normal renal blood flow of about 1.2 L/min. Rearranging these numbers (1.2 / 6.0), we find that the kidney is constantly fed 20% of the cardiac output or a renal fraction of .2. Thus, a very substantial portion of the total cardiac output flows through the kidneys.

Considering the fact that each kidney in a normal 70 Kg man weighs about 130 – 170 g, the large magnitude of the normal blood flow through the kidney becomes even more apparent. With a total flow of 1200 ml/min. and 300 g of kidney, the average flow per gram of kidney weight is about 400 ml/min/100g (see Table 1). This is several times greater per unit weight of organ than the blood flow through most other organs. For example, liver tissue has a blood flow of about 20 ml/min/100g and resting muscle has a flow of about 27 ml/min/100g. During various stress conditions or disease, this renal fraction can vary considerably and be markedly affected.

Blood flow to the kidneys will be dependent on a number of important systemic factors. Clearly if there is a problem with volume (dehydration, hemorrhage) or cardiac output (congestive heart failure, myocardial infarct) then blood flow is diminished. In less obvious ways hypoalbuminemia (cirrhosis, nephrotic syndrome, and starvation) affects the intravascular volume so that the effective blood (volume) flow is diminished despite many of these patients appearing total body fluid overloaded. Finally, hypotension from severe vasodilatation (anaphylactic shock, sepsis) would also diminish blood flow to the kidneys.

Table 1: 0₂ Consumption in the Kidney Compared to Other Organ Systems

Region or Organ	O2 Delivery ml/min/100 g	Blood Flow Rate ml/min/100 g	O2 Consumption ml/min/100 g	O2 Consumption/O2 Delivery (%)
Hepatoportal	11.6	58	2.2	18
Kidney	84.0	420	6.8	8
Renal outer medulla	7.6	190	6.9	79
Brain	10.8	54	3.7	34
Skin	2.6	13	0.38	15
Skeletal muscle	0.5	2.7	0.18	34
Heart	16.8	87	11.0	65

OXYGEN CONSUMPTION

Why do the kidneys receive a disproportionate amount of the cardiac output? Oxygen consumption by the kidneys is quite high and amounts to about 8% of the total oxygen consumption of the body. Oxygen delivery to any organ is directly dependent on hemoglobin content (blood) and cardiac output (blood flow). As in other tissues, an important function of blood flow to is to provide adequate oxygenation and nutrition. Therefore, the relatively high blood flow to the kidneys exists to feed its metabolic demands as well as to allow a high GFR.

In fact, the renal blood flow is so high that only a small percent of the available oxygen is extracted from the blood perfusing the kidneys. For example, with an arterial blood oxygen concentration of 20 ml/100 ml, the renal venous oxygen concentration would be 18.6 ml/100 ml of blood. The resultant arteriovenous oxygen difference of only 1.4 ml/100 ml of blood is much less than that of most other organs. For instance, the brain consumes over 4 times more of its O₂ delivery than the kidney as a whole. However, when one looks at certain regions of the kidney, such as the renal outer medulla, one sees that the per cent O₂ consumption can be higher than that of average brain tissue (Table 1).

This high degree of oxygen consumption subserves the very large metabolic demands that certain regions of the kidney have. Active solute reabsorption, particularly the active reabsorption of sodium chloride from the tubules, mandates a high metabolic rate. The thick ascending limb of Henle’s loop as well as the S3 segment of the proximal tubule resides within the renal outer medulla. As shown in fig 3, a close correlation exists between net transport of sodium (T_Na) by the tubules and renal oxygen uptake (V_O2). In these experiments, measurements of both were done in dogs undergoing diuresis.

Figure 3: O₂ Consumption as it Relates to Na Transport

In spite of the high rate of oxygen utilization, the very great renal blood flow relative to the oxygen consumption indicates that factors other than the oxygen requirements are responsible for the control of renal blood flow. Many investigators believe that renal blood flow is controlled primarily by factors involved in the regulation of the composition and volume of the extracellular fluid. These controlling factors can be understood more precisely by evaluating the forces that drive flow along the vascular network and yield the resultant intrarenal pressures. Forces that determine RBF can be represented using Ohm’s law of physics:

Ohm’s Law: Q (flow) = D P (pressure gradient) RBF = Blood Pressure R (resistance) Renal Vascular Resistance

Consider the forces that determine overall renal blood flow. The basic equation demonstrates that virtually all factors that influence total renal blood flow must do so by altering either the arterial blood pressure or the renal vascular resistance.

RENAL BLOOD FLOW: MEASUREMENT

No ideal technique exists for measurement of renal blood flow in man. An accepted method applies the Fick Principle to determine blood flow through clearance. Total renal blood flow can be measured by estimating the clearance of a substance that is completely removed from the blood in a single pass through the kidney. A substance “X” can be used to calculate renal blood flow (RBF) by the following equation.

Measurement

(i) Ux ^. V = (Ax – Vx) RBF where:

Rearranging terms: Ux = urine concentration

(ii) RPF = Ux ^. V Ax = renal artery concentration

(Ax – Vx) Vx = renal vein concentration

RPF = renal plasma flow

V = urine flow rate

Such a substance is para-aminohippurate (PAH) which is cleared from the blood by a combination of filtration and, primarily, tubular secretion. Therefore, PAH is not reabsorbed, metabolized or synthesized by the kidney. Furthermore, there is near complete secretion by the pars recta at low concentrations. To insure its blood concentration remains constant and below its transport maximum, it is infused at an appropriate rate through an I.V. The renal artery concentration of x will be the same as the peripheral venous concentration and the renal vein PAH concentration becomes negligible, since it is essentially all cleared. This is helpful since there is no simple way of measuring the renal vein concentration. It has been a useful investigative tool but is seldom used clinically. In actuality, it slightly underestimates flow. Therefore, if x = PAH, equation (ii) becomes simpler:

(iii) RPF = U_PAH ^. V [VPAH= 0]

A_PAH

Finally; correcting for the hematocrit:

(iv) RBF = RPF

1 – Hct

Renography

A substance, such as PAH labeled with a radioisotope, can be detected as it passes through the kidney by an appropriate recording system located outside the body. By measuring the density of tracer with the passage of time, a curve can be generated which indicates the rate of passage of tracer into the urine (radioisotope renography).

More sophisticated isotope “washout” techniques measure blood flow and distribution in more detail, but are not of clinical use except in specialized laboratories. Gamma camera scans can give a picture of the uptake and excretion of isotope, usually technetium, or iodine-labeled PAH. Finally, ultrasound and doppler flow techniques are rapidly being upgraded and developed as an noninvasive measure of flow.

RENAL HEMODYNAMICS

Renal function is dependent upon generous blood flow to the kidneys. As mentioned earlier, blood flow to the kidneys is dependent on systemic blood pressure. However, actual renal perfusion, and hence adequate glomerular perfusion, is further dependent on intra-renal vascular resistance. Autoregulatory mechanisms, through changes in vascular resistance, ensure that over a wide range of perfusion pressures renal blood flow remains stable and glomerular filtration can be maintained. These autoregulatory mechanisms are primarily local and intrinsic but systemic inputs also affect renal blood flow.

INFLUENCES OF RENAL VASCULAR RESISTANCE

Renal nerves Þ RVR Ü Hormones (epinephrine)

Ý

Intrinsic control systems

Blood flowing through the renal artery is distributed via a series of divisions (see fig 2) and ends by feeding the glomeruli through the afferent arterioles. After passing through the glomerular capillaries, blood leaves through the efferent arterioles to enter a second capillary network, the peritubular capillaries, which surround the tubules and then leave via renal venules.

Each of these capillary networks is exquisitely designed to serve the functional needs of the kidney. Every day 180 liters of fluid pass through the glomerular capillaries as filtrate. About 99% is recovered from the tubules and carried back into the general circulation via the peritubular capillaries. The remaining 1% continues on to its final presentation as urine. The generally accepted pressure gradient through this vascular system is shown in Figure 4. The pressure profile along the intrarenal vasculature (fig 4) starts with a mean arterial pressure of 100 mm Hg, and significantly drops between the renal artery and the glomerular capillaries. This is due to the pre-glomerular resistance of the afferent arteriole. Take note that the glomerular capillary pressure (Pgc) is much higher than any other capillary bed in the body (60 vs. 13 mm Hg). The increased hydrostatic pressure is a necessary phenomenon to insure the generation of filtrate and hence the glomerular filtration rate (GFR). A second resistance site is postglomerular and is located at the outflow site of the glomerulus in the efferent arterioles. The third site is the venous resistance located after the peritubular capillaries and most likely at the arcuate and interlobar veins.

Figure 4: Renal Vascular Pressure Gradients

The postglomerular or efferent arteriolar resistance also serves to maintain glomerular pressure and, in turn, is responsible for the pressure drop from the glomerular capillaries to the peritubular capillaries. Peritubular capillary pressure is regulated at around 15-20 mm Hg.

Both the preglomerular afferent and postglomerular efferent resistance vessels possess smooth muscle and therefore, can vary their level of vasoconstriction. Thus, most of the alterations in renal vascular resistance occur at these two sites. However, the available evidence indicates that the afferent resistance segment has the greatest role in the control of the intrarenal hemodynamics. Usually the venous resistance is not greatly altered.

Many control systems operate to regulate arterial pressure. Also, the phenomenon of autoregulation, which will be explained later, maintains renal blood flow in the face of arterial pressure fluctuations.

Therefore, the major variable that serves to regulate renal blood flow is the renal vascular resistance. In turn, renal vascular resistance can be affected by intrinsic intrarenal control mechanisms such as that responsible for the phenomenon of autoregulation. Extrinsic factors involved in autoregulation include renal nerve activity and hormones that can affect vascular contractility, as well as to a lesser extent the actual composition and viscosity of the blood perfusing the kidney.

AUTOREGULATION

The kidney itself continuously regulates distribution of flow within the renal tissue. This process is called autoregulation and is responsible for maintaining intra-renal blood flow over a wide range of systemic perfusion pressures (Figure 5). All tissues demonstrate this to some degree, but the kidney does it intrinsically with great precision and even when denervated. Selective vasoconstriction or dilatation must occur to maintain renal blood flow at a constant rate. These enable continued glomerular filtration to be maintained across a wide range of systemic blood pressures (Figure 6). The response of maintaining a constant blood flow over a wide range of arterial pressure is one manifestation of the phenomenon of autoregulation.

It should be emphasized that this mechanism can also regulate blood flow under other circumstances and in response to other stimuli.

Figure 5: GFR Maintenance Despite BP Changes

The mechanism behind autoregulation is felt to be via myogenic receptors that regulate AA vasoconstriction. These sense blood pressure changes through stretch receptors and respond accordingly through relaxation or constriction.

As mentioned, the requisite changes in resistance of autoregulation are predominantly localized to pre-glomerular sites. Since the glomerular capillary pressure (Pgc) i.e., filtration pressure is one important determinant of the amount of fluid that filters across the glomerular membrane into the proximal tubules, the phenomenon of renal autoregulation also serves to regulate the glomerular filtration rate by supporting the Pgc.

GFR = Pgc (filtration pressure) x K_uf (ultrafiltration coefficient)

The autoregulatory system accomplishes this by maintaining the glomerular capillary pressure around 60-70 mm Hg. The K_uf reflects available glomerular surface area. Simply put, the purpose of the autoregulation of RBF is to maintain GFR. For example, afferent arteriolar vasoconstriction would serve to protect the glomerulus from uncontrolled systemic hypertension, while AA vasodilatation would allow for greater blood flow into the glomerulus in times of hypotension. This ability to maintain renal perfusion pressure and Pgc is impaired when mean arterial pressure drops below 70 mmHg.

Figure 6: AA Vasoconstriction and Pgc

Without this control, the more subtle metabolic activities of the tubules could easily be overwhelmed by inappropriate changes in these physical forces. Therefore, a primary role of the renal autoregulatory mechanism is to regulate intrarenal hemodynamics and intrarenal pressures to levels that maintain an optimal balance with tubular metabolic functions.

Overall, the phenomenon of renal autoregulation serves to control the systemic hemodynamically determined inputs to the kidney.

Consequently, the relationship between renal arterial pressure and glomerular filtration rate is similar to that shown between pressure and renal blood flow and also demonstrates autoregulation. Filtration is the key step to entry into the tubule. Modification of the composition of the filtrate by reabsorption or secretion in the tubular system regulates the composition of the final urine and as a result maintains the strict requirements for ECF composition.

Ultrafiltration is characteristically high and nonselective and essentially throws the “baby out with the wash water”! The selective Figure 7:

tubular reabsorption that follows is responsible for the “baby’s retrieval” resulting in only 1% of filtrate’s excretion.

The tubule is long and follows a complex and winding course, commencing with Bowman’s capsule, and continuing until it joins with other tubules to form the major collecting ducts in the medullary papilla.

A schematic tubule is shown here following the tubule through its divisions: proximal tubule; loop of Henle; distal tubule; and collecting duct.

Figure 8: Tubule Anatomy

The length of the various segments of the tubule varies and the distal end of the nephron can be subdivided into cortical and medullary collecting ducts. Such subdivision is of little practical value to the clinical nephrologist. The subdivision of the loop of Henle is, however, of major physiologic importance in operating the “counter current” system which generates the medullary osmotic gradient that allows the collecting duct to reabsorb free water under the influence of antidiuretic hormone (ADH).

TUBULOGLOMERULAR FEEDBACK (TGF)

Figure 9: Anatomy of The Juxtaglomerular Complex

After forming the loop of Henle the ascending tubule comes back to nestle close to its own glomerulus at the juxtaglomerular apparatus (JGA). This brings us to the next site of autoregulation where feedback between tubule and glomerulus can occur. The juxtaglomerular apparatus is made up of specialized cells in the wall of the afferent arteriole and granular cells in the wall of the distal tubule (the macula densa). This area is innervated by adrenergic fibers and the granular cells carry renin in intracellular granules. The principle function of the JGA is adapting the GFR to early distal tubule fluid characteristics by modulating renin synthesis and release: this is known as the tubuloglomerular feedback (TGF) loop. Afferent arteriolar caliber is principally controlled by TGF. Besides altered sodium concentration at the macula densa of the distal tubule, release of renin can also be induced by changes in the blood flow patterns of the afferent arteriole, or by adrenergic stimulation. The afferent arteriole will respond to changes in renal perfusion pressure by either vasoconstriction or vasodilatation as a result of a direct myogenic response or TGF. Another intrinsic mechanism is angiotensin II effects which can be locally or systemically generated. As shown in fig 5 the use of an AII antagonist inhibits autoregulation. AII works primarily in the efferent arteriole to cause vasoconstriction. All these responses maintain downstream perfusion.

Figure 10: Tubuloglomerular Feedback

The precise mechanism of this TGF response has not been determined. Besides altered flow at the macula densa of the distal tubule, Na, NaCl and Cl concentrations have each been implicated as the key stimulus. One of these signals, most likely Cl^- concentration (as it relates to distal tubular flow), is sensed by the macula densa and a signal is transmitted to the afferent arteriole leading to changes in pre-glomerular resistance. Conversely, if distal tubular fluid flow falls, due to a decreased glomerular filtration rate, this leads to a decreased resistance and a restoration of glomerular filtration and tubular fluid flow.

At the level of the afferent arteriole, the effector point of the feedback loop, TGF may be mediated directly by adenosine and thromboxane, and indirectly by angiotensin II. The latter may act to sensitize the afferent arteriole to the actions of the other two vasoconstrictors: adenosine and thromboxane.

Figure 11: TGF Responses

Ý Distal Tubular Flow Þ Ý Afferent ßDistal Tubular Flow Þ ßAfferent
Arteriolar Resistance Arteriolar Resistance

ß ß

Return of DTF towards Ü ß GFR Return of DTF towards Ü ÝGFR control (lowers Pgc) control (raises Pgc)

The major function of autoregulation and TGF at the level of the organism is to prevent major losses of salt and water. Phylogenetically, the JGA / TGF system evolved in the nephrons of the first terrestrials, the early amphibians, in an effort to conserve salt and maintain extracellular volume and blood pressure. Their predecessors, the teleosts (bony fish), do not have such a system since it is not necessary in a constant salt-water milieu. These autoregulatory responses are seen almost immediately (especially the myogenic responses) and reflect sudden changes in blood flow to the kidneys.

Figure 12:

The importance of autoregulation is most likely seen in the hour to hour, or day to day volume changes experienced by normal people. In disease situations such as renal artery stenosis, where blood flow entering the kidney is compromised, GFR is largely supported by autoregulation.

In addition to the intrinsic mechanisms responsible for the autoregulatory control of renal vascular resistance, there also exist extrinsic mechanisms such as sympathetic nerve activity and certain circulating hormones such as the renin-angiotensin II (AII) axis. For example, Norepinephrine directly increases afferent arteriolar tone (R_A) while AII acts to increase efferent arteriolar tone (R_E). Post glomerular, efferent arteriolar vasoconstriction, mediated through AII, plays an invaluable role in maintaining adequate P_GC. Resistance here provides for a “back pressure” effect enhancing the glomerular hydrostatic pressure generated by flow permissive afferent arteriolar vasodilatation. AII release is dependent on renin release which results from blood flow changes in the afferent arteriole, adrenergic stimulation, as well as solute changes at the macula densa. The relative difference of these two resistances on either end of the glomerulus provides for optimum P_GC. Differences in AII receptor concentration in the efferent arteriole may explain its heightened response to AII.In counterbalance, renal vasodilator prostaglandins act to attenuate the vasoconstrictor effects described above. These mechanisms play a more significant role in long term maintenance of GFR in chronic disorders (such as severe congestive heart failure) where effective renal blood flow is reduced. It is in these patients that the use of aspirin, NSAIDs or ACE inhibitors can lead to renal failure due to effects on autoregulation and GFR. They act by dysregulating the fine balance of vasoconstriction vs. vasodilatation needed to maintain an already compromised GFR. Finally, the actual composition of the blood perfusing the kidney, for instance viscosity, may affect renal blood flow and renal function. The following table and figures serve as reviews.

Table 2: Regulation of Renal Hemodynamics Figure 13: Glomerular Arteriolar
Vasoconstriction

Intrinsic control mechanisms

Autoregulation

Myogenic response

Tubuloglomerular feedback

Local angiotensin II release

Extrinsic

Sympathetic innervation

Hormonal input

Renin/AII

Norepinephrine

Blood composition

Figure 14: Regulation of Glomerular Capillary Resistance

GLOMERULAR FILTRATION

The rules regulating the distribution of fluid between the plasma and interstitial compartments are well described by Starling’s hypothesis, which implies that, through a semi-permeable capillary wall, hydrostatic pressure shifts fluids outwards, whilst the oncotic pressure of the plasma albumin holds fluid within the capillary.

In abnormal states, however, imbalances occur due to changes in hydrostatic pressure, plasma albumin concentration or capillary permeability. Clinically, in various tissues and organ systems this leads to a spectrum of findings ranging from pulmonary edema and cirrhotic ascites to the more benign soft tissue swelling and dependent edema.

Starling’s hypothesis also applies to the glomerular capillary system as well. The result is the initial step towards urine formation: the separation of ultrafiltrate from plasma across the glomerular capillary wall.

As blood passes through the glomerular capillaries, continued filtration lowers the hydrostatic pressure, but at the same time leaves albumin in the capillary. As filtrate leaves without concomitant albumin loss, the concentration of the albumin relatively increases and therefore, so does the oncotic pressure. Filtration pressure is a balance between these two effects. Hydrostatic pressure predominates at the arteriolar end of the capillary, but becomes dissipated toward the venous end. Usually the outward and inward fluid shifts are approximately balanced.

Hydrostatic pressure forces fluid out of the glomerular capillary; oncotic pressure, due primarily to plasma albumin, holds fluid within the capillary. What governs which particles are also transferred depends on the permeability of the glomerular capillary wall and the size of the particles.

Figure 15:

In the glomerular capillaries, the relatively high hydrostatic pressure is predominantly responsible for the ultrafiltration of fluid into Bowman’s space. However, this pressure is counteracted in part by the hydrostatic pressure in Bowman’s space. Thus, a net hydrostatic pressure of about 45mm Hg exists across the glomerular capillaries.

The colloid osmotic pressure of plasma entering the glomerular capillaries is the normal value of about 26 mm Hg but increases steadily as protein-free fluid is filtered out of the vessels into the tubules. Colloid osmotic pressure rises about 35 mm Hg at the efferent end. The colloid osmotic pressure in Bowman’s space is near zero and thus, the average colloid osmotic pressure gradient across the glomerular capillaries is about 30 mm Hg.

This colloid osmotic pressure serves as a negative force and therefore counteracts the hydrostatic pressure force across the glomerular capillaries. The actual effective or net filtration pressure is the difference between the net hydrostatic pressure and the colloid osmotic pressure.

The normal value for effective filtration pressure is about 15 mm Hg and it is this pressure that is responsible for the transcapillary fluid movement from the glomerular capillaries into Bowman’s capsule and hence, the tubules.

At the peritubular capillary level, the hydrostatic pressure and colloid osmotic pressures of the plasma in the peritubular capillaries and of the tissue fluids surrounding them all contribute to the final net movement of fluid. These capillaries must reabsorb back into the vascular system the major bulk of the fluid transported out of the tubules into the tissue fluids. To arrive at the net force, the forces on both sides of the membrane have to be considered similar to what occurs at the level of the glomerular capillaries.

It should be emphasized that the effective pressure gradients at both the glomerular and the peritubular capillaries are relatively low when compared to the magnitude of the forces involved. The significant difference between these two capillary systems is that the hydrostatic pressure gradient predominates in the glomerular capillaries thus leading to an efflux of fluid, while the colloid osmotic pressure predominates in the peritubular capillaries allowing a net influx of fluid.

GLOMERULAR FILTRATION

Figure 16: Determinants of GFR

Filtration within the glomerular capillary bed is a passive process driven by the same 3 physical forces as filtration at every other capillary bed in the body:

1) Hydrostatic pressure (N.B. Because of the position of the glomerular capillary between the afferent and efferent arterioles, the hydrostatic pressure within this capillary is quite high)

2) Oncotic (colloid osmotic) pressure (determined by plasma proteins)

Ultrafiltration Coefficient (KUF). The KUF is a function of both surface area and hydraulic permeability. Recent studies have shown that surface area may vary greatly as mesangial cells contract in response to stimuli such as angiotensin II and parathyroid hormone.

The interaction of these factors is defined by the equation:

GFR = KUF [PGC – PB) – (pGC - pB)]

Where: KUF = ultrafiltration Coefficient

PGC = hydrostatic pressure in the glomerular capillary

PB = hydrostatic pressure in Bowman’s space

pGC = oncotic pressure in glomerular capillary

pB = oncotic pressure in Bowman’s space (~ 0)

This interaction can be defined schematically as follows.

Figure 17.

GLOMERULAR BARRIER

At the microscopic level, the filtration surface includes the capillary endothelial cell, the basement membrane and the foot processes of the epithelial cells derived from Bowman’s capsule. These are strongly negatively charged due to anionic amino acids, sialic acid residues and proteoglycans.

The “effective pore size” of the glomerular filter is a complex concept and depends on a range of factors which include the size, shape and charge of the filtered particles, as well as the characteristics of the glomerulus itself. Up to a molecular weight of about 5000 daltons there is no barrier to filtration, but above that, charge and shape become progressively more important determinants of filtration.

Figure 18:

The glomerular filtrate is defined as an ultrafiltrate of plasma, that is, a protein-free filtrate of plasma. All other solutes of smaller molecular size are present in the glomerular filtrate in the same concentration found in plasma with the exception of those bound to plasma proteins. There are three components to the barrier:

1) Endothelial cells

2) Basement membrane this is the glomerular basement membrane (GBM) which contains negatively charged glycoproteins. It is often the site of injury in glomerular diseases such as lupus nephritis and membranous nephropathy. It is considered the primary barrier to the filtration of larger molecules.

3) Epithelial podocytes with negatively charged slit pores. These cells also synthesize the connective tissue proteins that make up the GBM.

There is debate as to the exact location of the ultimate barrier; however, it may be generally stated that the endothelium restricts movement of cells and the combination of basement membrane and slit pores restricts movement of macromolecules.

Finally, there is a third cell, resident to the glomerulus, called the mesangial cell. It is situated in the interstitium of the glomerulus, or the mesangium, and though it is not part of the barrier, it plays an important role in the physiology of the GFR. Mesangial cells are considered to be specialized smooth muscle cells, which can alter glomerular surface area, and hence GFR, through their contraction. They also have macrophage-like properties and synthesize the matrix proteins of the mesangium. These properties and changes or expansion of the mesangial matrix have important pathogenic consequences in glomerular diseases.

The permselectivity of the glomerulus to macromolecules depends on three features:

1) Molecular size

2) Molecular charge

3) Molecular configuration

The interrelation of these factors is seen in the Figure 19. In man, the molecular configuration is less important than size or charge.

Figure 19.

EFFECTIVE MOLECULAR RADIUS

The figure 19 demonstrates that for uncharged dextrans, little filtration (Y-axis ) occurs above a radius of 44 A (X-axis), the approximate size of albumin. Because of the negative charge of the barrier, polyanionic dextrans (dextran sulfate) are relatively restricted and polycationic dextrans (DEAE) are relatively filterable.

In glomerular diseases such as minimal change nephrosis, the barrier with respect to charge is disrupted resulting in selective proteinuria and massive loss of negatively charged albumin. Conversely, in diabetic nephropathy, loss of both size barrier and charge barrier leads to nonselective proteinuria.

Table 3: Summary of Direct GFR Determinants and Factors That Influence Them

Direct determinants of GFR: GFR=K1 (PGC-PBC-pGC)	Major factors that tend to increase the magnitude of the direct determinant
K1*	1. Glomerular surface area because of relaxation of glomerular mesangial cells Results: GFR
PGC*	1. Renal arterial pressure 2. ¯ Afferent-arteriolar resistance (afferent dilation) 3. Efferent-arteriolar resistance (efferent constriction) Results GFR
PBC*	1. Intrabular pressure because of obstruction of tubule or extrarenal urinary system Results: ¯ GFR
pGC*	1. Systemic-plasma oncotic pressure (sets pGC or beginning of glomerular capillaries) 2. ¯ Total renal plasma flow (sets rate of rise of pGC along glomerular capillaries) Results: ¯ GFR

MEASURING RENAL FUNCTION

Several tests of renal function have been devised. Some depend upon the composition of the urine, some upon the composition of the blood and some upon a combination of the two.

The Concept of Clearance

The amount of any substance in the plasma that appears in the urine can be calculated by multiplying its concentration in the urine (U) by the volume of urine produced in a unit of time (V).

Amount of urine = U x V

The volume of plasma that would contain that amount can be calculated by dividing the amount (U x V) by the plasma concentration of substance (P).

Volume of plasma = U x V/P

In other words, clearance is the volume of plasma that would have to be completely “cleared” of the substance per unit of time to produce the amount seen in the urine. This is known as the “clearance” for that substance.

Glomerular Filtration Rate (GFR)

A “glomerular substance” is one which is: (1) freely filtered at the glomerulus, (2) neither reabsorbed nor secreted by the tubule.

As a result whatever is filtered appears in the urine.

The clearance of such a substance will thus be a measure of the rate at which it is filtered (GFR).

Insulin is the material that fits best with the definition of a glomerular substance but is not part of normal endogenous metabolism and therefore has to be infused. It is a nuisance to assay and is of limited practical use.

Radio labeled vitamin B₁₂, iothalamate, and EDTA can also be used and are easier to measure but also have to be infused.

The normal range for GFR is 1.25 – 2.10 mL/s 1.73 m2 and varies with age and sex.

Measurement of GFR

The basic formula for clearance (Cx) of a substance X is:

Cx = (Ux) ^. (V) Ux = urine concentration of X

Px V = urine volume / time

Px = plasma concentration of X

For most practical purposes creatinine is very close to being a glomerular substance and has the great advantage that it is present in the plasma at a constant level as an endogenous product of muscle metabolism.

In severe renal failure and heavy proteinuria, some creatinine may enter the tubular urine after filtration by secretion and in this situation its clearance may overestimate the GFR.

The clearance of any filtered substance can be calculated.

If the clearance is more than the GFR then the substance is filtered and then added to by tubular secretion.

The Plasma Creatinine

The concentration of creatinine in the plasma depends upon two factors.

(1) The rate of its production from muscle, which depends upon the lean body mass and, at least in the short term, is constant from day to day.

(2) The GFR which determines its rate of elimination by the kidney.

Diet may have some effect upon plasma creatine levels, but is small enough to be ignored. For a normal person, therefore, the plasma concentration reflects the GFR.

Thus the plasma creatinine is predictably related to the GFR unless there are major changes in muscle mass. Measurement of plasma creatinine is therefore a good indicator of GFR.

If the GFR falls by half, the plasma creatine will double. If the plasma creatinine is five times the baseline, then GFR is one fifth of normal.

For this reason in many clinical settings it is simpler merely to follow the plasma creatinine levels rather than to add the complication of clearance measurements.

In very small or very muscular individuals or where body weight is changing, then the added accuracy of a measured GFR may be necessary.

U creatinine = 100 mg/dl

V = 1440 min/day

P creatinine = 1.0 mg/dl

GFR (ml/min) = (100 mg/dl) ^. (1440 min/day) = 100 ml/min.

(1.0 mg/dl) ^. (1440 min/day)

Blood Urea

The blood urea is used much less as a measure of renal function because

(1) It varies with the dietary protein intake.

(2) it is reabsorbed in the tubules and is therefore not a glomerular substance. This results in a falsely low estimate of GFR.

3) The reabsorption varies with urine flow and its clearance becomes completely independent of the GFR at low urine flow rates.

However, both of these are approximations with significant potential for error. The limitations of creatinine and urea in assessing GFR are summarized in the table:

Table 4: Creatinine And Urea As Markers Of Renal Function

	Plasma Creatinine	Blood Urea Nitrogen
Production	Constant if muscle mass constant	Variable: protein intake, liver production, catabolic rate
Filtration	Complete	Complete
Reabsorption	None	40-60% depending on volume status, antidiuretic hormone
Secretion	< 10% with normal renal function.	None

MECHANISMS OF PROGRESSION OF RENAL DISEASE

Figure 20:

Progression to end stage renal disease (ESRD) is a common event in chronic renal failure (CRF) once the plasma creatinine (Pcr) exceeds 1.5-2.0 mg/dl. The rate of progression is relatively uniform in many patients, with the GFR declining in a linear pattern with time (figure 20). This figure looks at chronic pyelonephritis (CPN), medullary cystic disease (MKD) and chronic glomerulonephritis (CGN). In certain other disorders, such as diabetic nephropathy and untreated HIV nephropathy, the slope is sharper reflecting a more rapid decline.

As depicted, the GFR can be simply estimated from the reciprocal of the Pcr (1/Pcr). Remember the formula for the creatinine clearance (Ccr) = Ucr x urine flow rate/Pcr. In the steady state the urinary cr excretion (Ucr x urine flow rate) equals cr production from muscle, and therefore the Ccr or GFR varies with reciprocal of Pcr.

The factors responsible for progressive renal failure are not fully understood. Risk factors appear to be the severity of the primary disease and complications such as uncontrolled hypertension, urinary tract infection, obstruction, or the intrarenal deposition of calcium phosphate. However, even when these factors are controlled as with surgical correction of vesicoureteral reflux and prevention of infection, many disorders continue to progress to ESRD. Thus it has been suggested that after a critical point, a reduction in functioning nephron number eventually results in failure of the more normal nephron units. In fact, experimental and clinical evidence indicates that these events appear to be a predictable result of the glomerular hemodynamic response to widespread renal injury. These observations form the basis for current clinical studies that are attempting to reverse these hemodynamic changes and thus slow the progression to ESRD.

Glomerular Hyperperfusion and Progression of Experimental Renal Disease:

The simplest experimental model to study the hemodynamic response to reduce nephron number is provided by surgical nephrectomy were there is an increase in the single nephron glomerular filtration rate (SNGFR) in the remaining nephrons. Dilatation of the afferent and to a lesser degree efferent glomerular arterioles is responsible for this hyperfiltration by allowing an increase in both glomerular plasma flow and hydrostatic pressure. The magnitude of the elevation in remnant nephron GFR correlates with the amount of renal mass removed. Thus, the remnant nephron GFR in rats increases by about 40 – 50% with uninephrectomy and to more than twice normal with 80% surgical ablation.

The enhanced filtration by remnant nephrons is an adaptive response to nephron loss that serves to counteract the fall in renal function. However, the hemodynamic changes responsible for remnant nephron hyperfiltration ultimately results in glomerular injury and loss of nephrons. The remnant glomeruli undergoes a series of histologic changes consisting of glomerular hypertrophy, fusion of epithelial foot processes, and expansion of the mesangium. The latter leads to collapse of capillary lumens and areas of glomerular sclerosis. The pace of this injury is determined by the degree of nephron loss. Uninephrectomy results in moderate acceleration of the glomerular sclerosis normally seen in aging rates with two kidneys. However, this injury is seen within 2 weeks after 90 to 95% nephrectomy.

Table 5: Renal Hemodynamics In Normal And Nephrectomized Rats

Condition Single Nephron Glomerular Plasma Glomerular Hydrostatic

GFR, nL/min Flow, nL/min Pressure, mmHg

Normal 28 74 49

90 % nephrectomy 63 187 63

90 % nephrectomy 38 92 46

+ protein restriction

The mechanism responsible for the glomerular injury appears to involve increased capillary pressure and/or flow. Table 5 shows that with 90% nephrectomy the SNGFR, glomerular plasma flow and glomerular hydrostatic pressure increase significantly. If the same procedure is performed in rats receiving a low protein diet (25% of normal), a much lower increase in these glomerular variables occurs and the rats develop less glomerular morphologic changes and lower rates of proteinuria.

Figure 21:

The effects of intermittent (dashed lines) and ad libitum high-protein feeding (solid lines) on whole kidney and single nephron function are illustrated in figure 21. After ingestion of each meal, vasodilator mechanisms associated with protein feeding combine with diet-induced extracellular fluid volume expansion to increase the determinants of single nephron filtration (glomerular plasma flow and glomerular capillary pressure), and total RBF and GFR. The ad libitum ingestion of small but frequent protein containing meals is more in line with our present diets.

Similar responses to nephron loss are seen when the number of functioning nephrons is reduced by an active chronic disease such as experimental glomerulonephritis. The remnant glomeruli undergo the same increases in pressure and filtration rate that ultimately result in injury and further nephron loss. Again, if these rats are given a low protein diet, the progression of immunologic and diabetic glomerular disease in rats and the lupus-like nephropathy of the NZB/NZW mouse are ameliorated by the lowering of the intraglomerular pressure.

There is evidence that factors other than dietary protein may also protect against progression in CRF. Nephron loss leads to an increase in many tubular functions in these same nephrons where there is an increase in filtration rate. One such change is the rise in ammonia production by the nephron in an attempt to excrete the daily acid load. The local accumulation of ammonia can then activate the alternate complement pathway, leading to tubulointerstitial damage and worsening renal function. Lowering the acid load by administering sodium bicarbonate will lower the need to produce ammonia and minimize the tubulointerstitial injury. Since the daily acid load is generated mainly from protein metabolism, the protective effect of dietary protein restriction could be mediated in part by a reduction in ammoniagenesis.

Role of Systemic Hypertension:

Systemic hypertension, either primary or superimposed upon underlying renal disease, can cause glomerular injury as the pressure is transmitted to the glomerulus. In animal models of primary hypertension associated with renal vasoconstriction, as in the spontaneously hypertensive rat, there is little change in glomerular hemodynamics and little glomerular injury. However, when hypertension is associated with a normal or reduced renal vascular resistance, intraglomerular pressure and flow will increase resulting in glomerular sclerosis. Low protein diets can prevent or minimize this injury in the rat. Systemic hypertension may be more damaging when superimposed upon underlying renal disease where the compensatory mechanisms involve renal vasodilatation in the remaining nephrons. In these instances, there is afferent arteriolar dilatation, which now permits the hypertension to be transmitted directly to the glomerulus. Clinical correlates to these conditions are patients with immune mediated chronic glomerulonephritis and patients with insulin-dependent diabetes mellitus. In the latter, the renal vasodilatation results in an elevated GFR early in the disease state.

Another example of where these mechanisms can worsen the remnant nephrons is in hypertension induced by partially occluding on renal artery. This condition, termed renovascular hypertension or renal artery stenosis, results in glomerular injury in the remaining kidney that is perfused at a higher pressure. The renal vasodilatation in the contralateral kidney allows the hypertension to be directly transmitted to the glomeruli.

Figure 22:

The dependence of intrarenal pressure and flow on systemic blood pressure has important therapeutic implications. In animals with subtotal nephrectomy, diabetes mellitus, or glomerulonephritis, reducing the systemic BP returns intrarenal hemodynamics toward normal and minimizes the degree of glomerular injury. One such group of drugs, angiotensin converting enzyme inhibitors (ACE) such as enalapril or captopril, are particularly effective in lowering the glomerular capillary pressure by reversing the angiotensin II-mediated constriction of the efferent (postglomerular) arteriole (figure 22). These act to inhibit the conversion of angiotensin I to II. The net effect is that proteinuria and glomerular sclerosis are markedly diminished.

Clinically, other antihypertensives such as calcium channel blockers have been shown to curtail the progression of hypertensive renal failure. However, this is not seen in antihypertensive drug combinations (hydrochlorothiazide-reserpine-hydralazine) that simultaneously dilate the preglomerular afferent arteriole, allowing more of the systemic BP to be transmitted to the glomerulus. Despite a reduction of systemic BP, this does not provide the needed protection in the glomerulus.

Corticosteroids also produce renal vasodilatation and raise the intraglomerular pressure. This deleterious effect of chronic, high dose corticosteroid therapy could contribute to the diminished effectiveness of this regimen in comparison to cyclophosphamide therapy in patients with diffuse proliferative lupus nephritis.

Figure 23:

Mechanism of Hemodynamic Injury:

ACE inhibitors decrease RVR by dilating the efferent arteriole and can increase glomerular capillary permeability. As a result, the glomerular capillary pressure is reduced to near normal, but there are persistent elevations in glomerular plasma flow (due to renal vasodilatation) and filtration rate (due to increased water permeability). Preferential lowering of the intraglomerular pressure also occurs with dietary protein restriction although this response is mediated by afferent arteriolar constriction rather than efferent vasodilatation. These findings suggest that the elevation in glomerular capillary pressure and not hyperfiltration itself, is of primary importance in hemodynamically mediated glomerular injury. Increased glomerular plasma flow alone does not appear to be deleterious, but it may exacerbate the effect of intraglomerular hypertension.

Figure 23 illustrates the role of sustained increments in glomerular pressure and flows in the initiation and progression of glomerular sclerosis. Glomerular hypertension may act by causing mechanical disruption of normal vascular integrity and development of glomerular sclerosis. Proteinuria ensues with loss of both size and change-selective properties of the glomerular basement membrane. The combined effects of increased pressure and flow also enhance the transglomerular movement of macromolecules (albumin). There is increased mesangial deposition of macromolecules leading to mesangial expansion and sclerosis. The loss of nephrons from this process places the remaining ones at risk for subsequent disease.

Mechanism of Hemodynamic Changes:

The mechanism by which reduction in renal mass leads to structural and functional hypertrophy of the remaining nephrons is not fully understood. Uninephrectomy in the rat results in increased renal RNA and protein synthesis within hours, and kidney size and weight begin to increase within 1-2 days. Both the glomeruli and tubules hypertrophy and SNGFR increases due to arteriolar dilatation. By 2 weeks the whole kidney GFR and kidney weight rise by 40%.

In humans who have donated a kidney, the remaining kidney GFR and RBF increase by 40% within several weeks such that the total GFR is 70% of the prenephrectomy level. Factors responsible for this hypertrophy include age of the patient, with the best growth seen in younger patients, necessity to excrete protein metabolites, androgens, pituitary hormones (growth hormone), and other undefined growth factors.

Studies of the effect of protein intake on renal function in normal humans have shown that chronically changing from a low to a normal protein diet can raise the GFR by 15 to 20 ml/min or more. An acute protein load can raise GFR by 35 to 40 ml/min but this effect disappears once function is < 50%. This is probably due to the already maximally compensated increase in GFR in CRF. The mechanism of protein-mediated increases in GFR involves amino acids. The increase in GFR with meals facilitates the excretion of protein metabolites and other waste solutes.

Progression of Human Renal Disease:

Progressive loss of renal function, proteinuria, and glomerular sclerosis has been seen in humans with a variety of renal disorders, even after correction of the primary abnormality. Studies of renal transplant donors followed for > 10-15 years, have shown that the GFR is unchanged but there is an increased incidence of proteinuria and hypertension.

Implications for Therapy:

Probably the most important and proven therapeutic modality is control of systemic HT. Preliminary data suggests that ACE inhibitors may be effective by both lowering the systemic HT and lowering the intraglomerular hypertension by selectively dilating the efferent arteriole. Furthermore, ACE inhibitors and specifically captopril, may have other effects on matrix production thereby limiting sclerosis and progression of renal disease.

In diabetics, strict glucose control has also been shown to curtail the progression of nephropathy as measured by microproteinuria. This was true in patients with existing renal insufficiency as well.

Reversing intraglomerular hypertension may delay or prevent progressive renal disease in chronic glomerulonephritis, pyelonephritis, and polycystic kidney disease. One such intervention is dietary protein restriction. Clinical studies have been at best suggestive but not conclusive and the positive effects may be a result of reduction of phosphates and acidosis. The big concern in patients with renal insufficiency is that dietary restrictions may lead to harmful malnutrition. Recommended restriction of protein is 0.6 g/Kg of high biologic value protein per day and 700 mg of phosphorus, or 20 to 30g of protein supplemented with amino acids and their keto analogues). Presently it is suggested to assess the patient’s nutritional status prior to implementation of dietary restrictions. Limiting protein intake is less effective when the Pcr is > 8 mg/dl, if there is hypertension, or if nephrosclerosis is present.

PREGNANCY

Functional Changes

Pregnancy produces dramatic changes in volume and composition of the body fluids beginning very early in human pregnancy.

The plasma volume, the red cell mass and the albumin mass all increase. The increase in plasma volume (up 40%) is greatest and produces a dilutional fall in the hematocrit and the plasma albumin concentration. Total body volume increases between 6 and 8 liters.

The latter is one of the factors which lowers the threshold for edema in the pregnant state. Another factor is the effect of a large uterus compressing the veins in the pelvis thus raising venous pressure in the legs and encouraging ankle swelling.

Pregnancy has marked effects upon renal blood flow and glomerular filtration rate. Both rise very early in pregnancy with the GFR increasing up to 50% by the end of the first trimester (figure 24).

As a result, plasma concentration of components handled by filtration falls; for example normal serum creatinine in pregnancy is about 40-50 mmol/L.

Increased filtration is one of the factors causing glycosuria of pregnancy because an increased filtered load of glucose may exceed the tubular reabsorptive capacity.

Blood volume, cardiac output (up 30 – 40%) and peripheral resistance are all changed, and part of the weight gain in pregnancy is related to retention of sodium, which totals up to 800 mmol during the course of gestation. Volume changes might be expected to have an impact upon blood pressure however due to decreased peripheral resistance and decreased sensitivity to angiotensin II, blood pressure actually is mildly decreased (figure 25).

All these factors change in pregnancy. It is perhaps astonishing that, in spite of all these changes, the blood pressure and its minute-to-minute regulation remain stable in normal pregnancy.

Figure 24: Figure 25: