CLEARANCE UNITS

The units of clearance are often confusing to the first-time reader, so let us be sure of the meaning. First, the units are volume per time (not amount per time). The easiest way to think of this is to ask what volume of plasma contains the amount excreted in a given time. Suppose 5 mg of a substance is excreted per hour and 200 mL of plasma contains 5 mg. Then the clearance of the substance is 200 mL/h, ie, 200 mL of volume has been completely cleared of the substance. Again, note the units: volume per time (ie, the volume of plasma from which a substance is completely removed [cleared]).
The general meaning and specific renal meaning of clearance can be illustrated by comparing how the body handles 2 substances with similar-sounding names but very different properties: insulin and inulin. Insulin is the familiar pancreatic hormone involved in regulating blood sugar (glucose). It is a protein with a molecular weight of 5.8 kDa and is small enough to be freely filtered by the glomerulus. Once in Bowman’s space, it moves along with every other filtered substance into the proximal convoluted tubule, where it is largely taken up by endocytosis and degraded into its constituent amino acids. Very little insulin escapes this uptake, and very little of the filtered insulin survives to be excreted in the urine. Thus, the kidney takes part in clearing insulin from the blood; however, because so little appears in the urine, the specific renal clearance is very low (<1 mL/min). However, the body has additional mechanisms for clearing insulin, and its metabolic clearance rate is quite high (half-life less than 10 min). Let us contrast this with inulin. Inulin is a polysaccharide starch of about 5 kDa molecular weight. Like insulin, it is freely filtered by the glomerulus, but it is not reabsorbed or secreted by the nephron. All the inulin that is filtered flows through the nephron and appears in the urine. Thus, inulin’s renal clearance is relatively large. Inulin in the blood is not taken up by other tissues, and the kidneys are the only excretion route. As we will see, this makes inulin a very special substance with respect to assessing renal function.

Clearance

One of the crucial functions of the kidneys is to remove metabolic wastes and excess amounts of ingested substances from the blood. Nitrogenous wastes such as urea and creatinine are generated by metabolism and removed from the body by the kidneys. The average excretion rate normally reflects the rate at which these substances are added to the blood by metabolic processes, thus keeping the body in balance for these substances. Other substances, those that are crucial for normal function, including sodium and potassium, enter the body by ingestion and are also removed from the body by the kidneys. These excretion rates parallel ingestion rate in the long term but are altered transiently to reflect other needs, such as the regulation of blood pressure or plasma osmolality.
Ridding the body of a substance is often called clearance. This term in a biomedical context has both a general meaning and a specific renal meaning. The general meaning of clearance is simply that a substance is removed from the blood by any of several mechanisms. For instance, a drug may be cleared by excretion in the urine or the feces, or it may be transformed by the liver and other peripheral tissues to an inactive form. Renal clearance, on the other hand, means that the substance is removed from the blood and excreted in the urine.

AUTOREGULATION

It is extremely important for the kidneys to keep the GFR at a level appropriate for the body because, as we have emphasized, the excretion of salt and water is strongly influenced by the GFR. We have also emphasized that the GFR is strongly influenced by renal arterial pressure. A rise in blood pressure causes an increased excretion of salt and water, a process called pressure natriuresis, whereas a fall in blood pressure diminishes excretion. These changes in excretion are mediated partly via changes in GFR. The effect is so strong that urinary excretion would tend to vary widely with the ordinary daily excursions of arterial pressure. Also, vascular pressure in the thin-walled glomerular capillaries is higher than in capillaries elsewhere in the body and hypertensive damage ensues if this pressure is too high.
Therefore, both to protect the glomerular capillaries from hypertensive damage and to preserve a healthy GFR, changes in GFR and RBF are severely blunted by mechanisms that we collectively call autoregulation. Consider first a situation in which mean arterial pressure rises 20%. Such a modest rise occurs many times throughout the day in association with changes in excitement level and activity. Pretend, for the moment, that all renal vascular resistances remain constant. By the basic flow equation (Q = ΔP/R), RBF would rise 20% also (actually slightly more if pressure in the renal vein is unaffected). What would this do to GFR? It would rise much more than 20%, in fact almost 50%. This is because net filtration pressure would rise almost 50%. In effect, fractional changes in upstream pressure (in the renal artery) are magnified in terms of net filtration pressure. Why is this? At the beginning of the glomerulus, capillary hydrostatic pressure is about 60 mm Hg and the pressures opposing filtration sum to 36 mm Hg, yielding a net filtration pressure of about 24 mm Hg. With an increase in arterial pressure to 120 mm Hg, capillary pressure would rise to about 71 m Hg, but there would be no increase in the pressures that oppose filtration–plasma oncotic pressure and Bowman’s capsule pressure. Therefore, net filtration pressure would rise to 71 – 36 = 35 mm Hg (an increase of almost 50%). The higher net filtration pressure would cause a parallel increase in GFR. (In turn, this would raise plasma oncotic pressure at the distal end of the glomerulus, tending to reduce filtration somewhat, but the total effect is still a major rise in GFR.) This emphasizes the crucial role of glomerular capillary pressure in glomerular filtration.
Now, what actually happens in the face of changes in mean arterial pressure? As is the case in many organs, blood flow does not change in proportion to changes in arterial pressure. The changes are blunted. A rise in driving pressure is counteracted by a rise in vascular resistance that almost offsets the rise in pressure. The word “almost” is crucial here. Higher driving pressures do indeed lead to higher flow but not proportionally. Within the range of mean arterial pressures commonly found in the human body4), RBF varies only modestly when mean arterial pressure changes. This is partly a result of a direct reaction of the vascular smooth muscle to stretch or relaxation—or the myogenic response—and partly the result of intrarenal signals that we describe shortly. The myogenic response is very fastacting and protects the glomeruli from short-term fluctuations in blood pressure. In addition to keeping changes in RBF fairly small, autoregulatory processes also keep changes in GFR fairly small. Again, GFR does rise with an increase in arterial pressure, just not substantially.

Autoregulation of renal blood flow (RBF). A similar pattern holds for glomerular filtration rate.

How do the intrarenal processes work? Much is by way of a process with the clumsy name of tubuloglomerular feedback. Tubuloglomerular feedback is feedback from the tubules back to the glomerulus (ie, an influence of events in the tubules that is exerted on events in the glomeruli). But for now the essence of tubuloglomerular feedback can be summarized as follows: As the filtration rate in an individual nephron increases or decreases, the amount of sodium that escapes reabsorption in the proximal tubule and the loop of Henle also increases or decreases. More sodium filtered means more sodium remaining in the lumen of the nephron and more sodium flowing from the thick ascending limb into the distal tubule. Recall that at the division between these nephron segments lies the macula densa, a special group of cells in the nephron wall where the nephron passes between the afferent and efferent arterioles. The macula densa cells sense the amount of sodium and chloride in the lumen. They act, in part, as salt detectors. One result of changing levels of luminal sodium chloride is to increase or decrease the secretion of transmitter agents into the interstitial space that affect the filtration in the nearby glomerulus. High levels of sodium flowing past the macula densa cause a decrease in filtration rate; low levels of sodium flowing past allow a higher filtration rate. It is as though each nephron adjusts its filtration so that the right amount of sodium remains in the lumen to flow past the macula densa. How can it adjust its filtration? The transmitter agents released by the salt-sensing macula densa cells produce vasoconstriction of the afferent arteriole, thereby reducing hydrostatic pressure in the glomerular capillaries. These same agents also produce contraction of glomerular mesangial cells, thereby reducing the effective filtration coefficient. Both processes reduce the single-nephron filtration rate and keep it at a level appropriate for the rest of the nephron.
In conclusion, we emphasize that autoregulation blunts or lowers the RBF and GFR responses to changes in arterial pressure but does not totally prevent those changes.

Filtered Load

A term we use in other chapters is filtered load. It is the amount of substance that is filtered per unit time. For freely filtered substances, the filtered load is just the product of GFR and plasma concentration. Consider sodium. Its normal plasma concentration is 140 mEq/L, or 0.14 mEq/mL. (Note: 1 mEq of sodium is 1 mmol.) A normal GFR is 125 mL/min, so the filtered load of sodium is 0.14 mEq/mL × 125 mL/min = 17.5 mEq/min. We can do the same calculation for any other substance, being careful in each case to be aware of the unit of measure in which concentration is expressed. The filtered load is what is presented to the rest of the nephron to handle. A high filtered load means a substantial amount of material to be reabsorbed. The filtered load varies with plasma concentration and GFR. A rise in GFR, at constant plasma concentration, increases the filtered load, as does a rise in plasma concentration at constant GFR.

Direct Determinants of GFR

Variation in GFR is a crucial determinant of renal function.
Everything else being equal, a higher GFR means greater excretion of salt and water. Regulation of the GFR is straightforward in terms of physical principles but very complex functionally because there are so many regulated variables. The rate of filtration in any of the body’s capillaries, including the glomeruli, is determined by the hydraulic permeability of the capillaries, their surface area, and the net filtration pressure (NFP) acting across them.

Rate of filtration 5 hydraulic permeability × surface area × NFP

Because it is difficult to estimate the area of a capillary bed, a parameter called the filtration coefficient (Kf ) is used to denote the product of the hydraulic permeability and the area. The net filtration pressure is the algebraic sum of the hydrostatic pressures and the osmotic pressures resulting from protein—the oncotic or colloid osmotic pressures —on the 2 sides of the capillary wall. There are 4 pressures to contend with: 2 hydrostatic pressures and 2 oncotic pressures. These are referred to as Starling forces, named after the physiologist who first described them. Applying this to the glomerular capillaries:

NFP = (PGC - πGC) - (PBC - πBC),

where PGC is glomerular capillary hydraulic pressure, πBC is oncotic pressure of fluid in Bowman’s capsule, PBC is hydraulic pressure in Bowman’s capsule, and πGC is oncotic pressure in glomerular capillary plasma.
Because there is normally little protein in Bowman’s capsule, πBC may be taken as zero and not considered in our analysis. Accordingly, the overall equation for GFR becomes

GFR = Kf (PGC − PBC − πGC).

Net filtration pressure in the renal corpuscle equals glomerular-capillary hydraulic pressure (PGC) minus Bowman’s capsule hydraulic pressure (PBC) minus glomerularcapillaryoncotic pressure (πGC).

Estimated forces involved in glomerular filtration in humans (these are the same values shown in Table 2–1). Net filtration pressure (NFP) = PGC − πGC − PBC.

Note that the hydraulic pressure changes only slightly along the glomeruli; this is because the very large total cross-sectional area of the glomeruli collectively provides only a small resistance to flow. Importantly, note that the oncotic pressure in the glomerular capillaries does change substantially along the length of the glomeruli. Water is moving out of the vascular space and leaving protein behind, thereby raising protein concentration and, hence, the oncotic pressure of the unfiltered plasma remaining in the glomerular capillaries. Mainly because of this large increase in oncotic pressure, the net filtration pressure decreases from the beginning of the glomerular capillaries to the end. The average net filtration pressure over the whole length of the glomerulus is about 17 mm Hg. This average net filtration pressure is higher than found in most nonrenal capillary beds. Along with a high value for Kf , it accounts for the enormous filtration of 180 L of fluid/ day (compared with 3 L/day or so in all other capillary beds combined).
As we have noted, the GFR is not fixed but shows marked fluctuations in differing physiological states and in disease. If all other factors remain constant, any change in Kf , PGC, PBC, or πGC will alter GFR. However, “all other factors” do not always remain constant, and so other simultaneous events may oppose the effect of any one factor. To grasp this situation, it is essential to see how a change in any one factor affects GFR under the assumption that all other factors are held constant.
Table 2–2 presents a summary of these factors. It provides, in essence, a checklist to review when trying to understand how diseases or vasoactive chemical messengers and drugs change GFR. In this regard, it should be noted that the major



cause of decreased GFR in renal disease is not any change in these parameters within individual nephrons but rather simply a decrease in the number of functioning nephrons.

Kf
Changes in Kf can be caused by glomerular disease and drugs, but this variable is also subject to normal physiological control by a variety of chemical messengers. The details are still not completely clear, but these messengers cause contraction of glomerular mesangial cells. Such contraction may restrict flow through some of the capillary loops, effectively reducing the area available for filtration and, hence, Kf. This decrease in Kf will tend to lower GFR.

PGC
Hydrostatic pressure in the glomerular capillaries (PGC) is the most complex of the variables in the basic filtration equation because it is itself influenced by so many factors. We can help depict the situation by using the analogy of a leaking garden hose. If pressure feeding the hose (pressure in the pipes leading to the faucet) goes up or down, this directly affects pressure in the hose and, hence, the rate of leak. Resistances in the hose also affect the leak. If we kink the hose upstream from the leak, pressure at the region of leak falls, and less water leaks out. However, if we kink the hose beyond the leak, this raises pressure at the region of leak and increases leak rate. These same principles apply to PGC and GFR. First, a change in renal arterial pressure will cause a change in PGC in the same direction. If resistances remain constant, PGC will rise and fall as renal artery pressure rises and falls. This is a crucial point because a major regulator of renal function is arterial blood pressure. Second, changes in the resistance of the afferent and efferent arterioles have opposite effects on PGC. An increase in resistance upstream from the glomerulus in the afferent arteriole (like kinking the hose above the leak) will lower PGC, whereas an increase in resistance downstream from the glomerulus in the efferent arteriole (like kinking the hose beyond the leak) will increase PGC. In contrast, a decrease in afferent resistance (RA) (resulting from afferent arteriolar dilation) will tend to raise PGC. Similarly, a decrease in efferent resistance (RE) (caused by efferent arteriolar dilation) tends to lower PGC. It should also be clear that when RA and RE both change simultaneously in the same direction (ie, both increase or decrease), they exert opposing effects on PGC.
It is possible for both resistances to rise by the same fraction, with the result that there is no effect on PGC (even though, in this case, RBF would fall). In contrast, when they change in different directions, they cause additive effects on PGC (and can have no effect on RBF). The real significance of this is that the kidney can regulate PGC and, hence, GFR independently of RBF. The effect of changes in RA and RE.

PBC
Changes in this variable generally are of very minor physiological importance. The major pathological cause of increased hydraulic pressure in Bowman’s capsule is obstruction anywhere along the tubule or in the external portions of the urinary system (eg, the ureter). The effect of such an occlusion is to increase the tubular pressure everywhere proximal to the occlusion, all the way back to Bowman’s capsule. The result is to decrease GFR.

Effects of afferent- and/or efferent-arteriolar constriction on glomerular capillary pressure (PGC) and renal blood flow (RBF). The RBF changes reflect changes in total renal arteriolar resistance, the location of the change being irrelevant. In contrast, the changes in PGC are reflected in which set of arterioles the altered resistance occurs. Pure afferent constriction lowers both PGC and RBF, whereas pure efferent constriction raises PGC and lowers RBF. Simultaneous constriction of both afferent and efferent arterioles has counteracting effects on PGC but additive effects on RBF; the effect on PGC may be a small increase, small decrease, or no change. Vasodilation of only 1 set of arterioles would have effects on PGC and RBF opposite those shown in parts B and C. Vasodilation of both sets would cause little or no change in PGC, the same result as constriction of both sets but would cause a large increase in RBF. Constriction of 1 set of arterioles and dilation of the other would have maximal effects on PGC but little effect on RBF.

πGC
Oncotic pressure in the plasma at the very beginning of the glomerular capillaries is, of course, simply the oncotic pressure of systemic arterial plasma. Accordingly, a decrease in arterial plasma protein concentration, as occurs, eg, in liver disease, will lower arterial oncotic pressure and tend to increase GFR, whereas increased arterial oncotic pressure will tend to reduce GFR.
However, now recall that πGC is identical to arterial oncotic pressure only at the very beginning of the glomerular capillaries; πGC then progressively increases along the glomerular capillaries as protein-free fluid filters out of the capillary, concentrating the protein left behind. This means that net filtration pressure and, hence, filtration progressively decrease along the capillary length. Accordingly, anything that causes a steeper rise in πGC will tend to lower average net filtration pressure and hence GFR.
This steep increase in oncotic pressure tends to occur when RPF is low. It should not be hard to visualize that the filtration of a given volume of fluid from a small total volume of plasma flowing through the glomeruli will cause the protein left behind to become more concentrated than if the total volume of plasma were large. In other words, a low RPF, all other factors remaining constant, will cause the πGC to rise more steeply and reach a final value at the end of the glomerular capillaries that is higher than normal. This increase in average πGC along the capillaries lowers average net filtration pressure and, hence, GFR. Conversely, a high RPF, all other factors remaining constant, will cause πGC to rise less steeply and reach a final value at the end of the capillaries that is less than normal, which will increase the GFR.
Another way of thinking about this is in terms of filtration fraction: the ratio GFR/RPF. The increase in πGC along the glomerular capillaries is directly proportional to the filtration fraction (ie, the more volume that is filtered from plasma, the higher is the rise in πGC). Therefore, if you know that filtration fraction has changed, you can be certain that there has also been a proportional change in πGC and that this has played a role in altering GFR.

Formation of Glomerular Filtrate

As stated in Chapter 1, the glomerular filtrate is nearly protein-free2 and contains most inorganic ions and low-molecular-weight organic solutes in virtually the same concentrations as in the plasma.
In order to form a glomerular filtrate, filtered fluid must pass through the glomerular filtration barrier. The filtration barrier separates the blood from the urinary space that topologically connects to the outside world via the renal tubules, ureters, bladder, and urethra. The route that filtered substances takes from the blood through the filtration barrier of a renal corpuscle into Bowman’s space is a 3-step process: through fenestrae in the glomerular-capillary endothelial layer, through the basement membrane, and finally through slit diaphragms between podocyte foot processes. The fraction of endothelial surface area occupied by fenestrae is about 10%. Both the slit diaphragm and basement membrane are composed of an array of proteins, and while the basement membrane may contribute to selectivity of the filtration barrier, integrity of the slit diaphragms is essential to prevent excessive leak of plasma protein (albumin). Some protein-wasting diseases are associated with abnormal slit diaphragm structure. Selectivity of the barrier to filtered solute is based on both molecular size and electrical charge. Let us look first at size.
The filtration barrier of the renal corpuscle provides no hindrance to the movement of molecules with molecular weights less than 7000 Da (ie, solutes this small are all freely filtered). This includes all small ions, glucose, urea, amino acids, and many hormones. The filtration barrier almost totally excludes plasma albumin (molecular weight of approximately 66,000 Da). (We are, for simplicity, using molecular weight as our reference for size; in reality, it is molecular radius and shape that is critical.) The hindrance to plasma albumin is not 100%, however, and so the glomerular filtrate does contain extremely small quantities of albumin, on the order of 10 mg/L or less. This is only about 0.02% of the concentration of albumin in plasma and is the reason for the use of the phrase “nearly protein-free” earlier. (Note: Some small substances are partly or mostly bound to large plasma proteins and are thus not free to be filtered, even though, when not bound to plasma proteins, they can easily move through the filtration barrier. This includes hydrophobic hormones of the steroid and thyroid categories and about 40% of the calcium in the blood.)
For molecules with a molecular weight ranging from 7000 and 70,000 Da, the amount filtered becomes progressively smaller as the molecule becomes larger. Thus, many normally occurring plasma peptides and small proteins are filtered to a significant degree. Moreover, when certain small proteins not normally present in the plasma appear because of disease (eg, hemoglobin released from damaged erythrocytes or myoglobin released from damaged muscles), considerable filtration of these may occur.
Electrical charge is the second variable determining filterability of macromolecules. For any given size, negatively charged macromolecules are filtered to a lesser extent, and positively charged macromolecules to a greater extent, than neutral molecules. This is because the surfaces of all the components of the filtration barrier (the cell coats of the endothelium, the basement membrane, and the cell coats of the podocytes) contain fixed polyanions, which repel negatively charged macromolecules during filtration. Because almost all plasma proteins bear net negative charges, this electrical repulsion plays a very important restrictive role , enhancing that of purely size hindrance. (For example, when neutral dextrans, the same size as plasma albumin, are administered to experimental animals, they are found to be 5–10% filterable rather than albumin’s 0.02%.) In other words, if albumin were not charged or the filtration barrier were not charged, even albumin would be filtered to a considerable degree. Certain diseases that cause glomerular capillaries to become “leaky” to protein do so by eliminating negative charges in the membranes.
It must be emphasized that the negative charges in the filtration membranes act as a hindrance only to macromolecules, not to mineral ions or low-molecularweight organic solutes. Thus, chloride and bicarbonate ions, despite their negative charge, are freely filtered.

FLOW, RESISTANCE, AND PRESSURE IN THE KIDNEYS

The basic equation for blood flow through any organ is as follows:

Q=ΔP/R,

where Q is organ blood flow, ΔP is mean pressure in the artery supplying the organ minus mean pressure in the vein draining that organ, and R is the total vascular resistance in that organ. Resistance is determined by the blood viscosity and the lengths and radii of the organ’s blood vessels, the arteriolar radii being overwhelmingly the major contributor. As described by Poiseiulle’s law, resistance of a cylindrical vessel varies inversely with the fourth power of vessel radius. It takes only a 19% decrease or increase in vessel radius to double or halve vessel resistance. The radii of arterioles are regulated by the state of contraction of the arteriolar smooth muscle.
The presence of 2 two sets of arterioles (afferent and efferent) and 2 sets of capillaries (glomerular and peritubular) makes the vasculature of the cortex unusual. (The vasculature of the medulla is even more unusual, but we concentrate on the cortex for now.) Normally, the resistances of the afferent and efferent arterioles are approximately equal and account for most of the total renal vascular resistance. Resistance in arteries preceding afferent arterioles (ie, cortical radial arteries) plays some role also, but we concentrate on the arterioles. Vascular pressures (ie, hydrostatic or hydraulic pressure) in the 2 capillary beds are quite different. The peritubular capillaries are downstream from the efferent arteriole and have a lower hydraulic pressure. Typical glomerular pressures are near 60 mm Hg in a normal unstressed individual, whereas peritubular pressures are closer to 20 mm Hg. The high glomerular pressure is crucial for glomerular filtration, whereas the low peritubular capillary pressure is equally crucial for the tubular reabsorption of fluid.
To repeat, total RBF is determined mainly by the mean pressure in the renal artery and the contractile state of the smooth muscle of the renal arterioles of the cortex. Now for a simple but very important point: A change in arteriolar resistance produces the same effect on RBF regardless of whether it occurs in the afferent arteriole or efferent arteriole. Because these vessels are in series, a change in either one has the same effect on the total. When the 2 resistances both change in the same direction, the most common state of affairs, their effects on RBF will be additive. When they change in different directions— one resistance increasing and the other decreasing—they exert opposing effects on RBF. We see in the next section that the story is totally different for GFR.