Approximately ______ of fluid enter the nephrons in an average adult per day.
What substances are filtered but are almost completely reabsorbed?
What is an advantage and a disadvantage of non-selective filtration?
Any substance that is just filtered (not reabsorbed or secreted) will be excreted in precisely the amount that is filtered. If the substance is not water, then it will end up concentrated in urine (relative to plasma) because of the extensive water reabsorption along the renal tubules. The advantage of filtration is that no specific transport system is needed for something to be eliminated from the body - just allow it to be filtered and do not reabsorb it. The disadvantage of essentially nonselective filtration is that the body must expend considerable energy to reabsorb filtered substances that it needs to retain.
What are the typical values for the following parameters?:
Renal blood flow (RBF)
Renal plasma flow (RPF)
Glomerular filtration rate (GFR)
Urine flow rate
How is RPF calculated?
Renal Blood Flow-RBF 1.1 L/min
Renal Plasma Flow-RPF 625 ml/min
RPF = RBF x (1 – hematocrit). typical hematocrit is ~0.43, so RPF is 1.1x0.57.
Glomerular Filtration Rate-GFR 125 ml/min
Urine Flow Rate 1 ml/min
What is filtration fraction? How is it calculated and what does it mean?
Filtration Fraction: GFR/RPF 20%
Because freely filtered substances enter Bowman’s space along with water, this means that ~20% of all freely filtered solutes (Na, K, glucose, etc.) present in the plasma enter Bowman’s space.
T or F: RBF can be modulated without causing damage to the kidneys.
True. RBF far exceeds what is needed to service the kidney’s own metabolic needs. This means RBF may vary dramatically without compromising kidney cell viability. In other words, RBF can be modulated in response to other physiological needs without kidney cells paying a metabolic price.
What is the difference between glomerular capillaries and most capillaries in the body as it pertains to net filtration and reabsorption along the length of capillary beds?
Explain the differences in values of/changes in values of capillary hydrostatic and pressure colloid oncotic pressure between a between most capillaries in the body and in glomerular capillaries.
The glomerulus has both afferent and efferent arterioles. The renal circulation traverses two capillary beds in series, the glomerular capillaries then the peritubular capillaries. Most capillaries in the body (as you may recall from the Cardiovascular lectures), have a net filtration of fluid at their arterial end and net reabsorption at the venous end. This is not the case in the glomerular capillaries. Fluid is filtered into Bowman’s capsule along the entire length of the glomerular capillaries. Figure 2.1 (attached) compares filtration in skeletal muscle and glomerular capillaries to illustrate this point.
First, note that the average hydrostatic pressure (averaged over the entire X-axis) in the skeletal capillaries is ~25 mm Hg and average hydrostatic pressure in the glomerular capillaries (PGC) is ~55 mm Hg. This higher pressure arises from the fact that the afferent arterial diameter is usually larger than that of the efferent arteriole. Second, note that the hydrostatic pressure in skeletal capillaries changes with distance, while hydrostatic pressure in the glomerular capillaries is essentially constant. This is because there is relatively high resistance in skeletal capillaries so pressure falls substantially with distance. There is relatively low resistance in glomerular capillaries so it does not. The reason is that the glomerulus has many (30-50) parallel capillary loops which make resistance to blood flow very low. Third, note that the colloid oncotic pressure (COP) becomes greater with distance along the glomerular capillaries (see sloping line in Figure 2.1, part B). The COP changes with distance along the glomerular capillaries because a large volume of water (~20%) filters out concentrating the proteins/solutes left behind. Remember, fluid is filtered into Bowman’s capsule all along the entire length of the glomerular capillaries. Hydrostatic pressure in Bowman’s space (PBS) is also shown. Note that PBS is low and constant.
What is net filtration pressure (NFP) in glomerular capillaries? How is it calculated?
What is the filtration coefficient for glomerular capillaries? What is it dependent on?
How is GFR calculated? How are glomerular capillaries specialized for filtration?
Glomular filtration rate (GFR) will of course depend on NFP but it will also depend on the hydraulic permeability of the glomerular capillaries as well as their surface area. The filtration coefficient (Kf) is the product of capillary hydraulic permeability (i.e. permeability towater) and capillary surface area. The Kf, GFR and NFP are related as follows, GFR = Kf x NFP
In glomerular capillaries, hydraulic permeability and surface area are large compared to most other capillaries in the body. The hydraulic permeability is bigger because of the endothelial fenestrations of the glomerular capillaries. The surface area is bigger because of the extensive branching/looping of the glomerular capillaries. Consequently, the Kf of glomerular capillaries is high. This combined with a NFP that favors filtration along the entire length of the glomerular capillaries assures a high degree of fluid filtration at the glomerulus. Glomerular filtration amounts to roughly 180 L/day in a normal individual. Net filtration from all other capillaries in the body only amounts to only a few liters per day. The point here is that glomerular capillaries are specialized for filtration. These capillaries are different than those in other parts of the body. The magnitude of fluid filtered in the glomerular capillaries is huge. No reabsorption occurs in glomerular capillaries! The peritubular capillaries are the site of, and are specialized for, reabsorption.
Explain the impact of the following circumstances on GFR.
kidney stone (urinary tract obstruction)
blood pressure (MAP)
renal artery stenosis
starvation (or renal disease)
From the discussion above, it should be clear that anything that alters NFP or Kf will alter GFR (filtration). To illustrate this, some examples are given below:
- A urinary tract obstruction can increase the hydrostatic pressure in Bowman’s space (PBS). This could occur if there is kidney stone blocking a ureter. Building pressure up stream of the blockage may be transmitted up to the nephrons and Bowman’s capsule. The resulting increase in PBC would oppose filtration.
- An alteration in mean arterial pressure (MAP) can change the hydrostatic pressure in the glomerular capillaries (PGC) and thus of course would alter GFR. For example, increased MAP would increase GFR (all else being equal, which it isn’t). A MAPinduced change in GFR may be limited by autoregulation (see below).
- A renal artery stenosis (abnormal narrowing) can reduce PGC and thereby reduce GFR in the affected kidney.
- A reduction in the number of functional nephrons (perhaps due to disease) may result in a reduced Kf and thus reduce GFR. Each kidney contains ~1 million nephrons. Loss of half the nephrons in one kidney would substantially reduce the total surface area of glomerular capillary available for filtration. Remember that Kf is proportional to the surface area over which filtration occurs. Loss of nephrons reduces Kf which translates into reduced GFR.
- The role of the sympathetic nervous system in renal function will be described in more detail later. Mesangial cells sit between capillary loops in the glomerulus. These cells act as phagocytes and remove trapped material from the glomerulus. However, these cells also contain myofilaments and contract in response to simulation, much like vascular smooth muscle cells. Sympathetic activation can reduce both PGC (via arteriole vasoconstriction) and Kf (via mesangial cell contraction which closes down some glomerular capillary loops). These actions, individually or combined, would lead to a reduced GFR.
- Loss of plasma proteins (perhaps due to starvation or renal disease) will reduce the plasma oncotic pressure and, by itself, this will increase GFR (since plasma oncotic pressure opposes filtration).
What is autoregulation?
Is autoregulation intrinsic or extrinsic to the kidney? How may autoregulation be overidden?
List the 2 basic mechanisms of autoregulation.
Interestingly, RBF (and GFR) can remain relatively constant even when arterial blood pressure changes because of renal autoregulation. This autoregulation is an intrinsic property of the kidney and even occurs in kidneys that are isolated, denervated and perfused with blood in a lab. Figure 2.3 (attached) illustrates renal autoregulation showing RBF and GFR as a function of mean arterial blood pressure (MAP) over a wide range of pressures. Normal MAP (as you should recall from the cardiovascular lectures) is typically in the range of 80-180 mm Hg. In Figure 2.3, GFR and RBF values are normalized to 100% of normal so both can be seen easily on the same plot (remember, normal GFR is ~125 ml/min & normal RBF is ~1.1 L/min). Figure 2.3 shows that GFR and RBF remain essentially constant over the 80-180 range of MAP. Autoregulation fails at the extremes of MAP. For example, autoregulation fails when MAP falls significantly (below 80 mm Hg) and this will lead to a decrease in GFR. The mechanism of autoregulation (see paragraphs below) does not depend on innervation of the kidneys or on circulating hormones. It is an intrinsic property of the kidney. It is very important, however, to recognize that it can be over-ridden by extrinsic signals to the kidneys. These extrinsic signals include the actions of the sympathetic nervous system and circulating angiotensin II. Details concerning these extrinsic signals will be considered later on. Here, our focus is on intrinsic renal autoregulation. There are two basic mechanisms of intrinsic renal autoregulation. These are:
2. tubuloglomerular feedback
Explain the myogenic mechanism. Which mechanism of autoregulation (myogenic or tubuloglomerular) is more important?
The myogenic mechanism is one that is common to many arterioles and has already been described in the cardiovascular block. When arterioles are stretched due to increased blood pressure, they tend to contract and thereby increase their resistance to flow. Although the exact mechanism is uncertain, it is thought that stretch of the arterial wall “somehow” depolarizes the smooth muscle cells in the wall. This activates voltage-dependent Ca channels resulting in an influx of Ca into the cell. The resulting rise in intracellular Ca triggers contraction. Reduced stretch (at lower blood pressure) “somehow” hyperpolarizes the smooth muscle cells, reduces Ca entry and promotes relaxation. In the kidney, this myogenic mechanism is less important than tubuloglomerular feedback.
Explain the tubuloglomerular mechanism of autoregulation. State what cells are involved and the details of this process.
The tubuloglomerular feedback mechanism of renal autoregulation was described briefly before. In general, this is a negative-feedback system that stabilizes RBF and GFR. It is associated with the juxtaglomerular apparatus (JGA). The JGA consists of a juxtaposition of the distal tubule and the arterioles that control the blood supply at the glomerulus where the tubular fluid (inside that particular distal tubule) first formed. The JGA has one type of specialized cell that make up the macula densa as well as another type called granular cells.
The details of the tubuloglomerular feedback mechanism are still not yet entirely understood. It is known that an increase in blood pressure (MAP) will increase the hydrostatic pressure in the glomerular capillaries (PGC). This will increase GFR and ultimately increase the flow of tubular fluid through the nephron. Increased flow through the distal tubule is sensed by the macula densa. It is thought that the increased delivery of NaCl due to the increased flow is involved in the “sensing”. The precise “sensing” mechanism is still a bit of a mystery. In any event, increased flow in the distal tubule is “sensed” by the macula densa and this triggers the release of a vasoconstrictor molecule (from the macula densa cells). This molecule then diffuses locally around the JGA and ultimately causes the vasoconstriction of the afferent arteriole. There is some evidence (still inconclusive evidence) that the local vasoconstrictor is perhaps ATP (or possibly adenosine). A simplified summary of local tubulogomerular feedback is illustrated in Figure 2.5 attached). Note that granular cells are not involved in the local tubulogomerular feedback. Later you will learn that the granular cells secrete renin. This renin leaves the kidney and promotes production of angiotensin II. Angiotensin II is a potent vasoconstrictor (system-wide) but it is not part of the local tubulogomerular feedback mechanism.
What is the definition of clearance? How is clearance calculated?
What 2 renal parameters is clearance useful for estimating?
he plasma leaving the kidney through the renal veins lacks the substances that were left behind (in the kidney) to be eventually eliminated in the urine. In other words, the kidneys literally clear (or clean) these substances from the plasma that flows through them. Renal clearance of any substance is defined as the volume of plasma cleared of that substance by the kidneys per minute. Clearance can be calculated and is often used to evaluate how well the kidneys are working. To help you understand this concept, it is useful to keep the units of clearance (ml/min) in mind. Clearance units are volume per time. Clearance is not the amount of the substance removed but instead the volume of plasma from which it was removed.
See attached for clearance formula.
This formula is convenient because UX, PX and V can be measured in the lab. In our example above, assume that PX equals 0.1 mg/ml. If we plug all the values in, then we can calculate that CX is 300 ml/hr (i.e. 30 mg/hour divided by 0.1 mg/ml). The clearances of a few “special” substances have proven to be very useful in estimating crucial renal parameters such as GFR (glomerular filtration rate) and RPF (renal plasma flow). Thus, clearance (of these substances) represents important diagnostic tools in probing renal function.
What is inulin? What renal parameter is inulin clearance useful for estimating and why?
What is a drawback of using inulin?
Inulin is a polysaccharide that is freely filtered but not reabsorbed or secreted. This means that all the inulin that enters the nephron by filtration will be excreted in the urine. Thus, the volume of plasma cleared of inulin per unit time is the same as the glomerular filtration rate (GFR). Indeed, inulin clearance is the “gold standard” way to measure GFR. The factors that make inulin clearance ideal for measuring GFR are:
1. Inulin is freely filtered.
2. Inulin is neither reabsorbed nor secreted. The nephron tubules have no inulin transporters and inulin can not diffuse out of the tubules so all that is filtered will be excreted.
3. There are no enzymes in the tubules that break down or synthesize inulin.
The concept of inulin clearance is illustrated graphically in Figure 2.5A (attached). Inulin containing plasma is filtered. The plasma (not the inulin) is reabsorbed. Thus, the volume of plasma filtered was “cleared” of inulin. This is why inulin clearance can be used as a measure of GFR.
In order to calculate inulin clearance (CINULIN) and thus GFR, you will need to know the plasma and urine inulin concentrations (PINULIN & UINULIN) as well as urine flow rate (V). The main clinical drawback here is that inulin must be continuously infused while urine is collected (this is usually a day or so).
Clearance of a substance will be greater than GFR if a substance is filtered and also ____.
Clearance will be smaller than GFR if a substance is filtered and then _____.
Why clearance of a substance may vary is illustrated in Figure 2.5B (attached). Clearance will be greater than GFR if the substance is filtered and also secreted (right panel). Clearance will be smaller if the substance is filtered and then reabsorbed (left panel).
What is para-aminohippurate (PAH)? What renal parameters may its clearance be used to estimate? How good of an estimate is PAH clearance of these parameters and why?
Para-aminohippurate (PAH) is a freely filtered small organic anion that happens to be robustly secreted into the proximal tubule of the nephron. Its secretion can saturate. In other words, there is a maximum rate at which PAH can be secreted into the tubule (i.e. there is a transport maximum or TM). This concept of TM will be discussed more later. Like inulin, exogenous PAH must be infused into the patient. Robust PAH secretion results in nearly all (~90%) of the PAH in the plasma entering the kidney being excreted in the urine. Such a high value can not be reached simply by filtration alone because only ~20% of the plasma is filtered (recall filtration fraction = GFR/RPF = 0.2 usually). So, secretion must be involved here. The 90% value means that PAH clearance (CPAH) approaches RPF. Indeed, CPAH is often used to access RPF. The RPF estimated in this way is sometimes called the “effective renal plasma flow” (ERPF) to indicate that it is a slight underestimate of the true RPF. The concept of PAH clearance is graphically presented in Figure 2.6 (attached).
Two different situations are presented in Figure 2.6. Note that plasma PAH is doubled in part B of the figure. The RPF and GFR are the same but PAH clearance is not. Why? The PAH clearance in part A is a relatively good estimate of RPF because all the non-filtered PAH is secreted (i.e. all PAH in plasma entering the system ends up in the urine). In part B, the PAH secretion mechanism reaches its transport maximum (TM) and some non-filtered PAH escapes (33% in this case). Thus, not all PAH in plasma entering the system ends up in the urine. Consequently, PAH clearance in this particular situation is not such a good an estimate of RPF. The point is that PAH clearance predictions of RPF are best when the PAH secretion mechanism is not saturated (i.e. when its working below its TM). For practice, you may want to use the numbers given in Figure 2.6 and calculate CPAH, RPF and GFR for yourself. In this example (Figure 2.6 part A), GFR and RPF are 100 and 700 ml/min, respectively.
The example given in Figure 2.6 (part A) is idealized (i.e. 100% non-filtered PAH is secreted). In reality, there will always be some PAH in the renal venous blood even at very low plasma PAH values. The reason is related to kidney circulatory anatomy. The PAH secretory mechanism is in the proximal tubule and PAH in the peritubular capillaries surrounding the proximal tubule is secreted. However, some PAH containing plamsa goes to capillaries that perfuse other regions of the nephron (e.g. vasa recta). PAH in this plasma is not secreted and will escape back into the systemic circulation. Typically, this amounts to 10% of the PAH explaining the “nearly all 90%” value given above. This is why CPAH underestimates true RPF. If CPAH estimates RPF, then CPAH can be also be applied to estimate RBF (renal blood flow). Recall that:
RPF = RBF x (1 – hematocrit)
RBF = RPF / (1 – hematocrit) If the hematocrit is ~0.43, then (1-hemetocrit) = 0.57
CPAH is not usually performed in clinical situations.
What renal parameter is clearance of creatinine used to estimate? How good of an estimate is creatinine clearance of this parameter and why?
What are issues with using creatinine clearance as an estimate of this parameter?
CINULIN is the “gold standard” for clinical GFR determinations. Unfortunately, inulin is not a naturally occurring substance in the body. A CINULIN measurement requires administration of inulin into the blood at a rate that will keep its concentration in the plasma constant throughout the measurement period. This can be rather cumbersome. Measurement of CCREATININE is a practical alternative. Creatinine is the end product of creatine metabolism and is continuously dumped into the blood by skeletal muscle. Skeletal muscle creatinine production is constant and the rate of creatinine appearance in the blood is proportional to a person’s muscle mass (which is typically constant on the time frame needed here). Creatinine is freely filtered and not reabsorbed (i.e. just like inulin). A small amount (10-20%) is secreted by the proximal tubule and this makes it a nonideal inulin substitute that will slightly overestimate GFR (by 10-20%). However, this degree of error is often acceptable when a quick GFR assessment is desired. Indeed, CCREATININE is the most common method used for routine GFR assessments. In practice, however, it is far more common to simply measure plasma creatinine concentration (PCREATININE) and then use this value alone as an indicator of GFR. This is possible because there is a nice inverse correlation between PCREATININE and GFR. This correlation is shown in Figure 2.7 (attached).
Normal PCREATININE is 1 mg/dl and (from the plot above) normal GFR is ~125 ml/min (as expected). Suppose one day GFR suddenly falls to 50% normal. The plot above indicates that PCREATININE will approximately double.
In the example above, a single PCREATININE measurement is used as a reasonable indicator of GFR. As noted, this method is not completely accurate because some creatinine is secreted. Another issue is that a person’s normal PCREATININE may be unknown before a renal crisis arises so a sudden change may not be obvious. However, an abnormally high PCREATININE is always a red flag and suggests that there may be a renal problem.
If there is a 50% rduction in GFR, why doesn't Pcreatinine continue to rise?
A common question is the following. If there is a persistent 50% GFR reduction, then why doesn’t PCREATININE just continue to rise? Why does it stabilize at approximately double? The reason is that the amount of creatinine excreted in the urine is the same at (100% GFR & 1 mg/dl creatinine) as it is at (50% GFR & 2 mg/dl). This level of creatinine excretion equals the creatinine production in the body. This is an example of the balance concept. A substance is in balance when its input and output match.