101. Consider the following statements about peritubular capillaries.
I. They arise from the efferent arteriole.
II. They form a fine network around the renal tubule.
III. They carry filtrate from Bowman’s capsule into the PCT.
IV. Their position supports exchange between tubular fluid and blood.
ⓐ. I and III only
ⓑ. I, II and IV only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Blood leaves the glomerular capillary tuft through the efferent arteriole. This vessel then forms a fine network of peritubular capillaries around the renal tubule, making statements I and II valid. Their close association with tubular segments provides a vascular surface through which substances can later move between tubular fluid and blood, supporting statement IV. Statement III assigns them a tubular-flow function that they do not perform. Filtrate moves from Bowman’s capsule into the PCT through the lumen of the renal tubule, whereas peritubular capillaries contain blood. The correct set distinguishes the blood pathway from the filtrate pathway while recognising their close structural relationship. Peritubular capillaries remain part of the blood pathway and support exchange; they do not carry filtrate from Bowman’s capsule.
102. Arrange the vascular structures in the direction followed by blood around a nephron.
P. Peritubular capillary network
Q. Glomerular capillary tuft
R. Efferent arteriole
S. Afferent arteriole
ⓐ. \(Q \rightarrow S \rightarrow R \rightarrow P\)
ⓑ. \(S \rightarrow R \rightarrow Q \rightarrow P\)
ⓒ. \(R \rightarrow Q \rightarrow S \rightarrow P\)
ⓓ. \(S \rightarrow Q \rightarrow R \rightarrow P\)
Correct Answer: \(S \rightarrow Q \rightarrow R \rightarrow P\)
Explanation: Blood reaches a nephron through the afferent arteriole and then enters the glomerular capillary tuft. The capillaries reunite to form the efferent arteriole, so blood leaves the filtration bed through another arteriole rather than directly through a vein. The efferent vessel subsequently divides into a fine peritubular capillary network around the renal tubule; in long-loop nephrons, part of this post-glomerular circulation forms the vasa recta. The required order is \(S\rightarrow Q\rightarrow R\rightarrow P\). This sequence contains two capillary regions separated by an efferent arteriole. The glomerular bed is specialised for pressure-driven filtration, whereas the post-glomerular network supports reabsorption and secretion around tubular segments. Mixing this vascular route with the filtrate pathway would incorrectly insert Bowman’s capsule or the PCT into blood flow. Anatomical continuity therefore places the filtration bed before the exchange network surrounding the tubule. The sequence places the glomerular capillary bed between two arterioles and the exchange network after the efferent arteriole.
103. The efferent arteriole remains open immediately after leaving a glomerulus but fails to form its usual capillary network around the renal tubule. The most direct structural consequence is:
ⓐ. loss of much of the vascular exchange surface surrounding the tubule
ⓑ. absence of the capillary tuft enclosed by Bowman’s capsule
ⓒ. interruption of filtrate flow from the PCT into the loop of Henle
ⓓ. blockage of urine transport from the renal pelvis into the ureter
Correct Answer: loss of much of the vascular exchange surface surrounding the tubule
Explanation: The efferent arteriole normally forms peritubular capillaries after carrying blood away from the glomerulus. These fine vessels surround renal tubular segments and provide the nearby blood supply needed for exchange between the tubular and vascular compartments. Failure to form that network removes much of this exchange surface even though the glomerular tuft itself may remain present. The defect is vascular and occurs after glomerular outflow, so it does not directly break the physical continuity of the renal tubule or obstruct the ureter. The condition separates filtration at the glomerulus from later blood-tubule exchange around the nephron. The dependency chain for the glomerulus is unambiguous once the initiating event and final destination are identified.
104. Use the arrangement described below. Blood enters capillary tuft P through one arteriole and leaves through vessel Q. Vessel Q then divides into a fine network closely surrounding tubular structure R. The fine network is:
ⓐ. the glomerular capillary tuft
ⓑ. the renal pelvic circulation
ⓒ. the peritubular capillary network
ⓓ. the urinary collecting-duct system
Correct Answer: the peritubular capillary network
Explanation: Capillary tuft P is the glomerulus, and vessel Q is the efferent arteriole carrying blood away from it. The fine vessels formed from Q and distributed around tubular structure R are peritubular capillaries. Their identity is established by origin and position: they arise after the glomerular tuft and lie around the renal tubule. The glomerulus itself occurs before the efferent arteriole and remains enclosed by Bowman’s capsule. Collecting ducts and the renal pelvis carry tubular fluid or urine rather than blood. The description reconstructs the vascular relationship without requiring an external nephron diagram. The efferent arteriole must occur after glomerular capillaries and before peritubular capillaries, preserving one continuous renal vascular route. This post-glomerular network is positioned for exchange with tubular fluid after pressure-driven filtration has occurred upstream. It therefore links glomerular outflow with later reabsorptive and secretory exchange.
105. Compare two nephron-associated vessels.
| Feature | Vessel P | Vessel Q |
|---|
| Shape | Fine network around tubules | Minute U-shaped vessel |
| Relation | Surrounds several tubular regions | Runs parallel to the loop of Henle |
| Development in cortical nephrons | Present | Absent or highly reduced |
Vessels P and Q are respectively:
ⓐ. glomerulus and afferent arteriole
ⓑ. vasa recta and renal vein
ⓒ. peritubular capillaries and vasa recta
ⓓ. efferent arteriole and collecting duct
Correct Answer: peritubular capillaries and vasa recta
Explanation: Vessel P is described as a fine capillary network surrounding renal tubules, which identifies the peritubular capillaries. Vessel Q is minute, U shaped and runs parallel to the loop of Henle, identifying the vasa recta. The additional observation that Q is absent or highly reduced in cortical nephrons supports the same conclusion, since well-developed vasa recta are associated mainly with long-loop juxtamedullary nephrons. Both structures carry blood, but their arrangements differ. The peritubular network is broadly distributed around tubular segments, whereas the vasa recta follows the long medullary course of a loop of Henle. The long medullary course and U-shaped form distinguish vasa recta from the more widely distributed peritubular capillary network. Their different courses explain why vasa recta are especially important for medullary counter-current exchange, while peritubular capillaries chiefly support reabsorption and secretion around cortical tubules.
106. Assertion: Vasa recta are absent or highly reduced in cortical nephrons.
Reason: Cortical nephrons generally possess short loops of Henle that extend only slightly into the medulla.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true, whereas Reason is false
ⓓ. Assertion is false, whereas Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion
Explanation: Cortical nephrons generally possess short loops of Henle that enter the medulla only slightly. Their associated post-glomerular vessels therefore do not need a long, well-developed U-shaped course extending deep beside the loop. Vasa recta are consequently absent or highly reduced in these nephrons, so the assertion is true. The reason is also true and supplies the anatomical basis for that pattern. Juxtamedullary nephrons provide the contrast: their renal corpuscles lie near the cortex-medulla boundary, their loops descend deeply, and prominent vasa recta run parallel to those loops. This close vascular-tubular arrangement supports counter-current exchange and preservation of the medullary osmotic gradient. The reason directly explains the assertion by linking shallow loop penetration with reduced development of the accompanying medullary vessel.
107. A tracer is introduced into a minute vessel arising from the capillary network around a juxtamedullary nephron. The tracer descends into the medulla beside the loop of Henle, turns, and ascends toward the cortex. The vessel is identified as:
ⓐ. an afferent arteriole supplying a glomerulus
ⓑ. a collecting duct draining several DCTs
ⓒ. an efferent arteriole leaving the glomerular tuft
ⓓ. a vasa recta vessel accompanying the long loop
Correct Answer: a vasa recta vessel accompanying the long loop
Explanation: The traced vessel has three defining features of the vasa recta: it arises from the peritubular vascular network, forms a U-shaped course and runs parallel to the loop of Henle. Its descent into and return from the medulla also matches the arrangement associated with a juxtamedullary nephron. An afferent arteriole approaches a glomerulus rather than following the loop, while an efferent arteriole is the vessel immediately leaving the glomerular capillary tuft. A collecting duct may pass through the medulla but carries tubular fluid, not blood. The tracer evidence identifies both the vascular nature and spatial course of the vasa recta. Its hairpin course alongside a nephron loop is the vascular arrangement needed for medullary counter-current exchange. The tracer’s vascular origin also excludes a collecting duct, whose similar medullary course carries tubular fluid rather than blood.
108. Nephron P has a short loop that enters the medulla only slightly. Nephron Q has a long loop reaching deep into the medulla. Which vascular comparison is expected?
ⓐ. P has abundant vasa recta, while Q lacks peritubular vessels
ⓑ. Q has prominent vasa recta, whereas P has reduced or absent vasa recta
ⓒ. Both possess equally developed vasa recta regardless of loop length
ⓓ. P possesses vasa recta, while Q is supplied only by a renal vein
Correct Answer: Q has prominent vasa recta, whereas P has reduced or absent vasa recta
Explanation: Nephron P has the defining short-loop pattern of a cortical nephron. Vasa recta are absent or highly reduced in such nephrons. Nephron Q has a long loop extending deep into the medulla and corresponds to a juxtamedullary nephron, where a well-developed U-shaped vasa recta runs parallel to the loop. The comparison depends on matching vascular architecture with tubular depth. Both nephron types receive glomerular blood flow and possess peritubular circulation, but they differ markedly in development of the medullary vasa recta. The renal vein is an organ-level outflow vessel and does not replace the specialised vessels accompanying an individual long loop.
109. Match each urine-formation process with the principal direction of movement. Each Column II entry is used once.
| Column I | Column II |
|---|
| P. Glomerular filtration | I. Tubular compartment toward nearby blood |
| Q. Tubular reabsorption | II. Blood or tubular cells toward tubular fluid |
| R. Tubular secretion | III. Glomerular blood toward Bowman’s lumen |
| S. Final urine formation | IV. Result after all three processes modify tubular fluid |
ⓐ. \(\text{P-III, Q-I, R-II, S-IV}\)
ⓑ. \(\text{P-I, Q-III, R-IV, S-II}\)
ⓒ. \(\text{P-II, Q-IV, R-I, S-III}\)
ⓓ. \(\text{P-IV, Q-II, R-III, S-I}\)
Correct Answer: \(\text{P-III, Q-I, R-II, S-IV}\)
Explanation: Glomerular filtration moves water and filterable solutes from glomerular blood into Bowman’s lumen, so P maps to III. Reabsorption returns selected substances from tubular fluid toward the blood in surrounding capillaries, linking Q with I. Secretion adds selected substances from blood or tubular cells into the tubular fluid, placing R with II. Final urine is the product remaining after filtration has supplied the initial fluid and tubular reabsorption and secretion have altered its composition, so S maps to IV. The movement directions are the defining distinction among the three processes and should not be inferred merely from whether a substance is useful or wasteful. All correspondences linked to glomerular filtration must be simultaneously valid, which makes the complete pattern more informative than any isolated pair.
110. A small solute first crosses from glomerular blood into Bowman’s capsule. Most of it later moves from the PCT lumen into nearby capillaries, while another ion is transferred from blood into the tubular lumen. These movements are respectively:
ⓐ. secretion, filtration and reabsorption
ⓑ. reabsorption, secretion and filtration
ⓒ. filtration, reabsorption and secretion
ⓓ. filtration, secretion and reabsorption
Correct Answer: filtration, reabsorption and secretion
Explanation: The first movement begins in glomerular blood and ends in Bowman’s capsule, so it is glomerular filtration. The second moves a previously filtered solute out of the PCT lumen and back toward nearby capillary blood, identifying tubular reabsorption. The final movement adds an ion from blood into the tubular lumen and is tubular secretion. The classification follows compartment and direction rather than the chemical identity of the substance. A single molecule may be filtered and later reabsorbed, while another substance may enter tubular fluid through secretion after the initial filtrate has formed. These processes jointly determine the composition of final urine. Compartment and direction consistently classify all three movements and explain their combined effect on final urine.
111. Arrange the stages in a simplified account of urine formation.
P. Selective reabsorption and secretion modify tubular fluid.
Q. Glomerular pressure forms filtrate in Bowman’s capsule.
R. Modified fluid passes through later tubules and collecting ducts.
S. Final urine enters the kidney’s drainage passages.
ⓐ. \(P \rightarrow Q \rightarrow S \rightarrow R\)
ⓑ. \(Q \rightarrow P \rightarrow R \rightarrow S\)
ⓒ. \(Q \rightarrow R \rightarrow P \rightarrow S\)
ⓓ. \(S \rightarrow Q \rightarrow P \rightarrow R\)
Correct Answer: \(Q \rightarrow P \rightarrow R \rightarrow S\)
Explanation: Pressure in the glomerular capillaries first drives filterable plasma components into Bowman’s capsule, producing the initial filtrate. As this fluid enters the renal tubule, selective reabsorption returns useful substances and much water toward blood, while secretion adds selected ions and wastes to tubular fluid. These two tubular processes may occur in overlapping nephron regions, so they are grouped as modification rather than treated as universally separate consecutive phases. The processed fluid then passes through later tubular segments and collecting ducts, where its water and ionic composition may be adjusted further. Only after nephron and collecting-duct processing does the fluid enter the calyces and renal pelvis as final urine. The dependency is therefore fixed at the broader level: filtration must create tubular fluid before reabsorption or secretion can modify it, and drainage through kidney passages follows completion of that processing. This gives the order \(Q\rightarrow P\rightarrow R\rightarrow S\).
112. A filtered substance moves from Bowman’s capsule through the tubule and is then transferred completely into the surrounding capillary blood. Its renal handling is best classified as:
ⓐ. secretion into the tubule without prior filtration
ⓑ. filtration followed by secretion into the tubule
ⓒ. reabsorption followed by renewed filtration
ⓓ. filtration followed by full reabsorption into blood
Correct Answer: filtration followed by full reabsorption into blood
Explanation: Entry from glomerular blood into Bowman’s capsule is filtration. Once the substance is present in tubular fluid, movement from the tubular compartment into surrounding capillary blood is reabsorption. The word completely indicates that all of the filtered amount is recovered under the stated condition, so none of that amount remains for excretion. Secretion would require movement in the opposite direction, from blood or tubular cells into the tubule. This relationship shows that presence in the initial filtrate does not guarantee presence in final urine. Final excretion depends on the combined effects of filtration, reabsorption and secretion rather than filtration alone.
113. Assertion: Glomerular filtration is the first step in urine formation.
Reason: Blood pressure in the glomerular capillaries drives filterable plasma components into Bowman’s capsule.
ⓐ. Both Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓑ. Assertion is true, whereas Reason is false
ⓒ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓓ. Assertion is false, whereas Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion
Explanation: Urine formation begins when glomerular capillary pressure forces water and small dissolved substances across the filtration barrier into Bowman’s capsule. This creates the initial glomerular filtrate before tubular reabsorption and secretion alter it. Both statements are true, and the reason describes the mechanism producing the first tubular fluid. Filtration is not initiated by the collecting duct or by movement of substances from the tubule back into blood. The pressure-driven transfer from vascular to capsular compartment establishes the material upon which all later nephron segments act, directly explaining why glomerular filtration occupies the first position. Pressure-driven entry into Bowman’s capsule creates the initial filtrate, so the reason directly explains why filtration is first.
114. In an isolated renal corpuscle, glomerular capillary pressure is experimentally reduced while the filtration barrier remains intact and all other relevant conditions are unchanged. The expected immediate result is:
ⓐ. increased movement of filtrate into Bowman’s capsule
ⓑ. reduced formation of filtrate in Bowman’s capsule
ⓒ. conversion of tubular secretion into filtration
ⓓ. direct transport of urine into the afferent arteriole
Correct Answer: reduced formation of filtrate in Bowman’s capsule
Explanation: Glomerular filtration is driven by pressure within the glomerular capillaries. Reducing that pressure weakens the force moving water and small solutes across the intact filtration barrier into Bowman’s capsule. The immediate outcome is a reduction in filtrate formation. The experiment keeps the barrier unchanged, so the effect can be attributed to altered driving pressure rather than structural leakage. Tubular secretion is a separate process occurring through tubular cells and does not become filtration when glomerular pressure changes. Urine also cannot move into the afferent arteriole, which supplies blood to the glomerulus. The result demonstrates the dependence of the first urine-forming step on glomerular blood pressure. With barrier integrity controlled, the fall in filtrate volume is attributable to reduced glomerular pressure. This isolates driving pressure as the altered determinant of GFR.
115. Use the arrangement described below. Compartment P is the lumen of a glomerular capillary. Compartment Q lies between the walls of Bowman’s capsule. An arrow passes from P across the capillary wall, intervening membrane and capsular epithelium into Q. The arrow represents:
ⓐ. filtration into Bowman’s capsule
ⓑ. secretion into the renal tubule
ⓒ. reabsorption into capillary blood
ⓓ. transport through the ureter
Correct Answer: filtration into Bowman’s capsule
Explanation: Compartment P contains glomerular blood, while Q is the capsular space receiving the initial filtrate. Movement from P to Q crosses the filtration barrier and is glomerular filtration. The three structures mentioned in the route correspond to the glomerular endothelium, basement membrane and epithelium of Bowman’s capsule. Reabsorption would move substances from tubular fluid back toward blood, while secretion would add selected substances to tubular fluid through tubular cells. Ureteric transport occurs much later and carries urine from the kidney toward the bladder. Direction, location and crossed layers together identify the process without requiring a visible diagram. The blood-to-capsular direction across all three barrier layers therefore identifies filtration into Bowman’s space. The receiving compartment is capsular space, so the arrow cannot represent reabsorption into post-glomerular blood. Filtration is the only listed process that moves fluid in this direction at the renal corpuscle.
116. The kidneys receive \(1200\,\text{mL min}^{-1}\) of blood, equal to approximately one-fifth of the volume pumped by one ventricle per minute. The estimated ventricular output is:
ⓐ. \(240\,\text{mL min}^{-1}\)
ⓑ. \(1200\,\text{mL min}^{-1}\)
ⓒ. \(2400\,\text{mL min}^{-1}\)
ⓓ. \(6000\,\text{mL min}^{-1}\)
Correct Answer: \(6000\,\text{mL min}^{-1}\)
Explanation: Renal blood flow is \(1200\,\text{mL min}^{-1}\), which is stated to represent approximately \(\frac{1}{5}\) of ventricular output. If ventricular output is \(V\), then \(\frac{1}{5}V=1200\,\text{mL min}^{-1}\). Multiplying by \(5\) gives \(V=6000\,\text{mL min}^{-1}\), or \(6.0\,\text{L min}^{-1}\). The result is consistent with the usual relation: renal blood flow of about \(1100\text{–}1200\,\text{mL min}^{-1}\) corresponds to a total ventricular output of roughly \(5500\text{–}6000\,\text{mL min}^{-1}\). This calculation concerns blood delivered to the kidneys, not the much smaller volume of glomerular filtrate produced per minute. The units remain millilitres per minute throughout, so no time conversion is required. The fivefold step follows directly from renal flow being one of five equal parts of the total output, and the final unit conversion provides an independent numerical check.
117. Let \(R\) represent renal blood flow and \(F\) represent glomerular filtration rate under normal conditions. Which relation correctly distinguishes the two quantities?
ⓐ. \(R=F\), since all blood entering the kidneys becomes filtrate
ⓑ. \(R\gt F\), since only part of the delivered fluid enters Bowman’s capsule
ⓒ. \(R\lt F\), since filtrate is formed repeatedly from the same blood each minute
ⓓ. \(R=0\) whenever filtration occurs in the glomeruli
Correct Answer: \(R\gt F\), since only part of the delivered fluid enters Bowman’s capsule
Explanation: Renal blood flow \(R\) is the total volume of blood delivered to the kidneys per unit time, approximately \(1100\text{–}1200\,\text{mL min}^{-1}\). Glomerular filtration rate \(F\) is the volume of filtrate formed by both kidneys per unit time, approximately \(125\,\text{mL min}^{-1}\). The supplied blood contains cells, plasma proteins and fluid that remain largely within the vascular system; only part of the plasma water and its small dissolved substances crosses the filtration barrier into Bowman’s capsules. Hence \(R\gt F\). The comparison concerns two different compartments and cannot be treated as equality merely because filtration begins from renal blood. Blood not filtered at a particular passage continues through efferent arterioles and later renal capillaries before leaving through renal veins. No filtration-fraction calculation is required, but the representative values independently confirm the direction of the inequality: the delivered vascular flow is several times greater than the filtrate formed each minute.
118. Examine the measurements obtained from a healthy adult.
| Measurement | Value |
|---|
| Blood delivered to both kidneys | \(1150\,\text{mL min}^{-1}\) |
| Glomerular filtrate formed | \(125\,\text{mL min}^{-1}\) |
Which interpretation is supported?
ⓐ. Renal blood flow and GFR are distinct, with blood flow being much greater
ⓑ. GFR measures all blood entering both kidneys before filtration begins
ⓒ. Every millilitre of blood entering the kidneys becomes tubular fluid
ⓓ. The smaller value represents blood leaving through the renal vein
Correct Answer: Renal blood flow and GFR are distinct, with blood flow being much greater
Explanation: The first value measures blood supplied to both kidneys each minute, while the second measures the filtrate entering Bowman’s capsules during the same period. The numerical comparison shows that renal blood delivery is much larger than GFR. Blood contains cells, proteins and fluid, and most of it remains in the vascular pathway rather than becoming filtrate. The GFR value refers only to filterable fluid formed across all glomeruli. The smaller measurement is not renal venous outflow and should not be interpreted as the total blood remaining after filtration. The table prevents a common error: treating renal blood flow and filtrate flow as interchangeable quantities merely on the basis that both are reported per minute. Comparing \(1200\,\text{mL min}^{-1}\) with \(125\,\text{mL min}^{-1}\) also confirms that only a minority of delivered fluid contributes to newly formed filtrate during each minute.
119. Match each component of the glomerular filtration barrier with its position. Each Column II entry is used once.
| Column I | Column II |
|---|
| P. Glomerular endothelium | I. Middle layer between capillary and capsular epithelium |
| Q. Basement membrane | II. Layer facing the glomerular blood |
| R. Bowman’s-capsule epithelium | III. Outer filtration layer containing podocytes |
| S. Bowman’s lumen | IV. Space receiving the filtrate |
ⓐ. \(\text{P-I, Q-II, R-IV, S-III}\)
ⓑ. \(\text{P-III, Q-I, R-II, S-IV}\)
ⓒ. \(\text{P-II, Q-III, R-IV, S-I}\)
ⓓ. \(\text{P-II, Q-I, R-III, S-IV}\)
Correct Answer: \(\text{P-II, Q-I, R-III, S-IV}\)
Explanation: The glomerular endothelium forms the filtration layer directly facing blood in the capillary lumen, so P maps to II. The basement membrane lies between the capillary lining and the epithelium of Bowman’s capsule, linking Q with I. The capsular epithelial layer contains podocytes and forms the outer cellular part of the barrier, placing R with III. Filtrate that crosses all three layers enters Bowman’s lumen, so S maps to IV. The mapping follows the route from blood to tubular space and distinguishes an actual barrier layer from the receiving compartment. Bowman’s lumen is not a membrane but the space into which filtered fluid collects. Endothelium, basement membrane and podocyte layer are identified by their order from blood to Bowman’s space, while filtration slits lie between podocyte processes.
120. Arrange the filtration structures in the order crossed by fluid moving from glomerular blood into Bowman’s lumen.
P. Epithelium of Bowman’s capsule
Q. Basement membrane
R. Glomerular endothelium
ⓐ. \(Q \rightarrow R \rightarrow P\)
ⓑ. \(P \rightarrow Q \rightarrow R\)
ⓒ. \(R \rightarrow Q \rightarrow P\)
ⓓ. \(R \rightarrow P \rightarrow Q\)
Correct Answer: \(R \rightarrow Q \rightarrow P\)
Explanation: Fluid begins in the lumen of a glomerular capillary. It first crosses the endothelial lining of that capillary, represented by R. The basement membrane lies immediately beyond the endothelium and forms the middle filtration layer, so Q follows. The final cellular layer is the epithelium of Bowman’s capsule, represented by P. After crossing it, the filtrate enters Bowman’s lumen. The order is based on physical position along the blood-to-filtrate route. Reversing the sequence would place the capsular epithelium in direct contact with capillary blood or move the basement membrane outside both cellular layers, neither of which matches renal-corpuscle organisation. Each filtration layer must be crossed before fluid reaches the capsular space and then the PCT. The three layers act together as a barrier, but their order is fixed by movement from vascular blood toward the urinary space. Each layer occupies a fixed blood-to-filtrate position, so reversing any pair would misplace a cellular or extracellular component of the filtration barrier.
