Glomerular filtration

Glomerular filtration US Medical PG Question 1: Which region of the nephron reabsorbs the highest percentage of filtered bicarbonate?

• A. Collecting duct
• B. Thick ascending limb
• C. Distal tubule
• D. Proximal tubule (Correct Answer)

Glomerular filtration Explanation: ***Proximal tubule*** - The **proximal convoluted tubule (PCT)** reabsorbs approximately 80-90% of the **filtered bicarbonate** through a process involving **carbonic anhydrase** and the **Na+/H+ exchanger**. - This vital function ensures that the majority of bicarbonate, a key buffer, is returned to the blood to maintain **acid-base balance**. *Collecting duct* - While the collecting duct does have the ability to reabsorb and secrete bicarbonate, its contribution is minor compared to the PCT, primarily for fine-tuning acid-base balance. - Cells in the collecting duct, particularly **Type A intercalated cells**, are important for secreting acid (H+) in acidosis and therefore reabsorbing bicarbonate, but not the bulk of it. *Thick ascending limb* - The primary role of the **thick ascending limb** is the reabsorption of **sodium**, **potassium**, and **chloride** to create a concentrated interstitium, not significant bicarbonate reabsorption. - It is largely impermeable to water and is relatively impermeable to bicarbonate. *Distal tubule* - The **distal convoluted tubule (DCT)** reabsorbs a small percentage of filtered bicarbonate, but its main role is regulated reabsorption of **sodium** and **calcium**, and secretion of **potassium** and **hydrogen ions**. - Its contribution to bicarbonate reabsorption is much less significant than that of the proximal tubule.

Glomerular filtration US Medical PG Question 2: A healthy 30-year-old female has a measured creatinine clearance of 100 mL/min. She has a filtration fraction (FF) of 25%. Serum analysis reveals a creatinine level of 0.9 mg/dL and an elevated hematocrit of 0.6. Which of the following is the best estimate of this patient’s renal blood flow?

• A. 1.2 L/min
• B. 600 mL/min
• C. 800 mL/min
• D. 400 mL/min
• E. 1.0 L/min (Correct Answer)

Glomerular filtration Explanation: ***1.0 L/min*** - The **renal plasma flow (RPF)** can be calculated by dividing the **creatinine clearance (which approximates GFR)** by the **filtration fraction (FF)**: RPF = GFR / FF = 100 mL/min / 0.25 = 400 mL/min. - To find the **renal blood flow (RBF)**, we use the formula RBF = RPF / (1 - Hematocrit). Given RPF = 400 mL/min and Hematocrit = 0.6, RBF = 400 mL/min / (1 - 0.6) = 400 mL/min / 0.4 = 1000 mL/min, or **1.0 L/min**. *1.2 L/min* - This value would result if the hematocrit were lower (e.g., 0.5) or if the GFR or FF were different, leading to an incorrect RPF or RBF calculation. - It does not align with the provided values when applying the standard physiological formulas relating GFR, FF, RPF, and hematocrit. *600 mL/min* - This value might be obtained if the hematocrit was significantly underestimated or if the RPF calculation was incorrect in determining the RBF. - It arises from using an incorrect formula or misinterpreting the relationship between plasma flow and blood flow. *800 mL/min* - This result would occur if the calculation for RPF or the subsequent RBF was erroneous, possibly by using an incorrect denominator in the RBF formula. - For example, if RPF was incorrectly assumed to be 320 mL/min and divided by 0.4 (1-Hematocrit). *400 mL/min* - This value represents the calculated **renal plasma flow (RPF)**, not the **renal blood flow (RBF)**. - RBF is always higher than RPF because it includes both plasma and cellular components of blood.

Glomerular filtration US Medical PG Question 3: A patient is receiving daily administrations of Compound X. Compound X is freely filtered in the glomeruli and undergoes net secretion in the renal tubules. The majority of this tubular secretion occurs in the proximal tubule. Additional information regarding this patient's renal function and the renal processing of Compound X is included below: Inulin clearance: 120 mL/min Plasma concentration of Inulin: 1 mg/mL PAH clearance: 600 mL/min Plasma concentration of PAH: 0.2 mg/mL Total Tubular Secretion of Compound X: 60 mg/min Net Renal Excretion of Compound X: 300 mg/min Which of the following is the best estimate of the plasma concentration of Compound X in this patient?

• A. 2 mg/mL (Correct Answer)
• B. 3 mg/mL
• C. There is insufficient information available to estimate the plasma concentration of Compound X
• D. 1 mg/mL
• E. 0.5 mg/mL

Glomerular filtration Explanation: ***2 mg/mL*** * The **net renal excretion of Compound X (300 mg/min)** is the sum of the filtered load and the net tubular secretion. * Given that Compound X is **freely filtered** and undergoes **net secretion (60 mg/min)**, we can calculate the filtered load and subsequently its plasma concentration. * **Net excretion = Filtered load + Net tubular secretion** * **300 mg/min = Filtered load + 60 mg/min** * **Filtered load = 300 mg/min - 60 mg/min = 240 mg/min** * Since **Filtered load = Glomerular Filtration Rate (GFR) * Plasma concentration (P_X)**, and GFR is estimated by **inulin clearance (120 mL/min)**: * **240 mg/min = 120 mL/min * P_X** * **P_X = 240 mg/min / 120 mL/min = 2 mg/mL**. *3 mg/mL* * This value would imply a significantly higher filtered load or a different contribution from tubular secretion. * Calculations using this plasma concentration would not align with the provided excretion and secretion rates. *There is insufficient information available to estimate the plasma concentration of Compound X* * The problem provides all necessary values: **Inulin clearance (GFR)**, **net tubular secretion of Compound X**, and **net renal excretion of Compound X**. * These parameters are sufficient to determine the filtered load and thus the plasma concentration of Compound X. *1 mg/mL* * A plasma concentration of 1 mg/mL would result in a lower filtered load than calculated and would not account for the observed net renal excretion. * **Filtered load = 120 mL/min * 1 mg/mL = 120 mg/min**. Total excretion would then be 120 mg/min + 60 mg/min = 180 mg/min, which contradicts the given 300 mg/min. *0.5 mg/mL* * This plasma concentration would lead to an even lower filtered load, making it impossible to achieve the *net renal excretion of Compound X* given the tubular secretion. * **Filtered load = 120 mL/min * 0.5 mg/mL = 60 mg/min**. Total excretion would be 60 mg/min + 60 mg/min = 120 mg/min, which is much lower than the given 300 mg/min.

Glomerular filtration US Medical PG Question 4: On cardiology service rounds, your team sees a patient admitted with an acute congestive heart failure exacerbation. In congestive heart failure, decreased cardiac function leads to decreased renal perfusion, which eventually leads to excess volume retention. To test your knowledge of physiology, your attending asks you which segment of the nephron is responsible for the majority of water absorption. Which of the following is a correct pairing of the segment of the nephron that reabsorbs the majority of all filtered water with the means by which that segment absorbs water?

• A. Distal convoluted tubule via passive diffusion following ion reabsorption
• B. Distal convoluted tubule via aquaporin channels
• C. Thick ascending loop of Henle via passive diffusion following ion reabsorption
• D. Proximal convoluted tubule via passive diffusion following ion reabsorption (Correct Answer)
• E. Collecting duct via aquaporin channels

Glomerular filtration Explanation: ***Proximal convoluted tubule via passive diffusion following ion reabsorption*** - The **proximal convoluted tubule (PCT)** is responsible for reabsorbing approximately **65-70% of filtered water**, making it the primary site of water reabsorption in the nephron. - This water reabsorption primarily occurs **passively**, following the active reabsorption of solutes (especially **sodium ions**), which creates an osmotic gradient. *Distal convoluted tubule via passive diffusion following ion reabsorption* - The **distal convoluted tubule (DCT)** reabsorbs a much smaller percentage of filtered water (around 5-10%) and its water reabsorption is largely **regulated by ADH**, not primarily simple passive diffusion following bulk ion reabsorption. - While some passive water movement occurs, it is not the main mechanism or location for the majority of water reabsorption. *Distal convoluted tubule via aquaporin channels* - While aquaporin channels do play a role in water reabsorption in the DCT, particularly under the influence of **ADH**, the DCT is not the segment responsible for the **majority of all filtered water absorption**. - The bulk of water reabsorption occurs earlier in the nephron, independently of ADH for the most part. *Thick ascending loop of Henle via passive diffusion following ion reabsorption* - The **thick ascending loop of Henle** is primarily involved in reabsorbing ions like Na+, K+, and Cl- but is largely **impermeable to water**. - Its impermeability to water is crucial for creating the **osmotic gradient** in the renal medulla, which is necessary for later water reabsorption. *Collecting duct via aquaporin channels* - The **collecting duct** is critically important for **regulated water reabsorption** via **aquaporin-2 channels** under the influence of **ADH**, allowing for fine-tuning of urine concentration. - However, it reabsorbs only a variable portion (typically 5-19%) of the remaining filtered water, not the **majority of all filtered water**.

Glomerular filtration US Medical PG Question 5: A scientist is studying the excretion of a novel toxin X by the kidney in order to understand the dynamics of this new substance. He discovers that this new toxin X has a clearance that is half that of inulin in a particular patient. This patient's filtration fraction is 20% and his para-aminohippuric acid (PAH) dynamics are as follows: Urine volume: 100 mL/min Urine PAH concentration: 30 mg/mL Plasma PAH concentration: 5 mg/mL Given these findings, what is the clearance of the novel toxin X?

• A. 1,500 mL/min
• B. 600 mL/min
• C. 300 mL/min
• D. 60 mL/min (Correct Answer)
• E. 120 mL/min

Glomerular filtration Explanation: ***60 ml/min*** - First, calculate the **renal plasma flow (RPF)** using PAH clearance: RPF = (Urine PAH conc. × Urine vol.) / Plasma PAH conc. = (30 mg/mL × 100 mL/min) / 5 mg/mL = 600 mL/min. - Next, calculate the **glomerular filtration rate (GFR)**, which is the clearance of inulin. GFR = RPF × Filtration Fraction = 600 mL/min × 0.20 = 120 mL/min. Toxin X clearance is half of inulin clearance, so 120 mL/min / 2 = **60 mL/min**. *1,500 ml/min* - This value is likely obtained if an incorrect formula or conversion was made, possibly by misinterpreting the units or the relationship between GFR, RPF, and filtration fraction. - It significantly overestimates the clearance for a substance that is cleared at half the rate of inulin. *600 ml/min* - This value represents the **renal plasma flow (RPF)**, calculated using the PAH clearance data. - It does not account for the filtration fraction or the fact that toxin X clearance is half of inulin clearance (GFR). *300 ml/min* - This value would be obtained if the renal plasma flow (RPF) was incorrectly halved, or if an intermediate calculation was misinterpreted as the final answer. - It does not align with the given filtration fraction and the relationship between toxin X and inulin clearance. *120 ml/min* - This value represents the **glomerular filtration rate (GFR)**, which is equal to the clearance of inulin (RPF × Filtration Fraction = 600 mL/min × 0.20 = 120 mL/min). - The question states that the clearance of toxin X is **half** that of inulin, so this is an intermediate step, not the final answer.

Glomerular filtration US Medical PG Question 6: A 73-year-old male is brought in by ambulance after he was found to be lethargic and confused. He has not been routinely seeing a physician and is unable to recall how he came to be in the hospital. His temperature is 99°F (37°C), blood pressure is 150/95 mmHg, pulse is 75/min, and respirations are 18/min. His past medical history is significant for poorly controlled diabetes and longstanding hypertension, and he says that he has not been taking his medications recently. Labs are obtained and shown below: Serum: Na+: 142 mEq/L Cl-: 105 mEq/L K+: 5 mEq/L HCO3-: 16 mEq/L Urea nitrogen: 51 mg/dL Glucose: 224 mg/dL Creatinine: 2.6 mg/dL Which of the following changes would most likely improve the abnormal parameter that is responsible for this patient's symptoms?

• A. Increased Bowman's space hydrostatic pressure
• B. Decreased filtration coefficient
• C. Increased Bowman's space oncotic pressure
• D. Decreased glomerular capillary hydrostatic pressure
• E. Increased glomerular capillary hydrostatic pressure (Correct Answer)

Glomerular filtration Explanation: ***Increased glomerular capillary hydrostatic pressure*** - This patient presents with **acute kidney injury (AKI)** evidenced by **elevated creatinine (2.6 mg/dL)** and **BUN (51 mg/dL)**, causing uremic symptoms of **lethargy and confusion** - The "abnormal parameter" is the **reduced GFR** causing azotemia and uremia - To improve AKI and restore adequate filtration, **GFR must be increased** - **Increasing glomerular capillary hydrostatic pressure** increases the net filtration pressure: **NFP = (PGC - PBS) - (πGC - πBS)**, where PGC is the primary driving force for filtration - In prerenal AKI (likely in this patient with poor medication compliance for hypertension), restoring adequate renal perfusion pressure is the therapeutic goal - While chronic hyperfiltration can contribute to long-term diabetic/hypertensive nephropathy, the **acute management priority** is restoring adequate GFR to clear uremic toxins *Decreased glomerular capillary hydrostatic pressure* - This would **decrease the net filtration pressure**, thereby **reducing GFR** - Lower GFR would worsen azotemia and uremic symptoms - This is the opposite of what's needed to improve acute kidney injury *Increased Bowman's space hydrostatic pressure* - This **opposes filtration** by increasing back-pressure against the glomerular capillaries - Would **decrease GFR** and worsen the AKI - Occurs pathologically in urinary tract obstruction *Decreased filtration coefficient* - The filtration coefficient (Kf) represents the permeability and surface area of the glomerular capillaries - **Decreasing Kf reduces GFR**, worsening kidney function - This represents glomerular damage, not a therapeutic intervention *Increased Bowman's space oncotic pressure* - This would theoretically **increase net filtration pressure** and GFR - However, this is **physiologically implausible** as Bowman's space normally contains minimal protein (filtrate is protein-free) - Significant protein in Bowman's space indicates severe glomerular damage with proteinuria, not a mechanism to improve function

Glomerular filtration US Medical PG Question 7: A 58-year-old man presents to the Emergency Department after 3 hours of intense suprapubic pain associated with inability to urinate for the past day or two. His medical history is relevant for benign prostatic hyperplasia (BPH) that has been under treatment with prazosin and tadalafil. Upon admission, he is found to have a blood pressure of 180/100 mm Hg, a pulse of 80/min, a respiratory rate of 23/min, and a temperature of 36.5°C (97.7°F). He weighs 84 kg (185.1 lb) and is 175 cm (5 ft 7 in) tall. Physical exam, he has suprapubic tenderness. A bladder scan reveals 700 ml of urine. A Foley catheter is inserted and the urine is drained. Initial laboratory tests and their follow up 8 hours after admission are shown below. Admission 8 hours after admission Serum potassium 4.2 mmol/L Serum potassium 4.0 mmol/L Serum sodium 140 mmol/L Serum sodium 142 mmol/L Serum chloride 102 mmol/L Serum chloride 110 mmol/L Serum creatinine 1.4 mg/dL Serum creatinine 1.6 mg/dL Serum blood urea nitrogen 64 mg/dL Serum blood urea nitrogen 62 mg/dL Urine output 250 mL Urine output 260 mL A senior attending suggests a consultation with Nephrology. Which of the following best justifies this suggestion?

• A. Estimated glomerular filtration rate (eGFR)
• B. Urine output (Correct Answer)
• C. Serum creatinine (SCr)
• D. Serum blood urea nitrogen (BUN)
• E. Serum potassium

Glomerular filtration Explanation: ***Urine output*** - The patient's **urine output is severely reduced** at 260 mL over 8 hours (approximately **32.5 mL/hour**), which constitutes **oliguria** (defined as <0.5 mL/kg/hr; this patient at 84 kg should produce ≥42 mL/hr). - Despite **relief of the post-renal obstruction** via Foley catheterization, the persistent oliguria indicates **intrinsic kidney injury** rather than simple mechanical obstruction. - The combination of **oliguria persisting after decompression** + **rising serum creatinine** (1.4→1.6 mg/dL) meets **KDIGO criteria for Stage 2 AKI** (urine output <0.5 mL/kg/hr for ≥12 hours). - This requires **urgent nephrology consultation** to assess for acute tubular necrosis (ATN), guide fluid management during potential post-obstructive diuresis, and consider renal replacement therapy if oliguria worsens. *Serum creatinine (SCr)* - The serum creatinine **rose from 1.4 to 1.6 mg/dL** despite bladder decompression, which is concerning and suggests intrinsic renal injury. - However, creatinine is a **lagging indicator** of kidney function - it takes 24-48 hours to reflect acute changes in GFR, whereas **urine output is a real-time indicator** of kidney function. - While the rising creatinine supports the need for nephrology involvement, **urine output is the more immediate and actionable parameter** that prompted the attending's suggestion at this early time point. *Estimated glomerular filtration rate (eGFR)* - eGFR is **calculated from serum creatinine** using equations that assume steady-state conditions, which **do not apply in acute kidney injury**. - In the **acute setting with rapidly changing kidney function**, eGFR calculations are unreliable and can significantly overestimate or underestimate true GFR. - Clinicians rely more on **urine output and serial creatinine measurements** rather than eGFR when managing AKI. *Serum blood urea nitrogen (BUN)* - The BUN decreased slightly from 64 to 62 mg/dL, remaining elevated but showing minimal change after catheterization. - Elevated BUN can reflect **pre-renal azotemia, dehydration, or upper GI bleeding** and is less specific for intrinsic kidney injury than oliguria. - The **BUN:Cr ratio** is approximately 40:1 (64/1.6), suggesting a **pre-renal component**, but this alone doesn't justify urgent nephrology consultation as strongly as the persistent oliguria does. *Serum potassium* - Serum potassium levels remain **normal** (4.2→4.0 mmol/L) and do not indicate a metabolic emergency. - While **hyperkalemia** is a common complication of AKI that would warrant nephrology involvement, this patient's potassium is well-controlled and not the driving concern at this time.

Glomerular filtration US Medical PG Question 8: A researcher is investigating the effects of a new antihypertensive medication on renal physiology. She gives a subject a dose of the new medication, and she then collects plasma and urine samples. She finds the following: Hematocrit: 40%; Serum creatinine: 0.0125 mg/mL; Urine creatinine: 1.25 mg/mL. Urinary output is 1 mL/min. Renal blood flow is 1 L/min. Based on the above information and approximating that the creatinine clearance is equal to the GFR, what answer best approximates filtration fraction in this case?

• A. 10%
• B. 17% (Correct Answer)
• C. 33%
• D. 50%
• E. 25%

Glomerular filtration Explanation: ***17%*** - First, calculate **GFR** using the creatinine clearance formula: GFR = (Urine creatinine × Urinary output) / Serum creatinine = (1.25 mg/mL × 1 mL/min) / 0.0125 mg/mL = **100 mL/min**. - Next, calculate **Renal Plasma Flow (RPF)** from Renal Blood Flow (RBF) and Hematocrit: RPF = RBF × (1 - Hematocrit) = 1000 mL/min × (1 - 0.40) = **600 mL/min**. - Finally, calculate **Filtration Fraction (FF)** = GFR / RPF = 100 mL/min / 600 mL/min = 0.1667 = **16.7%, which approximates to 17%**. - This is the correct answer based on the physiological calculations and represents a normal filtration fraction. *10%* - This would correspond to a filtration fraction of 0.10, which would require either a GFR of 60 mL/min (lower than calculated) or an RPF of 1000 mL/min (higher than calculated). - This value is too low given the provided parameters and doesn't match the calculation from the given data. *25%* - This value would suggest FF = 0.25, requiring a GFR of 150 mL/min with the calculated RPF of 600 mL/min. - This is higher than the calculated GFR of 100 mL/min and doesn't match the given creatinine values. *33%* - This would imply FF = 0.33, requiring a GFR of approximately 200 mL/min with RPF of 600 mL/min. - This is significantly higher than the calculated GFR and would represent an abnormally elevated filtration fraction. *50%* - A filtration fraction of 50% is unphysiologically high and would indicate severe pathology. - This would require a GFR of 300 mL/min with the calculated RPF, which is impossible given the provided creatinine clearance data.

Glomerular filtration US Medical PG Question 9: A 48-year-old woman comes to the physician for a follow-up examination. At her visit 1 month ago, her glomerular filtration rate (GFR) was 100 mL/min/1.73 m2 and her renal plasma flow (RPF) was 588 mL/min. Today, her RPF is 540 mL/min and her filtration fraction (FF) is 0.2. After her previous appointment, this patient was most likely started on a drug that has which of the following effects?

• A. Inhibition of the renal Na-K-Cl cotransporter
• B. Constriction of the afferent arteriole
• C. Relaxation of urinary smooth muscle
• D. Constriction of the efferent arteriole (Correct Answer)
• E. Inhibition of vasopressin

Glomerular filtration Explanation: ***Constriction of the efferent arteriole*** - The previous GFR was 100 mL/min and RPF was 588 mL/min. For the follow-up, RPF is 540 mL/min and FF is 0.2. The new GFR can be calculated as FF × RPF = 0.2 × 540 = **108 mL/min**. - The patient shows **increased GFR** (100→108 mL/min) with **decreased RPF** (588→540 mL/min), resulting in an **increased filtration fraction**. - Medications that **constrict the efferent arteriole**, such as **NSAIDs**, produce this pattern by blocking prostaglandin synthesis. Prostaglandins normally cause vasodilation (predominantly of the afferent arteriole). When blocked, there is relatively more **efferent arteriolar constriction**, which increases glomerular hydrostatic pressure, thereby **increasing GFR while reducing overall RPF**. *Inhibition of the renal Na-K-Cl cotransporter* - This effect describes **loop diuretics** (e.g., furosemide), which increase sodium excretion and water diuresis. - Loop diuretics typically cause a **decrease in GFR** due to reduced fluid volume and lower filtration pressure, which contradicts the slight increase in GFR observed. *Constriction of the afferent arteriole* - **Afferent arteriole constriction** (e.g., by NSAIDs in high doses or norepinephrine) would decrease blood flow into the glomerulus, leading to a **decrease in both RPF and GFR**. - While RPF decreased in this case, GFR actually increased, making this option incorrect. *Relaxation of urinary smooth muscle* - Relaxation of urinary smooth muscle is characteristic of drugs like **alpha-blockers** (e.g., tamsulosin) or antimuscarinics used for conditions like benign prostatic hyperplasia or overactive bladder. - This effect primarily impacts urine flow out of the bladder and does **not directly affect GFR or RPF** in the way described. *Inhibition of vasopressin* - Vasopressin (ADH) inhibition leads to **increased water excretion** and is seen with drugs like **vasopressin receptor antagonists** (vaptans) or ethanol. - While it affects fluid balance, it typically causes a **decrease in GFR** due to hypovolemia and has no direct mechanism to increase GFR with decreased RPF as observed.

Glomerular filtration US Medical PG Question 10: A 75-year-old woman is brought to a physician’s office by her son with complaints of diarrhea and vomiting for 1 day. Her stool is loose, watery, and yellow-colored, while her vomitus contains partially digested food particles. She denies having blood or mucus in her stools and vomitus. Since the onset of her symptoms, she has not had anything to eat and her son adds that she is unable to tolerate fluids. The past medical history is unremarkable and she does not take any medications regularly. The pulse is 115/min, the respiratory rate is 16/min, the blood pressure is 100/60 mm Hg, and the temperature is 37.0°C (98.6°F). The physical examination shows dry mucous membranes and slightly sunken eyes. The abdomen is soft and non-tender. Which of the following physiologic changes in glomerular filtration rate (GFR), renal plasma flow (RPF), and filtration fraction (FF) are expected?

• A. Decreased GFR, decreased RPF, decreased FF
• B. Decreased GFR, decreased RPF, no change in FF
• C. Increased GFR, increased RPF, increased FF
• D. Increased GFR, decreased RPF, increased FF
• E. Decreased GFR, decreased RPF, increased FF (Correct Answer)

Glomerular filtration Explanation: ***Decreased GFR, decreased RPF, increased FF*** - Due to **dehydration** from diarrhea and vomiting, there is a decrease in blood volume leading to decreased renal blood flow and **renal plasma flow (RPF)**. - The body responds to hypovolemia by activating the renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system, which cause **preferential efferent arteriolar constriction** (more than afferent constriction). This helps maintain glomerular hydrostatic pressure despite reduced renal perfusion. - As a result, **GFR decreases** but proportionally **less than RPF decreases**, causing the **filtration fraction (FF = GFR/RPF) to increase**. - In this patient with significant dehydration (tachycardia, hypotension, dry mucous membranes), both GFR and RPF are reduced, but FF is elevated due to compensatory mechanisms. *Decreased GFR, decreased RPF, decreased FF* - While GFR and RPF will decrease due to dehydration, the **filtration fraction is expected to increase**, not decrease. - A decreased FF would imply GFR fell proportionally more than RPF, which contradicts the physiologic response where efferent arteriolar constriction helps preserve GFR relative to RPF. *Decreased GFR, decreased RPF, no change in FF* - With significant fluid loss and compensatory mechanisms (efferent arteriolar constriction via angiotensin II), a change in **filtration fraction** is expected. - The body actively alters arteriolar tone to prioritize GFR maintenance, which directly increases FF. *Increased GFR, increased RPF, increased FF* - This pattern suggests **hypervolemia** or increased renal perfusion, which directly contradicts the patient's severe dehydration. - Both GFR and RPF are expected to decrease in volume depletion, not increase. *Increased GFR, decreased RPF, increased FF* - An increase in GFR is physiologically impossible given the patient's severe volume depletion and reduced renal perfusion. - While FF does increase in dehydration, this occurs in the context of **both GFR and RPF being decreased**, not with an increased GFR.

Glomerular filtration Flashcard 1 of 10

Question: Does the basement membrane of the glomerular filtration barrier normally permit plasma proteins? _____

Answer: No

Glomerular filtration Flashcard 2 of 10

Question: How does non-enzymatic glycation of the efferent renal arteriole affect GFR?_____

Answer: Increased GFR (hyperfiltration)

Glomerular filtration Flashcard 3 of 10

Question: Filtered load (mg/min) can be calculated using the equation: Filtered load = _____

Answer: GFR (mL/min) * [Px] (mg/mL)

Glomerular filtration Flashcard 4 of 10

Question: Cortisol results in a(n) _____ glomerular filtration rate (GFR)

Answer: increased

Glomerular filtration Flashcard 5 of 10

Question: The renal clearance of albumin is normally _____ because albumin is not filtered across glomerular capillaries

Answer: zero

Glomerular filtration Flashcard 6 of 10

Question: What does PBS represent in regards to GFR (fluid exchange)? _____

Answer: Bowman's space hydrostatic pressure

Glomerular filtration Flashcard 7 of 10

Question: What is the effect of prerenal azotemia on GFR? _____

Answer: Decreased GFR

Extra Information:   Watch Renal Function Tests & Acute Kidney Injury (AKI) [https://dashboard.sketchy.com/study/medical/courses/medical-pathophysiology/units/medical-pathophysiology-renal/videos/medical-pathophysiology-renal-acute-kidney-injury-renal-function-tests-and-acute-kidney-injury-aki?utm_source=anki&utm_medium=partnership&utm_campaign=february_update&utm_content=medical] https://onlinemeded.org/spa/nephrology/acute-kidney-injury/acquire?ref=anki Other: Goljan: 

Glomerular filtration Flashcard 8 of 10

Question: What effect does increased plasma protein concentration (πGC) have on filtration fraction? _____

Answer: Decreased FF

Extra Information:  Watch associated Bootcamp video [https://app.bootcamp.com/med-school/nephrology/videos/fluid-and-filtration-physiology?index=4] https://onlinemeded.org/spa/renal/glomerular-filtration/acquire?ref=anki 

Glomerular filtration Flashcard 9 of 10

Question: What effect does dehydration have on RPF? _____

Answer: Decreased RPF

Extra Information:  Watch associated Bootcamp video [https://app.bootcamp.com/med-school/nephrology/videos/fluid-and-filtration-physiology?index=4] https://onlinemeded.org/spa/renal/glomerular-filtration/acquire?ref=anki 

Glomerular filtration Flashcard 10 of 10

Question: If Cx < GFR, there is net tubular _____ of X

Answer: reabsorption

Extra Information:  Watch associated Bootcamp video [https://app.bootcamp.com/med-school/nephrology/videos/fluid-and-filtration-physiology?index=6] https://onlinemeded.org/spa/renal?ref=anki