A 45-year-old man presents with a 3-day history of right-sided flank pain due to a lodged ureteral stone. What changes would be expected to be seen at the level of glomerular filtration?

Increase in Bowman's space hydrostatic pressure

Increase in glomerular capillary oncotic pressure

Increase in Bowman's space oncotic pressure

Increase in filtration fraction

No change in filtration fraction

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?

Proximal convoluted tubule via passive diffusion following ion reabsorption

Distal convoluted tubule via passive diffusion following ion reabsorption

Distal convoluted tubule via aquaporin channels

Thick ascending loop of Henle via passive diffusion following ion reabsorption

Collecting duct via aquaporin channels

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?

Decreased GFR, decreased RPF, increased FF

Decreased GFR, decreased RPF, decreased FF

Decreased GFR, decreased RPF, no change in FF

Increased GFR, increased RPF, increased FF

Increased GFR, decreased RPF, increased FF

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?

Constriction of the efferent arteriole

Inhibition of the renal Na-K-Cl cotransporter

Constriction of the afferent arteriole

Relaxation of urinary smooth muscle

A 76-year-old woman presents to the office with a generalized weakness for the past month. She has a past medical history significant for uncontrolled hypertension and type 2 diabetes mellitus. Her temperature is 37.0°C (98.6°F), blood pressure is 135/82 mm Hg, pulse is 90/min, respiratory rate is 17/min, and oxygen saturation is 99% on room air. Physical exam shows no remarkable findings. Her last recorded glomerular filtration rate was 30 mL/min. A radiograph of the patient’s hand is given. Which of the following lab findings is most likely to be found in this patient?

Increased PTH, decreased calcium, increased phosphate

Increased PTH, decreased calcium, decreased phosphate

Normal PTH, increased calcium, normal phosphate

Increased PTH, increased calcium, decreased phosphate

Increased PTH, increased calcium, increased phosphate

Activation of the renin-angiotensin-aldosterone system yields a significant physiological effect on renal blood flow and filtration. Which of the following is most likely to occur in response to increased levels of Angiotensin-II?

Decreased renal plasma flow, increased filtration fraction

Decreased renal plasma flow, decreased filtration fraction

Decreased renal plasma flow, increased glomerular capillary oncotic pressure

Increased renal plasma flow, decreased filtration fraction

Increased renal plasma flow, increased filtration fraction

Starling forces in glomerular filtration for USMLE Step 3: High-Yield Physiology Lessons

Starling Forces - The Pressure Players

📌 Mnemonic: PUSH forces (hydrostatic) vs. PULL forces (oncotic). Filtration happens when PUSH out > PUSH in + PULL in.

Forces Favoring Filtration	Forces Opposing Filtration
* Glomerular Hydrostatic Pressure ($P_{GC}$): Blood pressure in glomerular capillaries. Pushes fluid OUT of capillaries. Value: ≈ 60 mmHg	* Bowman’s Capsule Hydrostatic Pressure ($P_{BS}$): Fluid pressure in Bowman’s space. Pushes fluid back INTO capillaries. Value: ≈ 18 mmHg * Glomerular Oncotic Pressure ($\pi_{GC}$): Protein pressure in glomerular blood. PULLS fluid back INTO capillaries. Value: ≈ 32 mmHg

⭐ Under normal conditions, the filtrate is essentially protein-free, making the oncotic pressure in Bowman's space (π_BS) effectively zero.

The Filtration Equation - Summing It All Up

Net Filtration Pressure (NFP) is the net pressure driving filtration, determined by the sum of Starling forces.
- Equation: $NFP = (P_{GC} - P_{BS}) - (π_{GC} - π_{BS})$
- $P_{GC}$ & $π_{BS}$ favor filtration.
- $P_{BS}$ & $π_{GC}$ oppose filtration ($π_{BS}$ is normally negligible).
Glomerular Filtration Rate (GFR) is the total volume of fluid filtered from the glomeruli into Bowman's space per unit time.
- Equation: $GFR = K_f \times NFP$
- Kf: The filtration coefficient, reflecting capillary permeability and surface area.

⭐ The filtration coefficient (Kf) is not static; it decreases in glomerular diseases (e.g., glomerulonephritis, diabetic nephropathy) that reduce the surface area available for filtration.

Arteriolar Tone - Dialing the Pressure

By adjusting the resistance of the afferent and efferent arterioles, the kidney tightly controls glomerular capillary pressure ($P_{GC}$) and renal plasma flow (RPF), thereby regulating GFR. The relationship is captured by the Filtration Fraction: $FF = GFR / RPF$.

Arteriolar Action	Effect on RPF	Effect on $P_{GC}$	Effect on GFR
Afferent Constriction	↓↓	↓	↓
Afferent Dilation	↑↑	↑	↑
Efferent Constriction	↓	↑↑	↑
Efferent Dilation	↑	↓↓	↓

⭐ Angiotensin II preferentially constricts the efferent arteriole, a key mechanism to preserve GFR when renal perfusion is low.

Effects of afferent/efferent arteriole changes on GFR/RBF

Net filtration pressure (NFP) is the sum of Starling forces and dictates the glomerular filtration rate (GFR).

Glomerular hydrostatic pressure (PGC) is the primary force favoring filtration.

Glomerular oncotic pressure (πGC) is the primary force opposing filtration.

Afferent arteriole constriction (e.g., NSAIDs) ↓ PGC and ↓ GFR.

Efferent arteriole constriction (e.g., Angiotensin II) ↑ PGC and initially ↑ GFR.

Ureteral obstruction ↑ Bowman's capsule hydrostatic pressure (PBS), which ↓ GFR.

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