Questions: Glomerular Filtration Rate and Autoregulation

Question 1

A trauma patient arrives with a mean arterial pressure of 55 mmHg due to severe hemorrhage. What happens to glomerular filtration rate?

• A. GFR remains normal — autoregulation maintains filtration regardless of blood pressure
• B. GFR increases — lower pressure triggers compensatory hyperfiltration to maintain urine output
• C. GFR falls sharply — below ~80 mmHg, the afferent arteriole is maximally dilated and cannot compensate further, so filtration drops with perfusion pressure
• D. GFR is unaffected in the short term but falls progressively over 24–48 hours

Answer: (C)

Autoregulation maintains GFR between approximately 80–180 mmHg mean arterial pressure. At 55 mmHg, the system is operating below its autoregulatory floor — the afferent arteriole is already maximally dilated and cannot lower its resistance further. GFR falls with the falling perfusion pressure, urine output drops (oliguria or anuria), and metabolic waste accumulates. This is the physiology of pre-renal acute kidney injury. Option A is the classic misconception: autoregulation has limits, and severe hypotension overwhelms it.

Question 2

In tubuloglomerular feedback, GFR rises transiently. What is the sequence of events that returns GFR toward normal?

• A. Increased GFR → more water delivered to the collecting duct → ADH release → afferent arteriole vasoconstriction
• B. Increased GFR → more NaCl delivered to the macula densa → adenosine release → afferent arteriole constriction → reduced glomerular hydrostatic pressure → GFR normalized
• C. Increased GFR → higher Bowman's capsule pressure → opposition to filtration → GFR self-limited
• D. Increased GFR → more filtrate in the proximal tubule → increased tubular hydrostatic pressure → backpressure reduces net filtration

Answer: (B)

TGF is a closed negative-feedback loop operating within a single nephron. When GFR rises, more NaCl reaches the macula densa (the specialized cells at the junction of the thick ascending limb and distal tubule). These cells detect increased NaCl delivery and release adenosine (and reduce renin release), which constricts the afferent arteriole of the same nephron. This reduces glomerular hydrostatic pressure and brings GFR back toward normal. The signal travels from the distal end of the tubule back to the same nephron's glomerulus — a remarkable example of anatomical precision in physiological regulation.

Question 3

Even a 10% sustained increase in GFR without compensatory tubular reabsorption would cause catastrophic fluid loss, because 10% of the normal 180 L/day filtered load is 18 additional liters of fluid per day.

• T. True
• F. False

Answer: True

This arithmetic illustrates why GFR stability is physiologically critical. The kidney filters roughly 180 L of plasma per day, of which about 178.5 L is reabsorbed and only 1.5 L excreted as urine. A 10% increase in GFR would mean 198 L filtered — 18 extra liters that downstream tubular mechanisms would have to handle. Without autoregulation, routine blood pressure fluctuations from posture changes, exercise, and stress would cause dramatic swings in urine output and electrolyte loss. The extreme precision of autoregulation (keeping GFR constant across an 80-180 mmHg range) exists precisely because the margins are so thin.

Question 4

ACE inhibitors, which block angiotensin II formation, increase GFR by dilating the afferent arteriole and improving renal perfusion pressure.

• T. True
• F. False

Answer: False

This is a common misconception. Angiotensin II preferentially constricts the efferent arteriole (much more than the afferent), which maintains glomerular hydrostatic pressure and GFR when perfusion pressure is low. By blocking angiotensin II formation, ACE inhibitors dilate the efferent arteriole, reducing the resistance that sustains glomerular pressure. This lowers net filtration pressure and typically decreases GFR — which is why ACE inhibitors can cause acute kidney injury in patients whose kidneys depend on angiotensin II to maintain GFR (e.g., bilateral renal artery stenosis, severe heart failure). In clinical practice, creatinine is monitored when starting ACE inhibitors precisely because of this GFR-reducing effect.

Question 5

Why do the myogenic mechanism and tubuloglomerular feedback complement each other rather than being redundant? What aspect of autoregulation does each mechanism specialize in?

Model answer: The two mechanisms operate on different timescales and detect different signals. The myogenic mechanism is fast (seconds) and responds directly to pressure changes in the afferent arteriole wall: increased stretch causes immediate smooth muscle contraction, preventing the pressure rise from reaching the glomerulus. It is a local mechanical response requiring no chemical signaling. Tubuloglomerular feedback is slower (tens of seconds to minutes) and responds to the functional consequence of GFR change — the NaCl concentration actually delivered to the distal tubule. It closes the feedback loop between filtration output and filtration rate. Together, the myogenic mechanism provides rapid, pressure-sensitive upstream protection, while TGF provides flow-sensitive downstream correction that fine-tunes GFR based on actual tubular delivery. The combination is more robust than either alone.

Dual autoregulatory mechanisms are common in physiology when a single mechanism would be inadequate. The myogenic response can be overwhelmed by sustained hypertension or rapid pressure swings; TGF would be too slow to prevent glomerular damage during sudden pressure spikes. TGF can fail if tubular transport is impaired; the myogenic response is independent of tubular function. Their complementarity makes the system resistant to single points of failure.