A 72-year-old man is brought to the emergency department due to diarrhea and vomiting for the last 24 hours. The patient's daughter states that he has been unable to take in much fluid during this time. He has hypertension treated with hydrochlorothiazide, which he has not taken since the onset of symptoms. Blood pressure is 90/60 mm Hg and pulse is 105/min. Examination shows dry mucous membranes. Urinalysis reveals concentrated urine with a specific gravity of 1.030. Which of the following changes in renal plasma flow (RPF), glomerular filtration rate (GFR), and filtration fraction (FF) are most likely to be present in this patient as compared with the normal state?

Question

A 72-year-old man is brought to the emergency department due to diarrhea and vomiting for the last 24 hours.  The patient's daughter states that he has been unable to take in much fluid during this time.  He has hypertension treated with hydrochlorothiazide, which he has not taken since the onset of symptoms.  Blood pressure is 90/60 mm Hg and pulse is 105/min.  Examination shows dry mucous membranes.  Urinalysis reveals concentrated urine with a specific gravity of 1.030.  Which of the following changes in renal plasma flow (RPF), glomerular filtration rate (GFR), and filtration fraction (FF) are most likely to be present in this patient as compared with the normal state?

Quizplus · Accepted Answer

11ec160b_3a44_a0dd_9249_ef54fdd98490_MD0001_00 Filtration of fluid through the glomeruli depends on the renal plasma flow (RPF) and the glomerular filtration rate (GFR). The GFR is the volume of fluid filtered from the renal glomerular capillaries into the Bowman capsule per unit of time. The RPF is the volume of plasma delivered to the kidney per unit of time. The filtration fraction (FF) is the ratio of GFR to RPF (FF = GFR/RPF). On average, approximately one-fifth of the plasma that passes through the glomerular capillaries is filtered into the Bowman capsule. This patient is severely hypovolemic due to profuse diarrhea and vomiting. The decline in circulating blood volume is sensed by arterial and cardiac baroreceptors and triggers increased systemic arteriolar vasoconstriction. Renal vasoconstriction further lowers the RPF, which is already decreased due to the decline in circulating blood volume. Decreased RPF causes glomerular perfusion pressure to drop, lowering the GFR and reducing distal tubule sodium delivery. This stimulates secretion of renin and increased angiotensin II production. Angiotensin II preferentially constricts the efferent glomerular arteriole, which increases hydrostatic pressure in the glomerular capillaries to maintain GFR (autoregulation). Due to this compensatory mechanism, the decrease in GFR is less pronounced than the decrease in RPF, resulting in an increased FF (Choices B, C, and E). As RPF continues to decline, increasing glomerular oncotic pressure will eventually overwhelm the compensatory increase in hydrostatic pressure, leading to a precipitous drop in GFR and subsequent renal failure. (Choice D) Autoregulation improves but often cannot completely normalize the GFR in the setting of decreased renal perfusion pressure. If the net GFR were increased above normal, it would result in decreased renin formation and loss of the angiotensin II-driven arteriolar constriction required to maintain it. Educational objective: Hypovolemia results in a reduced renal plasma flow (RPF) and glomerular filtration rate (GFR). This leads to compensatory efferent arteriolar vasoconstriction, which raises the filtration fraction and maintains GFR at near-normal levels. As RPF continues to decline, increasing glomerular oncotic pressure will eventually overwhelm the compensatory increase in hydrostatic pressure, leading to a precipitous drop in GFR and renal failure.

A 72-Year-Old Man Is Brought to the Emergency Department Due