A 51-year-old man comes to the emergency department with sudden-onset, sharp, left flank pain; nausea; and vomiting. He has had no dysuria or hematuria. The patient has no past medical conditions and takes no daily medications. He does not use tobacco, alcohol, or illicit drugs. Temperature is normal. On examination, the patient appears to be in severe pain and cannot find a comfortable position on the bed. There is no abdominal rigidity or rebound, and no masses are palpable. Cardiopulmonary examination is normal. There is no peripheral edema. Imaging shows a 1-cm calculus in the left proximal ureter at the level of the L3 vertebra; the renal pelvis and proximal ureter are dilated, as shown in the exhibit. Which of the following is most likely increased in this patient's left kidney?
The glomerular filtration rate depends on the permeability of the capillary wall and the net ultrafiltration pressure. Net ultrafiltration pressure is a result of pressure gradients formed by Starling forces:
The hydrostatic pressure gradient is the difference between the hydrostatic pressure in the intraglomerular capillaries and the Bowman space. Typically, the hydrostatic pressure in the glomerular capillaries is markedly greater than the pressure in the Bowman space, favoring filtration.
The oncotic pressure gradient is the difference between the oncotic pressure in the intraglomerular capillaries and the Bowman space. Oncotic pressure is chiefly driven by large plasma proteins (eg, albumin), which do not freely filter across the glomerular capillary basement membrane due to both a size and charge barrier. Therefore, the oncotic pressure is negligible within the Bowman space, favoring absorption of fluid into the glomerular capillaries.
This patient has a kidney stone in the left ureter; the hydroureter and hydronephrosis suggest acute urinary tract obstruction. The resultant reflux of urine backward into the relatively noncompliant renal tubules results in increased renal tubular hydrostatic pressure. As the intraglomerular capillary hydrostatic pressure is unchanged, this results in a decreased hydrostatic pressure gradient leading to a reduction in glomerular filtration (Choices B and C).
(Choices A and E) Oncotic pressure in the Bowman space is negligible unless glomerular disease (eg, minimal change disease, diabetic nephropathy) allows proteins to leak into the Bowman space. The renal tubules are contiguous with the Bowman space and therefore also have negligible oncotic pressure. Acute urinary tract obstruction would not affect protein filtration at the glomerulus; therefore, the oncotic pressure in these regions remains essentially unchanged.
Educational objective:
Urinary tract obstruction causes reflux of urine into the renal tubules and increased tubular hydrostatic pressure. The intraglomerular capillary hydrostatic pressure is unchanged, resulting in a decreased hydrostatic pressure gradient across the glomerular capillary wall and a reduction in glomerular filtration. Oncotic pressure is maintained by large plasma proteins which are not filtered across the glomerular capillary basement membrane; it is unaffected by a urinary tract obstruction.