Renal Physiology: Glomerular Filtration and Tubular Function Flashcards

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GFR

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Glomerular filtration rate (GFR) is the volume of plasma filtered through the glomeruli into renal tubules per minute. Normal adult GFR is about 125 mL/min and is a primary measure of kidney function.

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Kf

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The glomerular ultrafiltration coefficient (Kf) equals capillary permeability multiplied by filtration surface area. Changes in Kf alter GFR by changing how readily plasma is filtered across the glomerular barrier.

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GBM charge

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The glomerular basement membrane (GBM) is negatively charged and repels negatively charged molecules. This charge selectivity reduces filtration of proteins like albumin despite their relatively small size.

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Inulin clearance

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Inulin clearance is the gold-standard measure of GFR because inulin is freely filtered, not reabsorbed or secreted, and not metabolized. Measuring inulin clearance requires exogenous administration and urine collections.

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Creatinine clearance

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Creatinine clearance estimates GFR using an endogenous compound produced by muscle. It slightly overestimates true GFR because a small amount of creatinine is secreted by renal tubules.

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Tubuloglomerular feedback

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Tubuloglomerular feedback (TGF) links $NaCl$ delivery sensed by the macula densa to adjustments in afferent arteriolar resistance. TGF helps autoregulate GFR by reducing or increasing glomerular filtration when tubular $NaCl$ is high or low.

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Glomerulotubular balance

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Glomerulotubular balance maintains a relatively constant fraction of the filtered load reabsorbed by the proximal tubule despite changes in GFR. It prevents distal overload when GFR fluctuates.

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Transport maximum (Tm)

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Transport maximum is the maximal rate at which a substance can be reabsorbed or secreted by the renal tubules. If the filtered load exceeds Tm, the excess is excreted in urine, as seen with glucose in uncontrolled diabetes.

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Renal threshold

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Renal threshold is the plasma concentration at which a substance first appears in urine. It often occurs below Tm because individual nephrons have variable transport capacities.

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Proximal tubule

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The proximal tubule reabsorbs the majority of filtered water, $Na^+$, bicarbonate, glucose, and amino acids. Fluid leaving the proximal tubule is typically iso-osmotic to plasma.

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Loop of Henle

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The loop of Henle creates the medullary osmotic gradient; the descending limb is water-permeable while the ascending limbs are impermeable to water. The thick ascending limb actively reabsorbs $Na^+$, $K^+$, and $2Cl^-$ via NKCC2.

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Thick ascending limb

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The thick ascending limb (TAL) is a major site for active $NaCl$ reabsorption via the NKCC2 cotransporter and is impermeable to water, making it the principal diluting segment. It is targeted by loop diuretics.

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Principal cell

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Principal cells in the late distal tubule and collecting duct mediate $K^+$ secretion and $Na^+$ reabsorption and respond to aldosterone and ADH. They play a central role in potassium balance and water handling.

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Type A intercalated

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Type A intercalated cells secrete $H^+$ and reabsorb $HCO_3^-$, contributing to acid excretion and systemic acid–base balance. They are important for correcting acidosis.

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ACE inhibitor

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ACE inhibitors dilate the efferent arteriole, reducing intraglomerular pressure and proteinuria, which slows progression of CKD. They can transiently reduce GFR and raise serum $K^+$, so monitoring is required.

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Renal clearance

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Renal clearance is the volume of plasma cleared of a substance per unit time, calculated from urine and plasma concentrations and urine flow. It reflects the combined effects of filtration, reabsorption, and secretion.

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Autoregulation

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Renal autoregulation maintains relatively constant renal blood flow and GFR across a wide range of systemic blood pressures. Mechanisms include the myogenic response and tubuloglomerular feedback.

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ADH

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Antidiuretic hormone (ADH) increases water permeability in the collecting duct, promoting water reabsorption and concentrating urine. ADH action helps maintain plasma osmolality and volume.

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Aldosterone

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Aldosterone increases $Na^+$ reabsorption and $K^+$ secretion in the distal nephron by upregulating ENaC channels and intracellular pumps. It is a key regulator of extracellular fluid volume and potassium homeostasis.

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Proteinuria

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Proteinuria indicates abnormal loss of proteins in urine, usually from increased glomerular permeability or loss of charge selectivity. Persistent proteinuria is a marker of glomerular disease and contributes to CKD progression.