Rishi Desai, MD, MPH

Tubular reabsorption and secretion of weak acids and bases

Chapter 62 Renal Physiology: Renal Reabsorption & Secretion  UREA RECYCLING osms.it/urea-recycling ▪ Urea: one of body’s waste products (byproduct of amino acid breakdown) ▪ Freely ﬁltered across kidneys’ glomerular capillaries, travels through renal tubule ▪ Part of reabsorbed urea secreted back into loop of Henle → “urea recycling” ▫ Helps establish corticopapillary gradient (reabsorbs water from kidneys back into blood) Four steps to urea recycling ▪ 50% of urea reabsorbed by simple diffusion in proximal convoluted tubule (leaving behind 50% of initial urea), together with water ▪ Urea from medullary interstitium secreted back into tubule in descending limb of loop  of Henle (resulting in 110% of initial urea in bottom of loop of Henle) ▫ Occurs due to higher urea concentration in medullary interstitium ▪ Ascending limb of loop of Henle, early distal convoluted tubule impenetrable to urea, water (urea levels stay same) ▪ 70% of initial urea reabsorbed into interstitium in late distal convoluted tubule, cortical, outer medullary collecting ducts (leaving behind 40% of initial urea to be excreted in urine) ▫ Occurs due to antidiuretic hormone (ADH)-induced water reabsorption through aquaporins → concentration gradient of urea towards interstitium  WEAK ACIDS & BASES NON-IONIC DIFFUSION osms.it/non-ionic_diffusion ▪ Many substances secreted by proximal tubule weak acids/bases ▪ Exist in uncharged (nonionic)/charged (ionized) forms; amount depends on pH of tubular ﬂuid ▫ Urine with low pH: nonionic forms dominate ▫ Urine with higher pH: ionized forms dominate  ▪ Nonionic weak acids, bases lipid soluble, able to passively diffuse back into blood from urine ▪ Ionized weak acids, bases not lipid soluble, remain in tubular ﬂuid to be excreted ▪ Excretion of unwanted substances, toxins accomplished by manipulating urine pH, promoting ionization  OSMOSIS.ORG 547   

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Tubular reabsorption and secretion of weak acids and bases

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