CH 26 Tubular Reabsorption and Tubular Secretion

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CHAPTER 26 • THE URINARY SYSTEM

tubular reabsorption

is a selective process that reclaims materials from tubular fluid and returns them to the bloodstream

Reabsorbed substances

include water, glucose, amino acids, urea, and ions, such as sodium, chloride, potassium, bicarbonate, and phosphate

tubular secretion

Some substances not needed by the body are removed from the blood and discharged into the urine

tubular fluid

filtered fluid that has entered the proximal convoluted tubule; composition of tubular fluid changes as it flows along the nephron tubule and through the collecting duct due to reabsorption and secretion

urine

fluid that drains from papillary ducts into the renal pelvis

apical membrane

contacts the tubular fluid

basolateral membrane

contacts interstitial fluid at the base and sides of the cell

paracellular route

between tubule cells; tight junctions do not completely seal off the interstitial fluid from the fluid in the tubule lumen

transcellular route

across tubule cells; substance passes from the fluid in the tubular lumen through the apical membrane of a tubule cell, across the cytosol, and out into interstitial fluid through the basolateral membrane

primary active transport

energy derived from hydrolysis of ATP is used to "pump" a substance across a membrane

secondary active transport

energy stored in an ion's electrochemical gradient, rather than hydrolysis of ATP, drives another substance across a membrane

secondary active transport

Symporters and antiporters

antiporters

move two or more substances in opposite directions across a membrane

symporters

move two or more substances in the same direction across the membrane

transport maximum (Tm)

maximum amount of a substance that can be reabsorbed per unit time

water reabsorption

obligatory water reabsorption and facultative water reabsorption

obligatory water reabsorption

90% of water reabsorption is obligatory; occurs via osmosis together with reabsorption of solutes; is not hormonally regulated; is the water reabsorbed with solutes in tubular fluid; occurs in the proximal convoluted tubule and the descending limb of the loop of Henle

facultative water reabsorption

10% of water reabsorption is facultative; varies according to body needs; is regulated by ADH

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