Solute Reabsorption

Most filtered solutes must be _____ to maintain normal body composition.

reabsorbed

Filtration

the movement of solutes and water from glomerular capillaries into Bowman’s space

Reabsorption

the movement of substances from the tubular lumen back into the blood

Secretion

transfer of substances from blood into the tubular lumen

Over 99% of filtered water and solutes are reabsorbed along the ____.

renal tubule

Passive transport

occurs down an electrochemical gradient, including diffusion and osmosis

ex: chloride following sodium movement

Solvent drag

mainly occurs in the proximal convoluted tubule, especially the paracellular route; water moves first, and solute follows (e.g. Na+, Cl-, Ca2+, Mg2+)

Primary active transport

direct use of ATP

ex: Na+/K+ -ATPase on the basolateral membrane

What is the significance of the sodium potassium pump?

maintains low intracellular sodium, driving most reabsorption

Secondary active transport

powered by ATP via sodium gradients (e.g. Na+—glucose)

sodium-solute exchangers (e.g. Na+/H+ exchanger)

Fractional excretion

the fraction of the filtered amount that is excreted in the urine

Low fraction exchange (FE) % of sodium means _____.

strong sodium reabsorption

High fractional exchange (FE) % of sodium means ____.

impaired tubular reabsorption

What is the physiological significance of fractional excretion rate?

Reflects tubular handling of a solute

Helps distinguish between pre-renal reduced GFR and tubular reabsorption or secretion defects due to kidney disease

The proximal tubule reabsorbs about _____% of the filtered load.

65-70%

The proximal tubule has ____ on the apical (luminal) side and ____ on the basolateral side that increase the surface area.

Dense microvilli (brush border)

infoldings