Loop of Henle & Initial Distal Tubule Transport

A. Role: establishes an appropriate osmotic environment for the adjustment of fluid balance

B. Active Reabsorption

Sodium ion    about 75% of amount entering loop
Chloride ion   about 70% of amount entering

C. Passive Reabsorption

Water              about 25% of amount entering

D. No Transport

Inulin
Creatinine

Summary: Because the reabsorption of sodium and chloride (the major dissolved substances) is much more rapid than the reabsorption of water, the tubular fluid becomes quite hypo-osmotic (100 mOsm/kg,  about one-third the osmolality of plasma). Thus, a hypo-osmotic fluid leaves the loop of Henle (and enters the distal tubule) at the rate of about 20 ml/min.

Later Distal Tubule & Colleting Duct Transport

A. Role: Final adjustment of the fluid to be excreted in order to maintain homeostasis

B. Reabsorption (variable)

1. Active transport

Na⁺              osmotic and electrolyte balance
Cl^-                osmotic and electrolyte balance

2. Passive electrochemical diffusion or osmosis

urea             body fluid balance
H₂O             osmotic and body fluid balance

3. Associated with renal metabolism

HCO₃^-       acid-base balance

C. Secretion (variable)

1. Active transport

K⁺            electrolyte balance
H⁺            acid-base balance

2. Associated with renal metabolism

NH₄⁺       acid-base balance

D. No Transport

1. Tubule impermeable

inulin
creatinine

Summary: leaving the collecting duct (to be excreted as urine) is a fluid of varying composition, varying osmolality (hyper-, hypo-, or iso-osmotic; range 100-1200 mOsm/kg), varying pH, and varying volume (0.2 to 20 ml/min), as required to satisfy homeostasis.