DISTAL TUBULE AND COLLECTING DUCTS

A. Initial Segment of Distal Tubule: properties similar to thick limb of loop of Henle in relation to NaCl and water transport

B. Final Segment of Distal Tubule and Collecting Ducts

1. Hypo-osmotic fluid (100 mOsm/kg) enters the later distal tubule at a rate of about 20-25 ml/min

2. As the fluid passes through the renal medulla in the collecting ducts, a large osmotic gradient exists between the tubular and interstitial fluids favoring water reabsorption

3. Rate of water reabsorption depends on the water permeability of the distal tubule and collecting ducts

4. Water permeability is controlled by antidiuretic hormone

5. Na+ is reabsorbed and K+ is secreted at a rate controlled by the hormone renin-angiotensin-aldosterone system

6. H+ is secreted to maintain body pH (acid-base balance)

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

Note: Will cover distal tubule and collecting duct function in detail when discussing water/osmotic control, electrolyte/volume regulation, and pH balance.