Renal

Question 1

Nephron osmolality1

Answer 1

• Water deprivation results in antidiuretic hormone (ADH) release from the posterior pituitary gland
• ADH stimulates V2 receptors on principal cells in the renal collecting ducts, activating a cAMP second-messenger G-protein system that leads to insertion of endosomes containing aquaporin 2 into the apical cell membrane.
• Aquaporin 2 is a water channel that spans the luminal membrane, enhancing the water permeability of the principal cells.
• When ADH levels are low, aquaporin 2 accumulates within pits along the luminal membrane and is returned to the cell cytoplasm through endocytosis

Question 2

Physio of tubular segments

Answer 2

presence of high ADH, the tubular fluid osmolarity follows this pattern:
1. In the proximal tubule, water is reabsorbed along with electrolytes. The tubular fluid in this segment remains isotonic with plasma (300 mOsm/L) whether the final urine is concentrated or diluted.

2. In the descending limb of the loop of Henle, free water is drawn out of the tubules into the renal interstitium (REABSORBED) and the tubular fluid becomes hypertonic (> 300 mOsm/L, typically reaching 1200 mOsm/L when ADH levels are high)
3. Electrolytes, but not water, are reabsorbed in the thin ascending limb of the loop of Henle and the tubular fluid becomes hypotonic (< 300 mOsm/L. The thick and thin ascending limbs are the primary region of urine dilution, achieved mainly through NaCl reabsorption.
4. distal convoluted tubule is relatively impermeable to water, so the tubular fluid remains hypotonic. Reabsorption of solutes continues to occur; thus, fluid in the distal tubules is the most dilute (lowest osmolarity, approaching 100 mOsm/L).
5. In the presence of ADH, the collecting duct is highly permeable to water. Water leaves the tubular fluid driven by the high osmolarity of the medullary interstitium, and hypertonic urine is formed (up to 1200 mOsm/L).
   The collecting duct system is the primary region of urine concentration through the mechanism of ADH-mediated water absorption.

In an animal deprived of water for 12 hours, ADH levels will be high. Thus, the distal tubule will contain the most dilute urine while the collecting ducts will contain the most concentrated urine.