Excretion

• Driving Force for Tubular Fluid Flow: Glomerular ultrafiltrate is transported down the nephron from Bowman's space to the renal pelvis primarily by a Hydrostatic Pressure Gradient.
• Pressure Dynamics: Glomerular filtration establishes a high hydrostatic pressure in Bowman's capsule ($\sim 15\text{--}20\text{ mmHg}$), whereas the renal pelvis maintains a near-zero pressure ($\sim 0\text{--}2\text{ mmHg}$), propelling fluid down the tubular network without ciliary propulsion or active peristalsis (Option 3).

Correct option: (3) Hydrostatic pressure gradient

• Stepwise Biosynthesis of Active Vitamin D:

• Skin: 7-dehydrocholesterol is converted by solar UV light into cholecalciferol (Vitamin $D_3$).
• Liver: Cholecalciferol undergoes 25-hydroxylation by hepatic 25-hydroxylase to form 25-hydroxyvitamin $D_3$ [calcidiol, $25(OH)D_3$].
• Kidney: Calcidiol is transported to the proximal convoluted tubules of the Kidney, where the mitochondrial enzyme $1\alpha$-hydroxylase (stimulated by parathyroid hormone) adds a second hydroxyl group to produce 1,25-dihydroxyvitamin $D_3$ [Calcitriol, $1,25(OH)_2D_3$].
  • Calcitriol is the biologically active hormonal form released by the kidney into the systemic circulation to regulate calcium and phosphate homeostasis.

• nalysis:
• (Kidney): Correct organ releasing active calcitriol.

• Target Site of Aldosterone Action: The mineralocorticoid Aldosterone exerts its greatest physiological regulatory action on the Principal Cells of the Cortical Collecting Duct (CCD).
• Mechanism: Binds intracellular mineralocorticoid receptors to upregulate the expression and apical membrane insertion of epithelial sodium channels (ENaC) and basolateral $\text{Na}^+/\text{K}^+$ ATPase pumps, stimulating active sodium reabsorption coupled to potassium and proton secretion.

Correct option: (4) Cortical collecting duct

• Renal Site of Aldosterone Action: Aldosterone stimulates active sodium reabsorption and potassium excretion in the Principal Cells of the Cortical Collecting Duct (CCD) and late distal convoluted tubule by inducing ENaC and $\text{Na}^+/\text{K}^+$ ATPase pumps (Option 1).

Correct option: (1) Cortical collecting duct