Kidney
stub — Seeded as a structural anchor for the urinary-system; not yet written out.
The kidney is the principal organ of phosphate excretion and the main target organ of the bone-mineral endocrine axis: it is where fgf23 (via its klotho co-receptor) downregulates phosphate reabsorption and suppresses vitamin D activation. Age-related decline in GFR, nephron number, and renal Klotho expression drives hyperphosphatemia and secondary hyperparathyroidism, tying renal aging to the musculoskeletal-system and cardiovascular-system. Renal aging features tubulointerstitial fibrosis, glomerulosclerosis, and a high senescent-cell burden.