renal pyramid

**Anatomy** - Each kidney has ~8–18 renal pyramids (medullary pyramids) that make up the renal medulla. - The base of each pyramid faces the cortex (corticomedullary junction). The apex, the renal papilla, projects into a minor calyx. - Renal columns (of Bertin) are extensions of cortex that lie between adjacent pyramids. - One pyramid plus its overlying cortex forms a renal lobe. - Urine drainage: collecting ducts → papillary ducts (of Bellini) → area cribrosa of the papilla → minor calyx → major calyx → renal pelvis → ureter. - The pyramids have a striated appearance because the tubules and vasa recta run parallel and straight.

**Histology/contents** - Descending and ascending limbs of the loops of Henle (mainly from juxtamedullary nephrons for the deep medulla). - Collecting ducts, lined by principal cells and intercalated cells. They are ADH-responsive via aquaporin-2. - Vasa recta, the capillary loops that run parallel to the loops of Henle. - Glomeruli and convoluted tubules are NOT in the pyramids. They are in the cortex.

**Physiology** - The medullary interstitium has an osmotic gradient (~300 mOsm/L at the corticomedullary junction to ~1200 mOsm/L at the papillary tip). - The gradient is generated by the countercurrent multiplier (loop of Henle, NKCC2 in the thick ascending limb) and by urea recycling from the inner medullary collecting duct. The vasa recta maintain it as countercurrent exchangers. - With ADH, aquaporin-2 inserts into the collecting duct, water is reabsorbed along the gradient, and concentrated urine results. - The medulla has low oxygen tension because blood flow is low (~10% of renal flow) and metabolic demand from active transport is high. The inner medulla and papillae are therefore susceptible to ischemia.

**Clinical correlations** - **Renal papillary necrosis:** ischemic coagulative necrosis of the pyramids/papillae. Causes: chronic NSAID/analgesic use (loss of vasodilatory prostaglandins), sickle cell disease/trait (hypoxic, hypertonic medulla promotes sickling), diabetes mellitus, acute pyelonephritis, urinary tract obstruction. Mnemonic: "POSTCARDS" (Pyelonephritis, Obstruction, Sickle cell, Tuberculosis, Cirrhosis, Analgesics, Renal transplant rejection, Diabetes, Systemic vasculitis). Findings: gross hematuria, flank pain, sloughed papillae in urine. Imaging may show a "ring sign" or "golf-tee" calyx. - **Medullary sponge kidney:** cystic dilation of the collecting ducts in the pyramids. Imaging shows a "paintbrush" or "bouquet of flowers" appearance on IVP. Associated with nephrolithiasis and hematuria. - **Autosomal recessive polycystic kidney disease:** dilated collecting ducts with radial cysts. - **Nephrocalcinosis** (e.g., from hypercalcemia or distal RTA) deposits calcium in the pyramids. - **Loop diuretics** (inhibit NKCC2) and **lithium** (nephrogenic DI) disrupt medullary concentrating ability. In nephrogenic DI, the collecting ducts are unresponsive to ADH. - **Medullary rays** are not the same as pyramids. They are collections of straight tubules that extend into the cortex.

**High-yield pearls** - Medulla = pyramids (the tubules are straight). Cortex = glomeruli + proximal and distal convoluted tubules. - Pyramid apex = papilla → minor calyx. - The vulnerable site for ischemia is the papilla/inner medulla.