Avian Kidneys: Structure, Function & Unique Excretion

Birds have two metanephric kidneys located in the pelvic fossa of the synsacrum (fused pelvic vertebrae), dorsally in the abdominal cavity.

Key Features

• No bladder: Urine flows directly from ureters → cloaca
• Uricotelic excretion: Nitrogenous waste excreted mainly as uric acid (semi-solid, low water loss)
• Nephron types:
  • Cortical (reptilian-type): glomerulus in cortex, short loop of Henle → cannot concentrate urine
  • Medullary (mammalian-type): glomerulus near medulla, long loop of Henle → can concentrate urine
• Renal portal system: Blood from hindlimbs can bypass kidneys via renal portal valve

Functions

• Water & electrolyte balance (conservation critical for flight)
• Acid-base regulation
• Excretion of uric acid, urea, creatinine, drugs/toxins
• Calcium/phosphate homeostasis (eggshell formation)
• Hormone production (erythropoietin, renin, vitamin D activation)

Cloacal Mixing

Urine (uric acid paste) + feces + reproductive products → expelled via cloaca. This reduces weight (no bladder) and water loss—key adaptations for flight.