Kidney, Nephron, Bladder & Urethra: Functional Anatomy Overview

Nephron – The Kidney's Functional Unit

• ~1 million per kidney.
• Components: glomerulus (filtration), proximal tubule, loop of Henle, distal tubule, collecting duct.
• Functions: Filters 180 L/day plasma → 1–2 L urine; reabsorbs 99% water, glucose, amino acids, electrolytes; secretes H⁺, K⁺, drugs; regulates acid-base, BP (renin), erythropoietin, vitamin D activation.

Bladder – Storage & Expulsion

• Distensible muscular reservoir (detrusor muscle).
• Capacity: 400–600 mL adults; urge sensation at 150–300 mL.
• Storage: Sympathetic (hypogastric) → detrusor relaxation, internal sphincter contraction.
• Voiding: Parasympathetic (pelvic) → detrusor contraction + internal sphincter relaxation; voluntary external sphincter (pudendal) relaxation.

Urethra – Conduit & Continence

• Male: 18–20 cm; prostatic, membranous, bulbar, penile segments. Dual role: urine + semen.
• Female: 4 cm; straight, shorter → higher UTI risk.
• Internal sphincter (smooth, involuntary) at bladder neck; external sphincter (skeletal, voluntary) in urogenital diaphragm.

Integrated Physiology

Kidneys produce urine continuously → ureters (peristalsis) → bladder stores → pontine micturition center coordinates voiding when socially appropriate. Disruption at any level causes dysfunction (retention, incontinence, reflux).