The Urinary System
The urinary system regulates blood volume, electrolytes, pH, blood pressure, and removes metabolic wastes. Each kidney contains about 1 million nephrons — functional units that filter blood and produce urine. Nephrology and urology cover conditions from UTIs and kidney stones to chronic kidney disease and dialysis.
Learning Objectives
- 1Describe the anatomy and function of the kidneys, ureters, bladder, and urethra
- 2Explain urine formation: filtration, reabsorption, secretion
- 3Identify combining forms for urinary system structures
- 4Recognize common urinary diseases and their terminology
- 5Interpret urinalysis results and renal function tests
1Kidney Anatomy
The kidneys are two bean-shaped organs in the retroperitoneal space. Each kidney contains:
Cortex (outer): Contains glomeruli and proximal/distal tubules
Medulla (inner): Contains loops of Henle and collecting ducts; pyramids and calyces
Renal pelvis: Funnel-shaped collecting chamber → ureter
The Nephron (functional unit, ~1 million per kidney):
1. Glomerulus: Capillary cluster — pressure filtration
2. Bowman's capsule: Collects filtrate
3. Proximal convoluted tubule (PCT): Reabsorbs 65% of filtrate
4. Loop of Henle: Concentrates urine
5. Distal convoluted tubule (DCT): Fine-tunes electrolytes
6. Collecting duct: Final concentration; ADH acts here
2Urine Formation
Three processes:
1. Filtration: Blood pressure forces water and small solutes from glomerulus → Bowman's capsule (180 L/day filtered)
2. Reabsorption: 99% of filtrate reabsorbed back into blood (glucose, amino acids, water, electrolytes)
3. Secretion: Additional wastes moved from blood into tubule (H⁺, K⁺, drugs, toxins)
Final urine: ~1–2 liters/day. Composition: water, urea, creatinine, uric acid, electrolytes
ADH (antidiuretic hormone from posterior pituitary): Increases water reabsorption → concentrated urine
Aldosterone (adrenal cortex): Increases Na⁺ reabsorption → increases water retention → increases blood pressure
3Common Urinary Conditions
UTI (Urinary Tract Infection): E. coli most common; cystitis (bladder) vs pyelonephritis (kidney)
Kidney Stones (Nephrolithiasis): Calcium oxalate most common; severe colicky flank pain
Chronic Kidney Disease (CKD): Progressive loss of GFR; staged 1–5
Glomerulonephritis: Immune-mediated inflammation of glomeruli → proteinuria, hematuria
Nephrotic Syndrome: Massive proteinuria, edema, hypoalbuminemia
Acute Kidney Injury (AKI): Sudden drop in GFR; prerenal, intrinsic, or postrenal causes
4Diagnostics and Treatment
Urinalysis (UA): pH, specific gravity, protein, glucose, RBCs, WBCs, casts, bacteria
Blood tests: BUN (blood urea nitrogen), creatinine, GFR (glomerular filtration rate), electrolytes
Imaging: Renal ultrasound, CT KUB (kidney-ureter-bladder), IVP (intravenous pyelogram)
Cystoscopy: Direct visualization of bladder and urethra
Dialysis (for ESRD):
Clinical Connections
- ►UTIs are the most common bacterial infection in women — 50–60% will have at least one in their lifetime
- ►Diabetes mellitus and hypertension are the two most common causes of chronic kidney disease
- ►The kidneys receive 20–25% of cardiac output — reflecting their critical filtration role