17. URINARY SYSTEM – ANATOMY AND PHYSIOLOGY OF URINARY SYSTEM

URINARY SYSTEM

The urinary system is formed by organs responsible for converting excess fluid and other substances into urine, its filtration, and its excretion from the body. These organs include kidneys, ureters, urinary bladder, and urethra. Urine excretes excess minerals, vitamins, and blood cells from the body.

PARTS OF URINARY SYSTEM

1. A pair of kidneys (forming urine)
2. A pair of ureters (transporting urine)
3. A urinary bladder (storing urine)
4. A urethra (carrying urine outside the body)

KIDNEYS

Kidneys are bean-shaped, positioned retroperitoneally in the superior lumbar region. The right kidney is slightly lower than the left. Mass: ~150g; size: 12 cm long, 6 cm wide, 3 cm thick. Kidneys produce urine and secrete hormones (calcitriol, erythropoietin, renin).

Layers of Kidneys

1. Renal Fascia: Outer layer; attaches kidneys to peritoneum and posterior abdominal wall
2. Adipose Capsule: Middle, thickest layer; holds kidneys in place and protects against physical trauma
3. Renal Capsule: Innermost thin layer; protects from infection and physical trauma

Internal Structure of Kidney

• Renal Cortex: Outer zone; light in colour, granular appearance
• Renal Medulla: Middle zone; darker, reddish-brown; contains medullary pyramids (cone-shaped tissue masses)
• Renal Pelvis: Funnel-shaped tube joining ureter at hilum; branches into major and minor calyces

Functions of Kidneys

1. Regulation of blood ionic composition (Na⁺, K⁺, Ca²⁺, Cl⁻, HPO₄²⁻)
2. Regulation of blood pH (excreting H⁺, conserving HCO₃⁻)
3. Regulation of blood volume (conserving water or eliminating excess urine)
4. Regulation of blood pressure (secreting renin)
5. Maintenance of blood osmolarity (~300 mOsm/litre)
6. Production of hormones (calcitriol, erythropoietin)
7. Regulation of blood glucose level (gluconeogenesis)
8. Excretion of wastes (ammonia, urea, bilirubin, creatinine, uric acid) and foreign substances

NEPHRONS

The nephron is the basic structural and functional unit of the kidney. Types: Cortical nephrons (renal corpuscles in superficial cortex) and Juxtamedullary nephrons (renal corpuscles near renal medulla).

Parts of a Nephron

• Renal Corpuscle: Glomerulus (capillary mass) + Bowman’s capsule (surrounds glomerulus; visceral layer has podocytes)
• Renal Tubule (3 cm long): Proximal Convoluted Tubule (PCT) → Loop of Henle → Distal Convoluted Tubule (DCT) → Collecting Duct

Juxtaglomerular Apparatus (JGA)

The JGA consists of macula densa (specialised cells at transition between ascending loop of Henle and DCT) and juxtaglomerular cells (modified smooth muscle cells in afferent/efferent arterioles). It regulates blood pressure and glomerular filtration rate.

URETERS

Ureters are paired tubes (25-30 cm long) through which urine flows from kidneys to urinary bladder. They begin at renal pelvis and terminate in the fundus of the bladder.

Layers of Ureter

• Tunica Adventitia (Fibrous Coat): Continuous with fibrous tunic of kidney and bladder
• Tunica Muscularis (Muscular Coat): Longitudinal and circular fibres
• Tunica Mucosa (Mucous Coat): Smooth coat with longitudinal folds

URINARY BLADDER

The urinary bladder is a hollow, muscular, distensible organ resting on the pelvic floor. Normal capacity: 400-600 ml. An empty bladder is pear-shaped.

Layers of Urinary Bladder

1. Mucous Membrane: Transitional epithelium; forms rugae (folds) when empty
2. Submucosa: Connective tissue with elastic fibres
3. Muscularis (Detrusor Muscle): Smooth muscle fibres interwoven in all directions; contracts to expel urine

URETHRA

The urethra is a tube-like structure transporting urine from the urinary bladder to the exterior. It is closed by the external urethral sphincter (voluntary muscle).

Male vs Female Urethra

• Female Urethra: 4 cm long; transports only urine
• Male Urethra: 20 cm long, S-shaped; transports urine and semen. Three regions: Prostatic urethra → Membranous urethra → Spongy urethra

PHYSIOLOGY OF URINE FORMATION

Three Processes of Urine Formation

1. Ultrafiltration (Glomerular Filtration): Passive process using hydrostatic pressure. Filtration membrane has three layers: fenestrated endothelium, basal lamina, and podocytes (filtration slits).
2. Tubular Reabsorption: Selective transepithelial process via transcellular route (4 steps) or paracellular route (through tight junctions).
3. Tubular Secretion: Removes unwanted substances (H⁺, K⁺, NH₄⁺, creatinine, drugs); controls blood pH.

Net Filtration Pressure (NFP) and GFR

NFP = GHP – (GCOP + CHP) = 50 – (30 + 10) = 10 mmHg
GFR (Glomerular Filtration Rate): ~125 ml/min; kidneys form ~180 litres of filtrate per day.

RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM (RAAS)

Stimuli for Renin Release

1. Increased sympathetic nerve activation (β₁-adrenoceptors)
2. Decreased renal artery hypotension
3. Decreased sodium delivery to distal tubules

Functions of Angiotensin II

1. Increases systemic vascular resistance (vasoconstriction)
2. Increases sodium and water retention
3. Stimulates aldosterone release from adrenal cortex
4. Stimulates ADH release from posterior pituitary
5. Stimulates thirst centers in the brain
6. Enhances adrenergic function
7. Stimulates cardiac and vascular hypertrophy

MICTURITION

When 300-400 ml of urine collects in the bladder, stretch-sensitive afferent autonomic nerve fibres are stimulated → detrusor muscle contracts → internal urethral sphincter relaxes → micturition. Conscious effort can override the reflex action for a limited time.

CLEARANCE TESTS

Blood Tests

• Serum Creatinine: >1.2 mg/dL (women) or >1.4 mg/dL (men) indicates kidney dysfunction
• Glomerular Filtration Rate (GFR): Normal ≥90; <60 indicates kidney dysfunction; <15 indicates risk of renal failure
• Blood Urea Nitrogen (BUN): Normal 7-20 mg/dL; rises as renal function declines

Imaging and Other Tests

• Ultrasound: Detects anomalies in kidney size, position, blockages (stones, tumours)
• CT Scan: X-ray imaging for structural issues and blockages
• Kidney Biopsy: Identifies disease process, assesses renal damage, diagnoses transplant failure
• Urinalysis: Dipstick test for protein, blood, pus, germs, sugar
• Microalbuminuria: Sensitive test for trace albumin in urine
• Creatinine Clearance: Compares blood and 24-hour urine creatinine levels
• Cystatin C: Blood protein marker of renal function