Urinary System

The urinary system, also known as the renal system, is responsible for filtering blood, regulating fluid and electrolyte balance, and eliminating waste products through the formation and excretion of urine. It consists of paired kidneys and ureters, the urinary bladder, and the urethra. The system is anatomically and functionally divided into the upper urinary tract (kidneys and ureters) and the lower urinary tract (bladder and urethra). The urinary system works in close coordination with the cardiovascular and endocrine systems to maintain homeostasis, including blood pressure regulation, pH balance, and erythropoiesis.

Organs of the Urinary System

• Kidneys
• Ureters
• Urinary Bladder
• Urethra

Kidneys

The kidneys are retroperitoneal, bean-shaped organs located on either side of the vertebral column between T12 and L3 vertebrae. The right kidney is usually slightly lower due to the position of the liver. Each kidney is surrounded by a fibrous capsule, perirenal fat, renal fascia, and pararenal fat, offering protection and structural support.

Gross Anatomy

• Renal Hilum: Medial opening for renal artery, vein, lymphatics, nerves, and ureter
• Renal Cortex: Outer region housing renal corpuscles and proximal/distal convoluted tubules
• Renal Medulla: Inner region made of renal pyramids and renal columns
• Renal Papilla: Apex of each pyramid, drains into a minor calyx
• Minor and Major Calyces: Collect urine from papillae and empty into the renal pelvis

Microscopic Structure

• Nephrons: Functional units composed of renal corpuscles and tubules
• Glomerulus: Capillary tuft where filtration begins
• Bowman’s Capsule: Encloses the glomerulus and receives filtrate
• Tubules: Proximal tubule, loop of Henle, distal tubule, and collecting duct

Ureters

The ureters are paired muscular tubes that transport urine from the renal pelvis to the urinary bladder. Each ureter measures approximately 25–30 cm in length and descends retroperitoneally along the psoas major muscle before entering the bladder posteriorly at the ureterovesical junction.

Course and Relations

Segment	Relation
Abdominal	Anterior to psoas major and genitofemoral nerve
Pelvic	Crosses pelvic brim at bifurcation of common iliac arteries
Terminal	Traverses bladder wall obliquely

Constriction Sites

• Ureteropelvic junction
• Pelvic brim (over iliac vessels)
• Ureterovesical junction

These are clinically important for ureteric stone lodging.

Urinary Bladder

The urinary bladder is a hollow, distensible muscular organ that stores urine until micturition. It lies in the true pelvis when empty and ascends into the abdomen when full. In males, it is anterior to the rectum and superior to the prostate; in females, it is anterior to the vagina and inferior to the uterus.

Anatomical Parts

• Apex: Directed anteriorly; attached to the median umbilical ligament
• Body: Central portion containing the distensible part of the bladder
• Fundus (base): Posterior wall; receives ureters at the ureteric orifices
• Neck: Inferior portion that opens into the urethra

Internal Features

• Trigone: Smooth triangular area between the ureteric and urethral orifices
• Detrusor Muscle: Smooth muscle responsible for bladder contraction
• Internal Urethral Sphincter: Involuntary control of micturition at the bladder neck

Urethra

The urethra is a fibromuscular tube that conveys urine from the bladder to the external environment. It differs significantly between sexes:

Male Urethra

• Length: Approximately 18–20 cm
• Parts: Prostatic, membranous, and spongy (penile) urethra
• Functions: Conveys urine and semen (dual function)

Female Urethra

• Length: Approximately 4 cm
• Course: Short, straight, and opens into the vestibule between the labia minora
• Function: Urinary only

Innervation

The urinary system receives both autonomic and somatic innervation.

Kidneys and Ureters

• Sympathetic: T10–L2 spinal segments via renal and aortic plexuses
• Parasympathetic: Vagus nerve (kidney), pelvic splanchnic nerves (ureter)

Bladder and Urethra

• Sympathetic: Hypogastric plexus (T11–L2); promotes urine storage
• Parasympathetic: Pelvic splanchnic nerves (S2–S4); initiates detrusor contraction
• Somatic: Pudendal nerve (S2–S4); controls external urethral sphincter

Blood Supply

• Kidneys: Renal arteries (branches of abdominal aorta)
• Ureters: Renal, gonadal, common iliac, and vesical arteries
• Bladder: Superior and inferior vesical arteries (from internal iliac artery)
• Urethra: Inferior vesical, vaginal, and pudendal arteries

Lymphatic Drainage

• Kidneys: Para-aortic (lumbar) lymph nodes
• Ureters: Lumbar, common iliac, external and internal iliac nodes
• Bladder: External and internal iliac nodes
• Urethra: Internal iliac (proximal), superficial inguinal (distal) nodes

Clinical Significance

• Urinary Tract Infections (UTIs): More common in females due to shorter urethra
• Renal Calculi: Stones may obstruct ureter at constriction points causing flank pain and hematuria
• Hydronephrosis: Urine backup due to obstruction can dilate the renal pelvis and damage nephrons
• Neurogenic Bladder: Nerve damage can impair detrusor contraction and sphincter control
• Prostatic Hypertrophy: In males, enlarged prostate can compress urethra and obstruct urine flow
• Incontinence: Weak pelvic floor or sphincter dysfunction can cause loss of voluntary urine control