Urinary System Overview

Introduction

The urinary system, also known as the excretory, genitourinary (GU), or urogenital (UG) system, is responsible for producing, storing, and eliminating urine. It plays a vital role in maintaining homeostasis by removing waste products and regulating fluid and electrolyte balance.

• Organs: Two kidneys, two ureters, one bladder, one urethra

• Primary Functions: Extraction of wastes, urine formation, urine transport, and elimination

Anatomy and Physiology of the Urinary System

Kidneys

The kidneys are purplish-brown, bean-shaped organs located in a retroperitoneal position from T12 to L3 vertebrae. They are essential for filtering blood, removing wastes, and maintaining fluid and electrolyte balance.

• Functions:

  • Remove wastes and extra fluid

  • Remove acid produced by cells

  • Maintain water, salts, and minerals balance

  • Produce urine

  • Produce hormones: erythropoietin, renin, angiotensin

  • Convert vitamin D to calcitriol

• External Structure: Concave and convex borders; hilum (notch); renal pelvis (collecting area)

• Internal Structure:

  • Cortex: Outer layer with arteries, veins, convoluted tubules, glomerular capsules

  • Medulla: Inner portion with renal pyramids and papillae projecting into calyces

Nephron Structure and Function

Nephrons are the structural and functional units of the kidney, responsible for regulating, controlling, and removing waste products from blood plasma.

• Renal Corpuscle: Glomerulus (capillary cluster) surrounded by Bowman capsule

• Renal Tubule: Proximal tubule, nephron loop (loop of Henle), distal tubule, collecting tubule

• Vital Functions:

  • Regulate fluid balance

  • Reabsorb water and electrolytes

  • Remove metabolic wastes (urea, uric acid, creatinine, ketone bodies)

• Glomerular Filtration: Filters excess fluid and waste into urine-collecting tubules

• Tubular Reabsorption: Allows selected electrolytes and water back into blood

• Tubular Secretion: Removes excess acid and other substances

Equation: Glomerular Filtration Rate (GFR) is a key measure of kidney function:

Ureters

Each kidney has a ureter, a narrow muscular tube that drains urine to the bladder. The ureter wall consists of three layers: mucous membrane, smooth muscle, and fibrous tissue.

Urinary Bladder

The bladder is a muscular, membranous sac in the anterior pelvic cavity, serving as a reservoir for urine. It consists of four layers: epithelium, smooth muscle, longitudinal muscle (detrusor urinae), and fibrous layer. The trigone is a triangular area near the base.

Urethra

The urethra is a musculomembranous tube extending from the bladder to the outside. The external opening is the urinary meatus.

• Male Urethra: ~20 cm, conveys urine and semen, divided into prostatic, membranous, and penile sections

• Female Urethra: ~4 cm, conveys urine only, meatus between clitoris and vaginal opening

Urine Formation and Properties

Urine Production

Urine is produced in three main steps: filtration, reabsorption, and secretion. The nephron is the site of these processes.

• Urine is 95% water and 5% solid substances

• Normal adult voids 1000–1500 mL daily; urge to void at 300–350 mL

• Normal urine: clear, yellow to amber, faint odor, specific gravity 1.003–1.030, slightly acidic pH

Urinalysis

Urinalysis (UA) is a laboratory test evaluating physical, chemical, and microscopic properties of urine. It is a valuable diagnostic tool for detecting metabolic, kidney, and liver disorders, as well as urinary tract infections (UTIs).

• Freshly voided specimens provide accurate results

• Bacteriologic cultures require catheterization

Common Disorders and Diagnostic Procedures

Urinary Tract Infections (UTIs)

UTIs are common, especially in children and sexually active women. Most are caused by Escherichia coli, Klebsiella, and Proteus.

• Infants: Fever, weight loss, nausea, increased urination, strong-smelling urine, diaper rash, failure to thrive

• Older Children: Frequent/painful urination, abdominal pain, hematuria, fever, chills, bedwetting

Cystitis and Interstitial Cystitis

Cystitis is common, especially in women due to shorter urethra. Interstitial cystitis is a painful inflammation of the bladder wall, mostly affecting women, and does not respond well to antibiotics.

Urinary Calculi (Stones)

Renal calculi (kidney stones) can form in the renal pelvis, ureter, bladder, or urethra, causing obstruction and pain.

Hematuria

Hematuria is the presence of blood in urine, often indicating infection, stones, or trauma.

Hydronephrosis

Hydronephrosis is the swelling of a kidney due to a build-up of urine, often caused by obstruction.

Dialysis

Dialysis is a treatment for kidney failure, using machines or peritoneal cavity to filter blood.

• Hemodialysis: Blood is filtered through a machine

• Peritoneal Dialysis: Uses the peritoneal cavity as a filter

Extracorporeal Shock Wave Lithotripsy (ESWL)

ESWL is a non-invasive procedure using shock waves to break kidney stones into smaller fragments for easier passage.

Percutaneous Ultrasonic Lithotripsy

A nephroscope is inserted into the renal pelvis, and ultrasound waves fragment stones, which are then removed.

Kidney Transplantation

Transplantation involves placing a healthy kidney from a donor into a patient with kidney failure.

Routes of Infection for Pyelonephritis

Pyelonephritis is a kidney infection that can occur via ascending infection from the urinary tract or through the bloodstream.

Medical Terminology

Key Terms and Word Building

• Albuminuria: Presence of albumin (protein) in urine

• Cystectomy: Surgical excision of the bladder

• Dysuria: Difficult or painful urination

• Nephrolithiasis: Condition of kidney stones

• Oliguria: Scanty urine production

• Urologist: Specialist in urinary system

Drug Highlights

Common Drug Classes

• Diuretics: Promote urine production

• Urinary tract antibacterials: Treat infections

• Urinary tract antiseptics: Prevent infection

Diagnostic and Laboratory Tests

Common Tests

• Blood Urea Nitrogen (BUN): Measures urea in blood

• Creatinine Clearance: Assesses glomerular filtration rate

• Urine Culture: Detects microorganisms

• Cystoscopy: Visual examination of bladder and urethra

• Intravenous Pyelography (IVP): X-ray visualization of urinary tract

• Renal Biopsy: Tissue removal for analysis

• Retrograde Pyelography: X-ray with contrast medium injected backward

• Serum Creatinine: Measures creatinine in blood

• Ultrasonography: Uses sound waves to visualize kidneys

Additional Information

• Urine formation begins in fetal development at 10 weeks gestation

• Newborn kidneys are immature and cannot concentrate urine

• Glomerular filtration and reabsorption mature by age 1–2 years

• Older adults may experience reduced bladder capacity and urge incontinence

• Benign prostatic hyperplasia (BPH) and prostate cancer can cause incontinence in men

Summary Table: Normal and Abnormal Constituents of Urine

Purpose: Classification of urine components for diagnostic purposes.

Additional info: Table entries inferred from standard urinalysis parameters.

Abbreviations and Acronyms

Common abbreviations used in urinary system terminology include:

• UA: Urinalysis

• BUN: Blood Urea Nitrogen

• IVP: Intravenous Pyelogram

• GFR: Glomerular Filtration Rate

• UTI: Urinary Tract Infection