The Urinary System: Structure, Function, and Clinical Relevance

Urinary System Overview

Introduction

The urinary system is essential for filtering blood, removing waste, and maintaining homeostasis of water, electrolytes, and acid-base balance. It consists of the kidneys, ureters, urinary bladder, and urethra.

Kidney Anatomy and Location

Location and Protective Structures

• Location: Retroperitoneal, between T12 and L3 vertebrae.

• Size: About the size of a bar of soap.

• Protection: Surrounded by an adipose capsule and fibrous capsule for protection and stabilization.

Gross Anatomy of the Kidney

• Renal Cortex: Outer region containing most nephrons.

• Renal Medulla: Inner region with renal pyramids and columns.

• Renal Pyramid: Cone-shaped tissues in the medulla.

• Renal Pelvis: Funnel-shaped structure collecting urine into the ureter.

• Calyces: Minor and major calyces collect urine from pyramids.

Kidney Functions

Major Functions

• Filtration: Removes waste from blood plasma.

• Homeostasis: Regulates osmolarity, blood volume, blood pressure, and acid-base balance.

• Secretion: Produces hormones such as renin (blood pressure regulation) and erythropoietin (stimulates RBC production).

• Detoxification: Metabolizes drugs and free radicals.

Nephron Structure and Types

Nephron Anatomy

Nephrons are the functional units of the kidney, responsible for urine formation. Each kidney contains about 1.2 million nephrons.

• Renal Corpuscle: Includes the glomerulus and Bowman’s (glomerular) capsule.

• Renal Tubule: Composed of the proximal convoluted tubule (PCT), nephron loop (Loop of Henle), distal convoluted tubule (DCT), and collecting duct (CD).

Types of Nephrons

• Cortical Nephrons: 85%, short loops, located near the kidney surface.

• Juxtamedullary Nephrons: 15%, long loops extending deep into the medulla, crucial for concentrating urine.

Renal Corpuscle and Filtration

Structure and Function

• Glomerulus: Capillary bed where filtration occurs; high blood pressure forces small molecules through the filtration membrane.

• Bowman’s Capsule: Surrounds the glomerulus and collects filtrate.

• Filtration Membrane: Consists of fenestrated endothelium, basement membrane, and podocyte filtration slits; allows passage of water, electrolytes, glucose, amino acids, and small molecules, but retains large proteins and blood cells.

Renal Tubule and Urine Formation

Segments of the Renal Tubule

• Proximal Convoluted Tubule (PCT): Major site of reabsorption for water, ions, and nutrients.

• Nephron Loop (Loop of Henle): Descending limb reabsorbs water; ascending limb reabsorbs electrolytes (Na+, Cl-, K+).

• Distal Convoluted Tubule (DCT): Further reabsorption and secretion, regulated by hormones.

• Collecting Duct (CD): Final concentration of urine, regulated by ADH and aldosterone.

Pathway of Filtrate

glomerular capsule → PCT → nephron loop → DCT → collecting duct → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra

Urine Formation: Processes

Overview of Processes

• Glomerular Filtration: Blood pressure forces water and solutes from glomerulus into Bowman’s capsule.

• Tubular Reabsorption: Useful substances are reabsorbed from the filtrate back into the blood.

• Tubular Secretion: Additional wastes are secreted from blood into the tubule.

• Water Conservation: Water is reabsorbed from the collecting duct, concentrating the urine.

Filtration Membrane and Pressure

Filtration Membrane Structure

• Fenestrated Endothelium: Pores exclude blood cells and large proteins.

• Basement Membrane: Proteoglycan gel excludes large molecules.

• Filtration Slits: Podocyte arms allow only small particles to pass.

Filtration Pressure

• High pressure in the glomerulus (due to wider afferent arteriole) drives filtration.

• Filtration pressure overrides reabsorption, ensuring continuous filtration.

Glomerular Filtration Rate (GFR) and Regulation

GFR Values and Importance

• Normal GFR: ~125 mL/min (180 L/day) in males; ~105 mL/min (150 L/day) in females.

• 99% of filtrate is reabsorbed; only 1–2 L urine excreted per day.

• High GFR: Increased urine output, risk of dehydration and electrolyte loss.

• Low GFR: Anuria (low urine output), waste retention.

Regulation of GFR

• Autoregulation: Myogenic mechanism and tubuloglomerular feedback stabilize GFR.

• Sympathetic Control: Constriction of afferent arterioles during stress reduces GFR and urine output.

• Hormonal Control: Renin-angiotensin-aldosterone system (RAAS) increases blood pressure and GFR.

Tubular Reabsorption and Secretion

Reabsorption in the PCT

• Most water, ions, glucose, amino acids, and vitamins are reabsorbed in the PCT.

• Peritubular capillaries have high osmolarity and low pressure, favoring reabsorption by osmosis and solvent drag.

Tubular Secretion

• Removes additional wastes (urea, uric acid, ammonia, drugs) from blood into tubule.

• Regulates acid-base balance by secreting H+ and bicarbonate ions.

Nephron Loop (Loop of Henle)

• Descending Limb: Permeable to water, not electrolytes.

• Ascending Limb: Permeable to electrolytes (Na+, Cl-, K+), not water.

• Creates a salinity gradient, allowing the collecting duct to concentrate urine.

DCT and Collecting Duct

• Regulated by hormones (aldosterone, ADH) for salt/water and acid/base balance.

• Aldosterone increases Na+ and water reabsorption, raising blood pressure and reducing urine volume.

• ADH increases water reabsorption, reducing urine volume during dehydration.

Urine Composition and Properties

Normal Characteristics

• Appearance: Clear to deep amber; yellow due to urochrome.

• Odor: Slightly aromatic; may change with diet or disease.

• Specific Gravity: 1.005–1.030 (correlates with osmolarity).

• pH: 4.5–8.2 (usually around 6.0).

• Chemical Composition: 95% water, 5% solutes (urea, NaCl, KCl, creatinine, uric acid).

Nitrogenous Wastes

• Urea: From amino acid catabolism.

• Uric Acid: From nucleic acid catabolism.

• Creatinine: From creatine phosphate catabolism.

Urine Volume and Disorders

Normal and Abnormal Volumes

• Normal: 1–2 L/day.

• Polyuria: >2 L/day (excessive urination).

• Oliguria: <500 mL/day (low output).

• Anuria: 0–100 mL/day (very low output).

Diabetes and Diuretics

• Diabetes Mellitus: Chronic polyuria with hyperglycemia and glycosuria.

• Diabetes Insipidus: ADH hyposecretion, leading to excessive dilute urine.

• Diuretics: Increase urine output, used to treat hypertension and heart failure (e.g., caffeine, alcohol).

Urinary Tract Anatomy: Ureters, Bladder, and Urethra

Ureters

• Muscular tubes (~25 cm) transporting urine from renal pelvis to bladder by peristalsis.

Urinary Bladder

• Muscular sac for urine storage; capacity 500–800 mL.

• Trigone: Triangular area between ureter and urethra openings, prone to infection.

• Rugae: Folds allowing distension.

Urethra

• Female: 3–4 cm, more prone to infections.

• Male: 18 cm, passes through prostate and penis.

• Internal Urethral Sphincter: Involuntary, smooth muscle.

• External Urethral Sphincter: Voluntary, skeletal muscle.

Micturition (Voiding Urine)

Control of Urination

• Bladder stretch receptors signal the micturition center when ~200 mL urine accumulates.

• Results in bladder contraction and relaxation of internal sphincter; voluntary control via external sphincter.

• Voluntary abdominal pressure can initiate urination if the bladder is not full.

Urinalysis and Clinical Relevance

Normal and Abnormal Constituents

• Normal: Urea, creatinine, uric acid, urobilinogen.

• Abnormal: Albumin (albuminuria), glucose (glucosuria), blood (hematuria), pus (pyuria), ketones (ketonuria), bilirubin (bilirubinuria), microbes (UTI).

Kidney Pathology

Common Disorders

• Trauma, Infection, Exercise, Hypoxia: Can cause proteinuria or hematuria.

• Polycystic Kidney Disease: Genetic disorder causing multiple cysts, leading to kidney failure.

• Kidney Stones (Renal Calculi): Hard granules of calcium, phosphate, uric acid, and protein; can block urine flow and cause severe pain.

• Renal Insufficiency: Inability to maintain homeostasis due to nephron loss; causes include hypertension, infections, trauma, toxins, autoimmune disease, and glomerulonephritis.

• Hemodialysis: Artificial filtration of blood for renal failure patients.

• Renal Cell Carcinoma: Kidney cancer originating in tubule lining; symptoms include pain, abnormal urine color, and weight loss.