Urinary System Overview

Introduction

The urinary system is essential for maintaining homeostasis by filtering blood, removing waste, and regulating fluid and electrolyte balance. It consists of the kidneys, ureters, urinary bladder, and urethra.

• Key Functions: Filtration of blood, elimination of waste, regulation of blood volume and pressure, acid-base balance, and hormone secretion.

• Major Organs: Kidneys, ureters, bladder, urethra.

Anatomy of the Kidneys

Kidney Location and Structure

The kidneys are retroperitoneal organs located between T12 and L3 vertebrae, protected by an adipose capsule. Each kidney is about the size of a bar of soap.

• Retroperitoneal: Positioned behind the peritoneum.

• Adipose Capsule: Provides protection and keeps kidneys in place.

Internal Anatomy of the Kidney

The kidney is divided into the renal cortex and renal medulla. The medulla contains renal pyramids, separated by renal columns. The renal pelvis collects urine from calyces and funnels it to the ureter.

• Renal Cortex: Outer region, contains nephrons.

• Renal Medulla: Inner region, contains pyramids.

• Renal Pyramid: Cone-shaped tissue in medulla.

• Renal Pelvis: Central cavity for urine collection.

• Major/Minor Calyx: Collect urine from pyramids.

Nephron: Functional Unit of the Kidney

Nephron Structure

Each kidney contains about 1.2 million nephrons, which are responsible for urine formation. Nephrons consist of a renal corpuscle and a renal tubule.

• Renal Corpuscle: Includes the glomerulus and Bowman's capsule.

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

• Cortical Nephrons: 85%, short loops, close to surface.

• Juxtamedullary Nephrons: 15%, long loops, deep in medulla.

Renal Corpuscle and Filtration

The renal corpuscle is where blood filtration occurs. The glomerulus is a capillary bed with high pressure, allowing small molecules to pass into the filtrate while retaining large molecules.

• Afferent Arteriole: Brings blood to glomerulus.

• Efferent Arteriole: Takes blood away from glomerulus.

• Bowman's Capsule: Surrounds glomerulus, collects filtrate.

Urine Formation

Filtration Membrane

The filtration membrane consists of fenestrated endothelium, basement membrane, and filtration slits formed by podocytes.

• Fenestrated Endothelium: Excludes blood cells and large molecules.

• Basement Membrane: Excludes large molecules.

• Filtration Slits: Allow only small particles to pass.

• Small Particles: Water, electrolytes, glucose, fatty acids, amino acids, nitrogenous wastes, vitamins.

• Large Particles: Albumin, formed elements of blood.

Filtration Pressure

Filtration is driven by high blood pressure in the glomerulus, maintained by the wider afferent arteriole compared to the efferent arteriole.

• Net Filtration Pressure: Difference between hydrostatic and osmotic pressures.

Glomerular Filtration Rate (GFR)

GFR is the rate at which filtrate is formed in the kidneys.

• Normal GFR: ≈125 ml/min (male), ≈105 ml/min (female).

• 99% of filtrate is reabsorbed. Only 1-2 L urine excreted per day.

• High GFR: Leads to dehydration and electrolyte depletion.

• Low GFR: Leads to anuria and waste reabsorption.

Regulation of Glomerular Filtration

Autoregulation

The kidneys regulate GFR through myogenic mechanisms and tubuloglomerular feedback.

• Myogenic Mechanism: Increased BP stretches afferent arteriole, causing constriction and stabilizing GFR.

• Tubuloglomerular Feedback: Macula densa in DCT monitors fluid and signals juxtaglomerular cells to constrict afferent arteriole.

Sympathetic and Hormonal Control

• Sympathetic Control: Strenuous exercise or shock causes afferent arterioles to constrict, reducing GFR and urine production.

• Hormonal Control: Renin-angiotensin-aldosterone system regulates blood pressure and GFR.

Tubular Reabsorption and Secretion

Reabsorption in the Proximal Convoluted Tubule (PCT)

Most water and solutes are reabsorbed in the PCT, including glucose, amino acids, and vitamins.

• Peritubular Capillaries: Favor reabsorption due to high osmolarity and low BP.

• Solvent Drag: Water absorption carries solutes with it.

Tubular Secretion

Tubular secretion removes wastes (urea, uric acid, ammonia, drugs) and regulates acid-base balance by secreting H+ and bicarbonate ions.

• Movement: From capillaries into tubule.

Nephron Loop (Loop of Henle)

• Descending Limb: Water reabsorbed, no electrolytes.

• Ascending Limb: Electrolytes reabsorbed (Na+, Cl-, K+), no water.

• Thick Segments: Impermeable to water, tubular fluid becomes dilute.

• Salinity Gradient: Allows collecting duct to concentrate urine.

DCT and Collecting Duct

Fluid reabsorption in the DCT and collecting duct is regulated by hormones, affecting salt/water and acid/base balance.

• Aldosterone: Promotes Na+ and water reabsorption, decreases urine volume, raises BP.

• ADH: Increases water reabsorption, decreases urine volume.

Urinalysis and Properties of Urine

Composition and Properties

Urine is typically odorless to deep amber, with yellow color due to urochrome.

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

• Appearance: Cloudy (pyuria) indicates infection; blood (hematuria) indicates trauma or disease.

• Odor: Bacteria degrade urea to ammonia; certain foods (asparagus) affect smell.

• Specific Gravity: Correlates to osmolarity.

• pH: Range 4.5-8.2, usually 6.0.

Nitrogenous Wastes

• Urea: From amino acid metabolism.

• Uric Acid: From nucleic acid catabolism.

• Creatinine: From creatine phosphate catabolism.

Urine Volume and Disorders

• Normal Volume: 1-2 L/day.

• Polyuria: >2 L/day.

• Oliguria: <500 mL/day.

• Anuria: 0-100 mL/day.

• Diabetes: Chronic polyuria, hyperglycemia, glycosuria.

• Diuretics: Increase urine output, decrease blood volume; used for hypertension and heart failure.

Ureters, Bladder, and Urethra

Ureters

Ureters are muscular tubes (about 25 cm long) that transport urine from the renal pelvis to the bladder via peristalsis.

Bladder

The bladder is a distensible organ with a capacity of 500-800 mL. The trigone is a triangular area formed by the openings of the ureters and urethra, susceptible to infection.

Urethra

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

• Male: 18 cm long.

• Internal Urethral Sphincter: Smooth muscle, involuntary control.

• External Urethral Sphincter: Skeletal muscle, voluntary control.

Micturition (Voiding Urine)

Process of Micturition

• Stretch receptors in the bladder signal the micturition center when about 200 mL urine accumulates.

• Results in bladder contraction and relaxation of internal sphincter.

• Voluntary control via external sphincter and cerebrum.

Abnormal Constituents and Pathology

Abnormal Constituents of Urine

• Albumin: Albuminuria.

• Glucose: Glucosuria.

• Blood: Hematuria.

• Pus: Pyuria.

• Ketones: Ketonuria.

• Bilirubin: Bilirubinuria.

• Renal Calculi: Kidney stones.

• Microbes: Urinary tract infection (cystitis).

Kidney Pathology

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

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

• Kidney Stones: Hard granules of calcium, phosphate, uric acid, and protein; can block renal pelvis or ureter.

• Renal Insufficiency: Inability to maintain homeostasis due to nephron destruction.

• Hemodialysis: Blood is filtered through a semipermeable tube with dialysis fluid to remove waste.

• Renal Cell Carcinoma: Cancer of kidney tubule lining; symptoms include abdominal pain, abnormal urine color, back/flank pain, swelling, and weight loss.

Summary Table: Urine Properties and Disorders

Additional info:

• Kidneys are the most commonly transplanted organ.

• Renin-angiotensin-aldosterone system is crucial for blood pressure regulation.

• Hemodialysis is a life-saving procedure for renal failure patients.