The Urinary System: Structure, Function, and Regulation

Urinary System: Overview

Introduction

The urinary system is essential for removing metabolic wastes, maintaining water and electrolyte balance, and regulating blood pressure and red blood cell production. It works in concert with other organ systems to eliminate various waste products from the body.

• Cellular wastes are toxic if not removed.

• Waste removal from cells is performed by blood and lymph.

• Major waste elimination systems:

  • Urinary system: Removes H2O, electrolytes, nitrogenous wastes, drugs.

  • Respiratory system: Removes CO2 and H2O.

  • Digestive system: Removes bilirubin.

  • Integumentary system: Removes H2O and electrolytes.

Functions of the Urinary System

• Waste removal: Excretes metabolic byproducts and toxins.

• Maintenance of water/electrolyte balance: Regulates osmolarity and ion concentrations.

• Regulation of body fluid volume and blood pressure: Adjusts blood volume and systemic pressure.

• Control of RBC production: Secretes erythropoietin to stimulate red blood cell synthesis.

Anatomy of the Urinary System

Macroscopic Anatomy

• Kidneys (2): Filter blood, reabsorb needed substances, secrete wastes.

• Ureters (2): Muscular tubes that convey urine from kidneys to bladder.

• Urinary bladder (1): Hollow organ that collects and stores urine.

• Urethra (1): Tube that conveys urine out of the body.

Kidney Anatomy

• Red-brown, bean-shaped organs.

• Enclosed in a thin fibrous capsule and surrounded by adipose tissue for protection.

• Located retroperitoneally (behind the peritoneum).

• Hilum: Indentation for passage of vessels, nerves, and ureter.

• Renal cortex: Outer layer.

• Renal medulla: Inner region containing renal pyramids (conical masses) and renal columns (extensions of cortical tissue).

• Nephrons: Structural and functional units of the kidney.

• Renal papillae: Tips of pyramids where urine is funneled into minor calyces.

• Minor calyces: Receive urine from collecting ducts.

• Major calyces: Receive urine from minor calyces.

• Renal pelvis: Expanded proximal end of ureter, receives urine from major calyces.

Ureters

• Hollow tubes (~25 cm long) extending from kidneys to urinary bladder.

• Retroperitoneal location.

• Muscular walls undergo peristalsis to propel urine.

• Flaps of mucosa at bladder openings prevent backflow.

Urinary Bladder

• Hollow, muscular, distensible sac in the pelvic cavity (subperitoneal).

• Attached to pelvic floor at the trigone.

• Four layers:

  • Inner mucosa (transitional epithelium for stretch).

  • Submucosa.

  • Middle smooth muscle (detrusor muscle).

  • Outer adventitia or serosa (visceral peritoneum on superior surface).

Urethra

• Males: Combined urogenital system; long urethra (~18 cm) with prostatic, membranous, and penile (spongy) subdivisions.

• Females: Separate reproductive and urinary systems; short urethra (~4 cm).

Microscopic Anatomy

• Nephron: Functional unit of the kidney (~1 million per kidney), mostly in the renal cortex.

• Consists of:

  • Renal corpuscle: Glomerulus + glomerular (Bowman's) capsule.

  • Renal tubule: Proximal convoluted tubule (PCT), nephron loop (loop of Henle), distal convoluted tubule (DCT).

• Collecting duct: Receives filtrate from multiple nephrons.

Urine Formation

Three Main Processes

1. Filtration: Blood pressure forces water and small solutes from glomerulus into glomerular capsule.

2. Reabsorption: Selective movement of substances from renal tubules back into blood (mainly in PCT).

3. Secretion: Additional substances actively transported from blood into renal tubules (mainly in DCT).

Glomerular Filtration

• Depends on blood pressure ("pressure filtration").

• Afferent arteriole brings blood to glomerulus; efferent arteriole carries it away.

• Small molecules and water move through glomerular walls into the capsule; fluid is called filtrate.

• Filtrate is similar to plasma (water, glucose, ions, amino acids, small nitrogenous wastes).

Tubular Reabsorption

• Selective movement of molecules from tubules back into blood.

• PCT reabsorbs water, glucose, amino acids, ions (up to 80% of filtrate).

• Filtrate becomes "tubular fluid" as it moves through nephron.

• Nephron loop and collecting duct concentrate urine and reabsorb water/ions.

• Microvilli increase surface area for absorption.

Tubular Secretion

• Primarily occurs in DCT.

• Additional molecules move from blood to renal tubules by active transport or osmosis.

• Substances secreted: water, ions (H+, K+, Na+, Cl-), some drugs (e.g., penicillin), creatinine (large nitrogenous waste).

Collecting Duct

• Receives fluid from numerous nephrons; fluid is now officially "urine".

• Empties through renal papillae into minor calyces.

• Hormones regulate reabsorption at the collecting ducts:

  • Aldosterone: Regulates Na+ reabsorption.

  • ADH: Regulates water reabsorption.

  • PTH: Regulates Ca2+ reabsorption.

Regulation of Fluid Volume

• Antidiuretic Hormone (ADH): Released when blood volume drops; increases permeability of collecting duct to water, resulting in increased water reabsorption and blood volume.

• Blood pressure: Required for glomerular filtration. If BP falls below a certain point, filtration stops, leading to accumulation of nitrogenous wastes.

• Aldosterone: Secreted by adrenal cortex; stimulated by low BP, high K+ (or low Na+). Acts on DCT to increase Na+ reabsorption and K+ excretion, resulting in increased water reabsorption and BP.

• Juxtaglomerular apparatus: Contact between DCT and afferent arteriole; releases renin when BP falls. Renin catalyzes production of angiotensin, which stimulates aldosterone release, raising BP.

Urine Pathway

Nephron → Collecting Duct (out renal papilla of renal pyramid) → Minor Calyx → Major Calyx → Renal Pelvis → Ureters → Urinary Bladder → Urethra → Outside Body

Composition of Normal Urine

• 95% water

• pH 6.0 (average)

• Urea (from protein metabolism)

• Uric acid (from nucleic acids and purine-rich foods)

• Creatinine (from creatine phosphate in muscles)

• Amino acids (trace amounts)

• Electrolytes (vary with diet)

Micturition (Urination)

• Bladder fills; stretch receptors respond with action potential.

• Impulse sent to spinal cord (sensory), then to urinary bladder (motor).

• Detrusor muscle contracts, forcing urine through internal urethral sphincter (smooth muscle).

• Voluntary relaxation of external urethral sphincter (skeletal muscle) allows urination.

Urine Volume

• Normal: 0.6 – 2.5 liters/day

• Depends on fluid intake, ambient/body temperature, humidity, emotional state, and respiration rate.

• Kidney failure: <30 cc/hour

Efficiency of the System

• Blood volume filtered ~36 times/day

• Total glomerular filtrate: 180 liters/day

• Total volume reabsorbed: 178 liters/day

Summary Table: Hormonal Regulation of the Urinary System

Hormone	Source	Stimulus	Main Effect
ADH (Antidiuretic Hormone)	Posterior pituitary	Low blood volume	Increases water reabsorption in collecting duct
Aldosterone	Adrenal cortex	Low BP, high K+	Increases Na+ reabsorption, K+ excretion
PTH (Parathyroid Hormone)	Parathyroid glands	Low blood Ca2+	Increases Ca2+ reabsorption

Key Equations

• Glomerular Filtration Rate (GFR):

  • $GFR = K_f \times (P_{GC} - P_{BS} - \pi_{GC})$

  • Where $K_f$ = filtration coefficient, $P_{GC}$ = glomerular capillary hydrostatic pressure, $P_{BS}$ = Bowman’s space hydrostatic pressure, $\pi_{GC}$ = glomerular capillary oncotic pressure.

Additional info: The notes above are expanded and clarified for academic completeness, with added definitions, examples, and a summary table for hormonal regulation. The key equation for GFR is included for exam preparation.