Urinary System: Structure, Function, and Regulation

Urinary System Overview

General Role and Waste Elimination

The urinary system is essential for removing toxic cellular wastes and maintaining homeostasis. Waste removal from cells is performed by blood and lymph, with different organ systems responsible for eliminating specific types of waste.

• Urinary system: Eliminates H2O, electrolytes, nitrogenous wastes, and drugs.

• Respiratory system: Removes CO2 and H2O.

• Digestive system: Eliminates bilirubin.

• Integumentary system: Removes H2O and electrolytes.

Functions of the Urinary System

• Waste removal: Excretion of metabolic byproducts.

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

• Regulation of body fluid volume and blood pressure: Controls blood volume and pressure via fluid management.

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

Anatomy of the Urinary System

Macroscopic Anatomy

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

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

• Urinary bladder (1): Hollow, muscular sac that stores urine.

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

Kidney Structure

• Shape and Location: Red-brown, bean-shaped; retroperitoneal; enclosed in a thin fibrous capsule and surrounded by adipose tissue.

• Hilum: Indentation for passage of vessels and nerves.

• Renal Cortex: Outer layer.

• Renal Medulla: Inner layer, contains renal pyramids (conical masses) and renal columns (extensions of cortical tissue).

• Nephrons: Structural and functional units of the kidney.

• Renal Papillae: Tips of pyramids.

• 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) extending from kidneys to 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 pelvic cavity.

• Attached to pelvic floor at trigone.

• Four layers: inner mucosa (transitional epithelium), submucosa, middle smooth muscle (detrusor), outer adventitia/serosa.

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 Structure

• Approximately 1 million nephrons per kidney, mostly in 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. Equation: $\text{Filtration Rate} = \text{GFR}$ (Glomerular Filtration Rate)

2. Reabsorption: Selective movement of molecules from renal tubules back into blood, especially in PCT. Equation: $\text{Reabsorption Rate} = \text{Amount Reabsorbed} / \text{Time}$

3. Secretion: Additional molecules move from blood into renal tubules, primarily in DCT.

Glomerular Filtration

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

• Afferent arteriole brings blood to glomerulus.

• Small molecules and water move into glomerular 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).

• 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 (water, ions, drugs, creatinine) move from blood to tubules via active transport or osmosis.

Collecting Duct

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

• Empties through renal papillae into minor calyces.

• Hormones regulate reabsorption:

  • 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.

• Results in increased water reabsorption and increased blood volume.

Urine Pathway

• Nephron → Collecting Duct → Renal Papilla → Minor Calyx → Major Calyx → Renal Pelvis → Ureters → Urinary Bladder → Urethra → Outside Body

Composition of Normal Urine

• 95% water

• pH 6.0 (average)

• Urea (from proteins)

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

• 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, temperature, humidity, emotional state, respiration rate.

• Kidney failure: <30cc/hour

Efficiency of the System

• Blood volume filtered 36 times/day

• Total glomerular filtrate: 180 liters/day

• Total volume reabsorbed: 178 liters/day

Regulation of Fluid Volume: Blood Pressure

• Blood pressure is required for glomerular filtration.

• If blood pressure falls below a certain point, filtration stops, leading to build-up of nitrogenous wastes.

• ADH, aldosterone (and renin), plasma proteins (e.g., albumin) all increase blood pressure.

Aldosterone

• Secreted by adrenal cortex.

• Stimulated by low blood pressure, high K+ (or low Na+).

• Acts on distal convoluted tubule to increase Na+ reabsorption and K+ excretion.

• Results in increased water reabsorption, blood volume, and blood pressure.

Juxtaglomerular Apparatus

• Contact between DCT and afferent arteriole.

• Afferent arteriole cells release renin when blood pressure falls.

• Renin catalyzes production of angiotensin.

• Angiotensin stimulates release of aldosterone, raising blood pressure.

Summary Table: Key Structures and Functions

Structure	Main Function
Kidney	Filtration, reabsorption, secretion
Ureter	Transport urine to bladder
Urinary Bladder	Store urine
Urethra	Eliminate urine from body
Nephron	Functional unit for urine formation

Additional info:

• Hormonal regulation (ADH, aldosterone, PTH) is crucial for maintaining homeostasis.

• Urine composition and volume are sensitive indicators of kidney health and systemic balance.