The Urinary System: Structure, Function, and Mechanisms

The Urinary System

Overview and Functions

The urinary system is essential for maintaining the composition of the body's extracellular fluids by filtering blood, removing wastes, and regulating fluid and electrolyte balance. It consists of the kidneys, ureters, urinary bladder, and urethra.

• Filtration: Kidneys filter 200–400 liters of fluid from the blood daily, removing toxins, metabolic wastes, and excess ions as urine.

• Regulation: Maintains blood volume, chemical composition, water-salt balance, and acid-base balance.

• Hormone Production: Produces renin (regulates blood pressure), erythropoietin (stimulates red blood cell production), and activates vitamin D.

Gross Anatomy of the Kidneys

Location and External Anatomy

The kidneys are bean-shaped organs located in a retroperitoneal position in the superior lumbar region, extending from the 12th thoracic to the 3rd lumbar vertebrae. The right kidney is slightly lower due to the presence of the liver.

• Lateral surface: Convex

• Medial surface: Concave, with a vertical cleft called the renal hilus leading to the renal sinus

• Structures entering/exiting at the hilus: Ureters, renal blood vessels, lymphatics, and nerves

Other Urinary System Organs

• Ureters: Paired tubes transporting urine from kidneys to bladder

• Urinary bladder: Temporary storage reservoir for urine

• Urethra: Transports urine from bladder out of the body

Internal Anatomy of the Kidneys

Regions and Structures

The kidney has three main regions: cortex, medulla, and renal pelvis.

• Cortex: Light-colored, granular, superficial region

• Medulla: Contains cone-shaped renal pyramids made of parallel bundles of urine-collecting tubules

• Renal columns: Inward extensions of cortical tissue separating the pyramids

• Renal pelvis: Flat, funnel-shaped tube within the renal sinus

• Major and minor calyces: Collect urine from papillae and empty into the renal pelvis

Blood and Nerve Supply

• About one-fourth (1200 mL) of systemic cardiac output flows through the kidneys each minute.

• Arterial and venous flow follow similar paths; nerve supply is via the renal plexus.

The Nephron: Functional Unit of the Kidney

Structure of the Nephron

Nephrons are the blood-processing units that form urine. Each nephron consists of a renal corpuscle and a renal tubule.

• Renal corpuscle: Includes the glomerulus (tuft of capillaries) and Bowman's (glomerular) capsule

• Glomerular endothelium: Fenestrated epithelium allowing solute-rich, protein-free filtrate to pass

• Bowman's capsule: Surrounds the glomerulus; has a parietal (structural) and visceral (podocyte) layer

Renal Tubule Segments

• Proximal convoluted tubule (PCT): Cuboidal cells with microvilli; reabsorbs water and solutes, secretes substances

• Nephron loop (Loop of Henle): Descending limb (thin, simple squamous cells, permeable to water), ascending limb (thick, cuboidal/columnar cells, permeable to solutes)

• Distal convoluted tubule (DCT): Cuboidal cells, mainly for secretion

• Collecting duct: Contains principal cells (water/salt balance) and intercalated cells (acid-base balance)

Types of Nephrons

• Cortical nephrons: 85% of nephrons, located in cortex, short nephron loops

• Juxtamedullary nephrons: Located at cortex-medulla junction, long loops deeply invade medulla, involved in concentrated urine production

Capillary Beds of the Nephron

Glomerulus and Peritubular Capillaries

• Each nephron has two capillary beds: glomerulus (filtration) and peritubular capillaries (reabsorption)

• Glomerulus is fed by an afferent arteriole and drained by an efferent arteriole

• Blood pressure in the glomerulus is high, promoting filtration

• Peritubular capillaries are low-pressure, porous, and adapted for absorption

• Vasa recta: Long, straight capillaries associated with juxtamedullary nephrons, maintain medullary osmotic gradient

Juxtaglomerular Apparatus (JGA)

Structure and Function

• Located where the distal tubule contacts the afferent arteriole

• Juxtaglomerular (JG) cells: Smooth muscle cells with renin-containing granules, act as mechanoreceptors

• Macula densa: Tall, closely packed distal tubule cells, function as chemoreceptors/osmoreceptors

• Mesangial cells: Control glomerular filtration rate

Mechanism of Urine Formation

Three Major Processes

1. Glomerular filtration: Passive process; hydrostatic pressure forces fluids and solutes through a membrane

2. Tubular reabsorption: Selective process; returns most filtrate to blood

3. Tubular secretion: Moves substances from blood into filtrate

Filtration Membrane

• Three layers: fenestrated endothelium, visceral membrane (podocytes), and fused basement membrane

• Allows passage of water and small solutes; blocks proteins and blood cells

Net Filtration Pressure (NFP)

• Responsible for filtrate formation

• Calculated as:

$NFP = HP_g - (OP_g + HP_c)$

• Where HP_g = glomerular hydrostatic pressure, OP_g = oncotic pressure, HP_c = capsular hydrostatic pressure

Glomerular Filtration Rate (GFR)

• Total amount of filtrate formed per minute by the kidneys

• Directly proportional to NFP, filtration membrane permeability, and surface area

• Regulated by renal autoregulation, neural controls, and the renin-angiotensin system

Tubular Reabsorption and Secretion

Reabsorption

• Most filtrate is reabsorbed into the blood via transcellular or paracellular routes

• All organic nutrients are reabsorbed; water and ion reabsorption is hormonally controlled (ADH, aldosterone, ANP, PTH)

• Sodium reabsorption is primarily active, via Na+-K+ ATPase pumps

Secretion

• Reverse of reabsorption; substances move from peritubular capillaries into filtrate

• Important for disposing of drugs, excess ions, and controlling blood pH

Regulation of Urine Concentration and Volume

Countercurrent Mechanism

• Keeps the solute load of body fluids constant (~300 mOsm)

• Countercurrent multiplier: Nephron loops create medullary osmotic gradient

• Countercurrent exchanger: Vasa recta preserve the gradient

Formation of Dilute and Concentrated Urine

• Dilute urine: Formed when ADH is not secreted; collecting ducts are impermeable to water

• Concentrated urine: ADH increases water reabsorption; urine osmolality rises

Physical and Chemical Characteristics of Urine

• Color: Clear, pale to deep yellow (urochrome pigment)

• Odor: Slightly aromatic; ammonia odor develops on standing

• pH: Slightly acidic (pH 6), range 4.5–8.0

• Specific gravity: 1.001–1.035, depending on solute concentration

• Chemical composition: 95% water, 5% solutes (urea, uric acid, creatinine, ions)

Ureters, Urinary Bladder, and Urethra

Ureters

• Slender tubes conveying urine from kidneys to bladder

• Enter bladder at an angle to prevent backflow

• Wall has three layers: transitional epithelium, smooth muscle, fibrous connective tissue

Urinary Bladder

• Muscular sac for temporary urine storage

• Located retroperitoneally on pelvic floor

• Wall has three layers: transitional epithelium, thick muscle, fibrous adventitia

• Trigone: Triangular area prone to infection

Urethra

• Muscular tube draining urine from bladder to outside

• Internal urethral sphincter (involuntary) and external urethral sphincter (voluntary)

• Female urethra: short, anterior to vaginal opening

• Male urethra: prostatic, membranous, and spongy regions