The Urinary System: Structure, Function, and Physiology

Chapter 24: The Urinary System

Module 24.1 Overview of the Urinary System

The urinary system is essential for excretion, homeostasis, and regulation of blood parameters. It consists of the kidneys and the urinary tract, which includes the ureters, urinary bladder, and urethra.

• Kidneys: Filter blood, remove metabolic wastes, and regulate fluid, electrolyte, acid-base, and blood pressure homeostasis.

• Location: Retroperitoneal, with an adrenal gland atop each kidney.

• Urinary Tract: Ureters transport urine to the bladder, which stores urine; the urethra expels urine from the body.

• Kidney Functions:

  • Removal of metabolic wastes

  • Regulation of fluid and electrolyte balance (osmolarity)

  • Acid-base balance (conservation/elimination of H+ and HCO3-)

  • Blood pressure regulation (via blood volume and renin secretion)

  • Regulation of erythropoiesis (erythropoietin release)

  • Metabolic functions: detoxification, vitamin D activation, gluconeogenesis

Module 24.2 Anatomy of the Kidneys

The kidneys are protected and held in place by connective tissue layers and have distinct internal regions for urine formation and drainage.

• External Anatomy:

  • Three layers: renal fascia (outer), adipose capsule (middle), renal capsule (inner)

  • Hilum: Entry/exit for vessels, nerves, and ureters

  • Renal sinus: Cavity with urine-draining structures and adipose tissue

• Internal Anatomy:

  • Three regions: renal cortex (outer), renal medulla (middle), renal pelvis (inner)

  • Renal columns: Extensions of cortex housing blood vessels

  • Nephrons: Over one million per kidney, consisting of renal corpuscle and renal tubule

  • Renal pyramids: Cone-shaped structures in medulla, ending in papillae

  • Urine drainage: Minor calyces → major calyces → renal pelvis → ureter

• Blood Supply:

  • Renal arteries (from abdominal aorta) supply ~25% of cardiac output

  • Arterial path: renal artery → segmental → interlobar → arcuate → interlobular (cortical radiate)

  • Capillary beds: afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries

  • Venous path: peritubular venules → interlobular → arcuate → interlobar → renal vein (no segmental veins)

• Nephron Microanatomy:

  • Renal corpuscle: glomerulus (fenestrated capillaries) + glomerular (Bowman's) capsule (parietal and visceral layers with podocytes)

  • Renal tubule: proximal tubule (with brush border), nephron loop (descending/thin and ascending/thick limbs), distal tubule (no brush border)

  • Juxtaglomerular apparatus (JGA): macula densa + JG cells, regulates blood pressure and filtration

  • Collecting system: cortical and medullary collecting ducts, papillary duct

• Types of Nephrons:

  • Cortical nephrons (80%): short loops, mostly in cortex

  • Juxtamedullary nephrons: long loops, vasa recta, crucial for urine concentration

Module 24.3 Overview of Renal Physiology

Renal physiology involves three main processes: glomerular filtration, tubular reabsorption, and tubular secretion.

• Glomerular Filtration: Selective filtration of blood based on size; cells and large proteins remain in blood, smaller substances enter filtrate.

• Tubular Reabsorption: Reclaiming water, glucose, amino acids, and electrolytes from filtrate back to blood; most occurs in proximal tubule and nephron loop.

• Tubular Secretion: Addition of substances from blood to filtrate for excretion; helps maintain homeostasis and remove toxins.

Module 24.4 Renal Physiology I: Glomerular Filtration

Filtration occurs across a specialized membrane in the renal corpuscle, driven by pressure gradients.

• Filtration Membrane:

  • Three layers: fenestrated endothelium, basal lamina, podocytes (filtration slits)

  • Allows passage of water, ions, small molecules; blocks cells and large proteins

• Glomerular Filtration Rate (GFR): Amount of filtrate formed per minute (~125 ml/min)

• Filtration Pressures:

  • Hydrostatic pressure (GHP): pushes water out, favors filtration (~50 mm Hg)

  • Colloid osmotic pressure (GCOP): pulls water in, opposes filtration (~30 mm Hg)

  • Capsular hydrostatic pressure (CHP): opposes filtration (~10 mm Hg)

  • Net Filtration Pressure (NFP):

  $NFP = GHP - (GCOP + CHP)$

• Regulation of GFR:

  • Autoregulation: myogenic mechanism (smooth muscle response), tubuloglomerular feedback (macula densa, NaCl concentration)

  • Hormonal: RAAS (renin-angiotensin-aldosterone system), angiotensin II, ANP

  • Neural: sympathetic nervous system (norepinephrine)

• Renal Failure: Decreased GFR leads to uremia (waste buildup), treated by dialysis

Module 24.5 Renal Physiology II: Tubular Reabsorption and Secretion

Reabsorption and secretion are mediated by carrier proteins and transport mechanisms, crucial for homeostasis.

• Transport Mechanisms:

  • Facilitated diffusion (passive), primary and secondary active transport (require energy)

  • Antiport and symport pumps; transport maximum (TM) when saturated

• Proximal Tubule:

  • Reabsorbs ions (Na+, K+, Cl-, SO42-, PO43-), nutrients (glucose, amino acids, vitamins, lactic acid), bicarbonate, and water (65%)

  • Sodium reabsorption via leak channels and carrier proteins

  • Bicarbonate reabsorption via Na+/H+ antiporter and carbonic anhydrase:

  $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

  • Obligatory water reabsorption via aquaporins

  • Secretion: H+, drugs, nitrogenous wastes

• Nephron Loop: Reabsorbs ~20% water, 25% Na+ and Cl-

• Distal Tubule and Collecting System:

  • By distal tubule: 85% water, 90% Na+ reabsorbed

  • Hormone regulation: aldosterone (Na+ reabsorption, K+ secretion), ADH (water reabsorption), ANP (Na+ excretion, inhibits aldosterone/ADH)

  • Medullary collecting system: impermeable to water without ADH, permeable to urea, intercalated cells secrete H+

• Acid-Base Balance:

  • When blood pH decreases: tubule cells deaminate glutamine, producing NH3 (secreted) and HCO3- (reabsorbed)

  • NH3 binds H+ to form NH4+; HCO3- buffers blood

  • When blood pH increases: less HCO3- reabsorbed, more excreted

Module 24.6 Renal Physiology III: Regulation of Urine Concentration and Volume

The kidneys adjust urine concentration and volume through obligatory and facultative water reabsorption, and countercurrent mechanisms.

• Osmolarity Changes:

  • New filtrate: iso-osmotic (300 mOsm)

  • Descending limb: water leaves, filtrate concentrated (up to 900 mOsm)

  • Ascending limb: ions pumped out, water stays, filtrate diluted (down to 100 mOsm)

  • Late distal tubule/collecting duct: facultative water reabsorption adjusts final concentration

• Dilute Urine: Produced when ADH is suppressed; collecting duct impermeable to water

• Concentrated Urine: Produced when ADH is present; countercurrent mechanism and medullary osmotic gradient allow water reabsorption

• Countercurrent Mechanism:

  • Countercurrent multiplier (nephron loop): NaCl actively transported, water follows, gradient established

  • Urea recycling: increases medullary osmotic gradient

  • Vasa recta: countercurrent exchanger maintains gradient

Module 24.8 Urine and Renal Clearance

Urine composition and renal clearance are important for diagnostic and functional assessment of the kidneys.

• Urine Composition: Water, ions (Na+, K+, Cl-, H+), phosphates, sulfates, metabolic wastes (urea, creatinine, ammonia, uric acid), small amounts of bicarbonate, calcium, magnesium

• Urinalysis: Examines color, odor, pH, specific gravity for disease detection

• Renal Clearance: Rate of removal of a substance from blood; used to estimate GFR

• Creatinine Clearance: Elevated levels indicate impaired kidney function

Module 24.9 Urine Transport, Storage, and Elimination

The urinary tract transports, stores, and eliminates urine, with specialized structures and reflexes for control.

• Ureters: 25–30 cm long, 3–4 mm diameter; three layers: adventitia, muscularis, mucosa

• Urinary Bladder: Hollow, distensible organ; three layers: adventitia, detrusor muscle, mucosa; trigone region; capacity 700–800 ml (males), less in females

• Urethra: Drains bladder; internal urethral sphincter (smooth muscle), external urethral sphincter (skeletal muscle, voluntary control)

• Male vs. Female Urethra:

  • Female: ~4 cm, opens between vagina and clitoris, only urine excretion

  • Male: ~20 cm, prostatic, membranous, spongy regions

• Micturition (Urination):

  • Reflex arc: stretch receptors → sacral spinal cord → parasympathetic efferents → detrusor contraction, internal sphincter relaxation

  • Micturition center in pons allows voluntary control

Table: Comparison of Nephron Types

Nephron Type	Location	Loop Length	Capillary Network	Main Function
Cortical	Renal cortex	Short	Peritubular capillaries	General filtration
Juxtamedullary	Cortex/medulla border	Long	Vasa recta	Urine concentration

Table: Filtration Pressures at the Glomerulus

Pressure	Value (mm Hg)	Effect
Glomerular Hydrostatic Pressure (GHP)	~50	Favors filtration
Glomerular Colloid Osmotic Pressure (GCOP)	~30	Opposes filtration
Capsular Hydrostatic Pressure (CHP)	~10	Opposes filtration
Net Filtration Pressure (NFP)	GHP - (GCOP + CHP)	Determines direction of filtration

Example: Renin-Angiotensin-Aldosterone System (RAAS)

• Low blood pressure → JG cells release renin

• Renin converts angiotensinogen to angiotensin I

• ACE converts angiotensin I to angiotensin II

• Angiotensin II: vasoconstriction, increased Na+ and water reabsorption, aldosterone release, increased thirst

• Result: Increased blood pressure and GFR

Additional info:

• Glomerular filtration rate (GFR) is a key indicator of kidney function; normal values are ~125 ml/min.

• Dialysis is a medical procedure that artificially removes wastes from the blood when kidneys fail.

• Urinalysis is a routine diagnostic test for kidney and metabolic diseases.