The Urinary System: Structure and Function

The Urinary System

Overview and Gross Anatomy

The urinary system is essential for maintaining the body's internal environment by regulating water, solute concentrations, and removing metabolic wastes. It consists of the kidneys, ureters, urinary bladder, and urethra.

• Kidneys: Major excretory organs located retroperitoneally in the superior lumbar region, between T12 and L5. The right kidney is slightly lower due to the liver.

• Ureters: Tubes that transport urine from the kidneys to the urinary bladder.

• Urinary bladder: Temporary storage reservoir for urine.

• Urethra: Tube that conveys urine from the bladder to the outside of the body.

Functions of the Kidneys

• Regulate total water volume and solute concentration in the body.

• Regulate ion concentrations in extracellular fluid.

• Maintain long-term acid-base balance.

• Excrete metabolic wastes, toxins, and drugs.

• Produce erythropoietin (stimulates RBC production) and renin (regulates blood pressure).

• Activate vitamin D and carry out gluconeogenesis if needed.

Location and External Anatomy of the Kidneys

The kidneys are retroperitoneal, lying between the dorsal body wall and the parietal peritoneum. The adrenal glands sit atop each kidney. The lateral surface is convex, while the medial surface is concave with a vertical renal hilum where vessels, nerves, and the ureter enter/exit.

Supportive Tissue Layers of the Kidney

• Renal fascia: Outer layer of dense fibrous connective tissue anchoring the kidney.

• Perirenal fat capsule: Fatty cushion protecting the kidney.

• Fibrous capsule: Transparent capsule preventing infection spread to the kidney.

Internal Gross Anatomy of the Kidney

Regions of the Kidney

• Renal cortex: Granular-appearing superficial region.

• Renal medulla: Deep to cortex, composed of cone-shaped medullary (renal) pyramids separated by renal columns (inward extensions of cortical tissue).

• Renal pelvis: Funnel-shaped tube continuous with the ureter; collects urine from major and minor calyces.

Blood Supply of the Kidneys

The kidneys receive about 25% of cardiac output each minute. Blood flows through a series of arteries and veins, with the renal artery entering and the renal vein exiting at the hilum.

• Arterial flow: Renal → Segmental → Interlobar → Arcuate → Cortical radiate (interlobular)

• Venous flow: Cortical radiate → Arcuate → Interlobar → Renal veins (no segmental veins)

Nephrons: The Functional Units of the Kidney

Structure of a Nephron

Each kidney contains over one million nephrons, which are responsible for forming urine. Each nephron consists of a renal corpuscle and a renal tubule.

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

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

Renal Corpuscle: Glomerulus and Glomerular Capsule

• Glomerulus: A network of fenestrated capillaries allowing efficient filtrate formation.

• Glomerular (Bowman's) capsule: Surrounds the glomerulus and consists of two layers:

  • Parietal layer: Simple squamous epithelium.

  • Visceral layer: Made of podocytes with foot processes and filtration slits, allowing filtrate to pass into the capsular space.

Renal Tubule and Collecting Duct

Proximal Convoluted Tubule (PCT)

• Composed of cuboidal cells with dense microvilli (brush border) to increase surface area.

• Functions in reabsorption of water and solutes, and secretion of substances into the filtrate.

Nephron Loop (Loop of Henle)

• U-shaped structure with descending and ascending limbs.

• Descending limb: Thin segment, permeable to water.

• Ascending limb: Thick segment, permeable to solutes but not water.

Distal Convoluted Tubule (DCT)

• Composed of cuboidal cells with few microvilli, confined to the cortex.

• Functions mainly in secretion rather than reabsorption.

Collecting Duct

• Receives filtrate from multiple nephrons and delivers urine through papillae into minor calyces.

• Principal cells: Maintain water and Na+ balance.

• Intercalated cells: Maintain acid-base balance of blood.

Classes of Nephrons

• Cortical nephrons: 85% of nephrons, almost entirely in the cortex.

• Juxtamedullary nephrons: Long nephron loops deeply invade the medulla; important for producing concentrated urine.

Nephron Capillary Beds

• Glomerulus: Fed and drained by arterioles; high blood pressure promotes filtration.

• Peritubular capillaries: Low-pressure, porous capillaries adapted for absorption.

• Vasa recta: Long, thin-walled vessels associated with juxtamedullary nephrons; important for forming concentrated urine.

Juxtaglomerular Complex (JGC)

The JGC regulates the rate of filtrate formation and blood pressure. It consists of:

• Macula densa: Chemoreceptors sensing NaCl content of filtrate.

• Granular (JG) cells: Mechanoreceptors sensing blood pressure; secrete renin.

• Extraglomerular mesangial cells: Pass signals between macula densa and granular cells.

Formation of Urine

Overview of Urine Formation

The kidneys process about 180 L of fluid daily, but only 1.5 L of urine is formed. Urine formation involves three main processes:

1. Glomerular filtration: Produces cell- and protein-free filtrate.

2. Tubular reabsorption: Returns 99% of substances from filtrate to blood.

3. Tubular secretion: Moves substances from blood to filtrate.

Step 1: Glomerular Filtration

• Passive process driven by hydrostatic pressure.

• Filtration membrane allows water and small solutes to pass; normally, no cells pass.

Pressures Affecting Filtration

• Outward pressure: Hydrostatic pressure in glomerular capillaries (HPgc), about 55 mm Hg.

• Inward pressures: Hydrostatic pressure in capsular space (HPcs, 15 mm Hg) and colloid osmotic pressure in capillaries (OPgc, 30 mm Hg).

• Net filtration pressure (NFP): The sum of these forces; determines glomerular filtration rate (GFR).

Glomerular Filtration Rate (GFR)

• GFR is the volume of filtrate formed per minute by both kidneys (normal: 120–125 ml/min).

• Directly proportional to NFP, total surface area available for filtration, and permeability of the filtration membrane.

Regulation of Glomerular Filtration

Intrinsic Controls (Renal Autoregulation)

• Maintains nearly constant GFR when mean arterial pressure is 80–180 mm Hg.

• Mechanisms: Myogenic (response to blood pressure changes) and tubuloglomerular feedback (macula densa senses NaCl).

Extrinsic Controls

• Regulate GFR to maintain systemic blood pressure.

• Mechanisms: Sympathetic nervous system and renin-angiotensin-aldosterone system.

Step 2: Tubular Reabsorption

• Selective process reclaiming most tubular contents and returning them to blood.

• Routes: Transcellular (through cells) and paracellular (between cells).

• Most active in the proximal convoluted tubule; hormonally regulated in distal tubule and collecting duct (aldosterone, ADH).

Step 3: Tubular Secretion

• Active process, mainly in the PCT, moving substances from blood into filtrate.

• Important for eliminating drugs, wastes, excess ions, and maintaining blood pH.

Regulation of Urine Concentration and Volume

• Kidneys maintain body fluid osmolality at ~300 mOsm by regulating urine concentration and volume.

• Countercurrent mechanisms (multiplier in nephron loop, exchanger in vasa recta) establish and maintain the medullary osmotic gradient.

• ADH and aldosterone adjust urine concentration and volume based on hydration status.

Transport, Storage, and Elimination of Urine

Ureters

• Slender tubes conveying urine from kidneys to bladder; enter bladder at an angle to prevent backflow.

• Wall has three layers: mucosa (transitional epithelium), muscularis (smooth muscle), and adventitia (fibrous connective tissue).

Urinary Bladder

• Muscular sac for temporary urine storage; can hold about 500 ml when moderately full.

• Wall layers: mucosa, thick detrusor muscle (three layers of smooth muscle), and fibrous adventitia.

Urethra

• Muscular tube draining the bladder; has internal (involuntary) and external (voluntary) sphincters.

• Female urethra is short (3–4 cm); male urethra is longer and carries semen as well as urine.

Micturition (Urination)

• Involves contraction of the detrusor muscle, opening of the internal urethral sphincter (ANS), and opening of the external urethral sphincter (somatic nervous system).

• In infants, urination is a reflex; in adults, it is controlled by brain centers.

Clinical Aspects

• Urinalysis: Examines urine for disease or illegal substances.

• Renal clearance: Volume of plasma cleared of a substance per minute; used to assess GFR and kidney function.

• Chronic renal disease: GFR < 60 ml/min for 3 months; renal failure: GFR < 15 ml/min.

• Renal calculi (kidney stones): Crystallized salts that may obstruct urine flow; risk factors include obesity and dehydration.

• Urinary incontinence: Inability to control urination; urinary retention: Inability to expel urine.