Chapter 25: The Urinary System

Overview

The urinary system is essential for maintaining the composition of the body's extracellular fluids. The kidneys play a central role in filtering blood, regulating fluid and electrolyte balance, and eliminating waste products through urine formation. This chapter explores the anatomy and physiology of the urinary system, the processes of urine formation, and the mechanisms that regulate urine concentration and volume.

25.1 Gross Anatomy of the Kidneys

Location and External Anatomy

• Kidneys are bean-shaped organs located retroperitoneally in the superior lumbar region.

• The renal hilum is a vertical cleft on the medial surface, leading into the renal sinus, where blood vessels, nerves, and lymphatics enter and exit.

• The kidneys are protected by three layers:

  • Renal fascia: Anchors the kidney and adrenal gland to surrounding structures.

  • Perirenal fat capsule: Cushions the kidney.

  • Fibrous capsule: Prevents infection from spreading to the kidney.

Internal Gross Anatomy

• The kidney has three regions: cortex (outer), medulla (middle), and renal pelvis (inner).

• Major and minor calyces collect urine and empty it into the renal pelvis.

Blood and Nerve Supply

• Blood flows into the kidneys via the renal arteries, branching into segmental, lobar, interlobar, arcuate, and cortical radiate arteries.

• Afferent arterioles branch from cortical radiate arteries and supply the glomeruli.

• Venous return traces the arterial path in reverse, draining into the renal veins.

• The renal plexus regulates blood flow and nephron activity by adjusting arteriole diameter.

25.2 Nephrons: The Functional Units of the Kidney

Structure of a Nephron

• Each nephron consists of a renal corpuscle (glomerulus and glomerular/Bowman's capsule) and a renal tubule.

• The glomerulus is a tuft of fenestrated capillaries for filtration.

• The glomerular capsule has a parietal layer (structure) and a visceral layer (podocytes for filtration).

Renal Tubule and Collecting Duct

• Renal tubule segments: Proximal convoluted tubule (PCT) (dense microvilli for absorption), nephron loop (descending and ascending limbs), and distal convoluted tubule (DCT) (few microvilli).

• The collecting duct contains principal cells (water/Na+ balance) and intercalated cells (acid-base balance).

Types of Nephrons

• Cortical nephrons (85%): Located mostly in the cortex.

• Juxtamedullary nephrons (15%): Located near the cortex-medulla junction, important for urine concentration.

Nephron Capillary Beds

• Each nephron is associated with two capillary beds:

  • Glomerulus: Specialized for filtration, maintained by afferent and efferent arterioles.

  • Peritubular capillaries: Surround renal tubules, absorb solutes and water.

  • Vasa recta: Parallel juxtamedullary nephron loops, maintain medullary osmotic gradient.

Juxtaglomerular Complex (JGC)

• Formed by the distal convoluted tubule and afferent arteriole.

• Macula densa: Chemoreceptors monitoring NaCl in filtrate.

• Granular cells: Mechanoreceptors monitoring blood pressure, secrete renin.

25.3 Overview of Urine Formation

• Glomerular filtration: Passive filtration of blood in the renal corpuscle, forming filtrate.

• Tubular reabsorption: Selective movement of substances from filtrate back to blood.

• Tubular secretion: Selective movement of substances from blood into filtrate for excretion.

• About 180 L of filtrate is formed daily, but less than 1% is excreted as urine.

25.4 Urine Formation, Step 1: Glomerular Filtration

Filtration Membrane and Pressures

• The filtration membrane consists of:

  • Fenestrated endothelium

  • Basement membrane (negatively charged)

  • Podocytes (visceral layer of capsule)

• Net filtration pressure (NFP) is determined by:

  • Hydrostatic pressure in glomerular capillaries (, ~55 mm Hg)

  • Hydrostatic pressure in capsular space ()

  • Colloid osmotic pressure in glomerular capillaries ()

• Equation:

Regulation of Glomerular Filtration Rate (GFR)

• GFR is proportional to NFP, surface area, and membrane permeability.

• Renal autoregulation: Myogenic and tubuloglomerular feedback mechanisms.

• Extrinsic controls: Sympathetic nervous system and renin-angiotensin-aldosterone system.

25.5 Urine Formation, Step 2: Tubular Reabsorption

Mechanisms of Reabsorption

• Active reabsorption: Requires ATP (e.g., Na+ transport).

• Passive reabsorption: Diffusion, facilitated diffusion, osmosis.

• Most nutrients (glucose, amino acids) are reabsorbed in the proximal convoluted tubule.

• Water reabsorption in the collecting duct is regulated by antidiuretic hormone (ADH).

• Transport maximums exist for substances with protein carriers; excess is excreted in urine.

Hormonal Regulation

• ADH: Increases water reabsorption, concentrates urine.

• Aldosterone: Increases Na+ reabsorption, dilutes urine.

• Atrial natriuretic peptide (ANP): Promotes Na+ loss, concentrates urine.

25.6 Urine Formation, Step 3: Tubular Secretion

• Occurs mainly in the proximal convoluted tubule, also in distal convoluted tubule and collecting duct.

• Removes unwanted solutes, excess K+, and helps regulate blood pH.

25.7 Regulation of Urine Concentration and Volume

Medullary Osmotic Gradient

• Countercurrent multiplier: Creates gradient via ion and water absorption in nephron loop.

• Countercurrent exchanger: Maintains gradient via vasa recta.

• ADH release increases water reabsorption, forming concentrated urine.

• Urea recycling contributes to the medullary osmotic gradient.

• Diuretics increase urine output by inhibiting Na+ reabsorption or acting osmotically.

25.8 Clinical Evaluation of Kidney Function

Renal Clearance

• Renal clearance: Volume of plasma cleared of a substance per unit time.

• Inulin is used to measure glomerular filtration rate (GFR).

• Clearance less than inulin: substance is reabsorbed; greater: substance is secreted; zero: completely reabsorbed.

Physical and Chemical Properties of Urine

• Fresh urine: Clear, pale to deep yellow (urochrome pigment), slightly aromatic.

• pH: Usually around 6 (range 4.5–8.0).

• Composition: 95% water, 5% solutes (mainly urea, creatinine, uric acid).

• Abnormal constituents indicate pathology (e.g., glucose in diabetes, proteins in kidney disease).

25.9 Ureters, Bladder, and Urethra

Ureters

• Muscular tubes transporting urine from kidneys to bladder.

• Wall layers: Inner mucosa, muscularis, outer adventitia.

Urinary Bladder

• Muscular sac for urine storage, located retroperitoneally on the pelvic floor.

• Wall: Outer adventitia, middle detrusor muscle, inner mucosa (with rugae for expansion).

• Capacity: ~500 mL; expands as it fills.

Urethra

• Muscular tube draining urine from the bladder.

• Length: 3–4 cm (female), ~20 cm (male).

• Two sphincters: Internal (involuntary, smooth muscle), external (voluntary, skeletal muscle).

Micturition (Urination)

• Requires detrusor contraction, internal and external sphincter relaxation.

• Controlled by spinal reflexes and centers in the pons (storage and micturition centers).

Developmental Aspects of the Urinary System

• Three sets of embryonic kidneys: Pronephros (degenerates), mesonephros (temporary), metanephros (forms adult kidney).

• Bladder and urethra develop from the urogenital sinus.

• Newborns void frequently; kidney function matures with age.

• Aging leads to decreased kidney function and bladder tone.

Recent Discoveries and Research Topics

• Artificial kidneys and organoids for disease modeling.

• Kidney disease increases risk of diabetes (elevated urea implicated).

• Suggested research: Effects of drugs, hypertension, shock, kidney stones, dialysis, and incontinence treatments on kidney function.