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Chapter 24: The Urinary System – Structure and Function

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Overview of the Urinary System

Functions of the Urinary System

The urinary system is essential for maintaining homeostasis by regulating the composition, pH, volume, and pressure of blood. It also removes metabolic wastes and produces important hormones.

  • Regulation of Blood Composition: Alters blood pH, volume, and pressure by controlling water and solute concentrations.

  • Ion Regulation: Maintains extracellular fluid (ECF) ion concentrations, including Na+, K+, Ca2+, Cl-, and HPO42-.

  • Acid-Base Balance: Excretes H+ and conserves HCO3- to regulate blood pH.

  • Excretion: Removes wastes, toxins, and drugs such as ammonia, urea, bilirubin, creatinine, and uric acid.

  • Hormone Production: Produces erythropoietin (stimulates RBC production) and renin (regulates blood pressure).

Components of the Urinary System

  • Kidneys (2): Organs that produce urine.

  • Ureters (2): Transport urine from kidneys to urinary bladder.

  • Urinary Bladder: Temporary storage reservoir for urine.

  • Urethra: Transports urine out of the body.

Kidneys

Location and Structure

The kidneys are paired, retroperitoneal organs located on either side of the vertebral column, between T12 and L3 vertebrae. The left kidney is slightly superior to the right due to the position of the liver.

  • Retroperitoneal: Positioned posterior to the peritoneum of the abdominal cavity.

  • Renal Hilum: Concave medial border where renal artery, vein, nerves, and ureter enter/exit.

Supportive Tissue Layers

  • Fibrous Capsule (Renal Capsule): Deep layer of dense irregular connective tissue; maintains kidney shape.

  • Perinephric Fat Capsule (Adipose Capsule): Middle layer of adipose tissue; protects from trauma.

  • Renal Fascia: Superficial layer; anchors kidney to surrounding structures and abdominal wall.

Internal Anatomy of the Kidney

  • Renal Cortex: Superficial portion; extends from the capsule to the base of the renal pyramids and between them (renal columns).

  • Renal Medulla: Deep portion; contains cone-shaped renal pyramids. The base faces the cortex, and the apex (renal papilla) faces the hilum.

  • Renal Pelvis: Funnel-shaped tube continuous with the ureter. Minor calyces collect urine from papillae; major calyces collect urine from minor calyces and empty into the renal pelvis.

Urine Flow: Renal pyramid → minor calyx → major calyx → renal pelvis → ureter

Blood and Nerve Supply

  • Blood Supply: Kidneys receive 20–25% of resting cardiac output via right and left renal arteries.

  • Nerve Supply: Renal nerves are part of the sympathetic division of the autonomic nervous system; regulate blood flow.

Renal Circulation

  • Arterial Blood Flow: Renal → Segmental → Interlobar → Arcuate → Cortical radiate (Interlobular)

  • Venous Blood Flow: Cortical radiate → Arcuate → Interlobar → Renal (no segmental vein)

Nephrons

Structure and Function

Nephrons are the structural and functional units of the kidney, responsible for urine formation. Each kidney contains about 1 million nephrons.

  • Renal Corpuscle: Site of filtration; includes the glomerulus (ball of capillaries) and glomerular capsule (Bowman's capsule).

  • Renal Tubule: Long passageway for filtered fluid; includes proximal convoluted tubule (PCT), nephron loop (loop of Henle), and distal convoluted tubule (DCT).

Renal Corpuscle Details

  • Glomerulus: Capillary network with leaky endothelium (fenestrations) for efficient filtrate formation.

  • Glomerular Capsule: Cup-shaped structure with two layers:

    • Parietal Layer: Simple squamous epithelium; forms outer wall.

    • Visceral Layer: Covers glomerular capillaries; consists of podocytes with foot processes that form filtration slits.

    Capsular Space: Separates the two layers; collects filtrate.

Renal Tubule Segments

  • Proximal Convoluted Tubule (PCT): Closest to corpuscle; site of reabsorption and secretion. Cells have microvilli for increased surface area.

  • Nephron Loop: U-shaped; consists of descending limb (thin) and ascending limb (thick).

  • Distal Convoluted Tubule (DCT): Farthest from corpuscle; functions more in secretion than reabsorption. Few microvilli.

Collecting Ducts

  • Receive filtrate from many nephrons.

  • Run through renal pyramids, giving them a striped appearance.

  • Deliver urine through renal papillae into minor calyces.

Classes of Nephrons

  • Cortical Nephrons: 80–85% of nephrons; short loops; efferent arteriole delivers blood to peritubular capillaries.

  • Juxtamedullary Nephrons: 15–20% of nephrons; long loops; efferent arteriole delivers blood to vasa recta (long, thin-walled vessels parallel to nephron loop; important for concentrated urine formation).

Juxtaglomerular Apparatus (JGA)

Structure and Function

The JGA is a specialized structure in each nephron that regulates filtrate formation and blood pressure. It involves modified portions of the nephron loop and afferent arteriole.

  • Macula Densa: Cells in the ascending limb of the nephron loop; chemoreceptors sense NaCl content of filtrate.

  • Juxtaglomerular Cells: Smooth muscle cells of the afferent arteriole; sense blood pressure and secrete renin in response to low blood pressure.

Physiology of the Kidney

Filtration and Urine Formation

The kidneys process about 180 L of fluid daily, but only 1.5 L of urine is formed. They filter the entire plasma volume about 60 times per day and consume 20–25% of the body's oxygen at rest.

  • Filtrate: Blood plasma minus proteins.

  • Urine: Produced from filtrate; contains metabolic wastes and unneeded substances (<1% of original filtrate).

Renal Physiology: Three Basic Processes

  • Glomerular Filtration: Water and solutes move from blood into the renal tubule.

  • Tubular Reabsorption: 99% of filtered water and solutes move from the renal tubule back into the blood.

  • Tubular Secretion: Wastes, drugs, excess ions, etc., move from blood into the renal tubule; any filtrate remaining in the collecting duct becomes urine.

Glomerular Filtration

Mechanism

Occurs in the renal corpuscle. Blood pressure forces water and dissolved solutes out of the blood and into the glomerular capsule. The composition of filtrate is similar to blood, but normally no cells or plasma proteins pass through the filtration membrane.

  • Filtration Membrane: Consists of fenestrated endothelium, basement membrane, and filtration slits formed by podocytes.

Key Equation

  • Net Filtration Pressure (NFP): Where:

    • GHP: Glomerular hydrostatic pressure

    • CHP: Capsular hydrostatic pressure

    • GCOP: Glomerular colloid osmotic pressure

Factors Affecting Filtration Rate

  • Mesangial Cell Contraction: Reduces surface area for filtration, decreasing rate.

  • Fenestrated Capillaries: More leaky than other tissues, increasing filtration.

  • Hydrostatic Pressure: High pressure in glomerular capillaries due to smaller efferent arteriole diameter.

Regulation of Glomerular Filtration Rate (GFR)

  • Renal Autoregulation: Kidneys adjust diameter of arterioles and capillaries to maintain GFR.

  • Neural Regulation: Sympathetic nervous system reduces GFR during stress (e.g., hemorrhage).

  • Hormonal Regulation: ANP increases GFR; aldosterone (RAA pathway) decreases GFR.

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