The Urinary System: Structure and Function (Chapter 25 Study Notes)

The Urinary System

Introduction

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

• Key Functions of Kidneys:

  • Regulate total body water volume and solute concentration

  • Regulate ion concentrations in extracellular fluid (ECF)

  • Ensure long-term acid-base balance

  • Excrete metabolic wastes, toxins, and drugs

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

  • Activate vitamin D (calcitriol) for calcium regulation

  • Carry out gluconeogenesis during prolonged fasting

• Components of the Urinary System:

  • Ureters: Transport urine from kidneys to bladder

  • Urinary bladder: Temporary storage for urine

  • Urethra: Transports urine out of the body

Location and External Anatomy of the Kidneys

The kidneys are retroperitoneal, bean-shaped organs located in the superior lumbar region (T12 to L3). The right kidney is slightly lower due to the liver. Each kidney has a convex lateral surface and a concave medial surface (renal hilum).

• Supportive Tissue Layers:

  • Renal fascia: Dense connective tissue anchoring the kidney

  • Perirenal fat capsule: Cushions the kidney

  • Fibrous capsule: Transparent capsule preventing infection

Internal Gross Anatomy of the Kidney

Each kidney has three distinct regions: cortex, medulla, and pelvis.

• Renal cortex: Light-colored, granular outer region

• Renal medulla: Darker, contains cone-shaped renal pyramids and renal columns

• Renal pelvis: Funnel-shaped tube; collects urine from major and minor calyces

• Urine Flow: Minor calyx → major calyx → renal pelvis → ureter → bladder

Blood and Nerve Supply

The kidneys receive a large blood supply to filter and adjust blood composition.

• Renal arteries: Deliver ~25% of cardiac output to kidneys

• Arterial flow: Abdominal aorta → renal artery → segmental artery → interlobar artery → arcuate artery → cortical radiate artery

• Venous flow: Cortical radiate vein → arcuate vein → interlobar vein → renal vein

• Nerve supply: Sympathetic fibers from renal plexus regulate blood flow and urine formation

Nephrons: The Functional Units of the Kidney

Nephrons are the structural and functional units that filter blood and form urine. Each kidney contains about 1 million nephrons.

• Components of a Nephron:

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

  • Renal tubule: Proximal convoluted tubule (PCT), nephron loop (loop of Henle), distal convoluted tubule (DCT)

  • Collecting duct: Receives filtrate from multiple nephrons

Renal Corpuscle

• Glomerulus: Tuft of fenestrated capillaries for efficient filtrate formation

• Glomerular capsule:

  • Parietal layer: Simple squamous epithelium

  • Visceral layer: Podocytes with foot processes forming filtration slits

Renal Tubule and Collecting Duct

• Proximal convoluted tubule (PCT):

  • Confined to cortex; cuboidal cells with microvilli (brush border) and mitochondria

  • Increases surface area for reabsorption and secretion

• Nephron loop (Loop of Henle):

  • U-shaped with descending (simple squamous) and ascending (cuboidal/columnar) limbs

• Distal convoluted tubule (DCT):

  • Confined to cortex; cuboidal cells with sparse microvilli

• Collecting duct:

  • Principal cells: Maintain water and Na+ balance

  • Intercalated cells: Maintain acid-base balance

Classes of Nephrons

• Cortical nephrons:

  • 85% of nephrons; located almost entirely in cortex

  • Short nephron loops

• Juxtamedullary nephrons:

  • Originate near cortex-medulla junction

  • Long nephron loops extend deep into medulla

  • Essential for producing concentrated urine

Example: Filtration and Urine Formation

Blood enters the glomerulus via the afferent arteriole, and filtration occurs through the filtration membrane. The filtrate passes through the renal tubule, where reabsorption and secretion adjust its composition before it is excreted as urine.

Additional info:

• Clinical conditions such as pyelonephritis (kidney infection) and trauma can affect kidney function.

• Nephron structure and function are foundational for understanding urine formation and renal physiology.