Urinary System Anatomy

Overview of the Urinary System

  • Structures:
    • Kidneys: Main organs responsible for filtering blood to form urine.
    • Ureters: Tubes that transport urine from the kidneys to the bladder.
    • Urinary Bladder: Stores urine until it is expelled from the body.
    • Urethra: Channel through which urine is conveyed outside the body.

Anatomy of the Kidneys

  • Location:
    • Posterior abdominal wall at vertebral levels T12 to L3.
    • Surrounded by a layer of renal fat padding.
  • Key Structures:
    • Renal hilum: Entry/exit point for blood vessels.
    • Cortex: Outer region of the kidney; contains renal columns.
    • Medulla:
    • Composed of renal pyramids.
    • Renal papillas release urine into minor calyces.
    • Renal sinus: Space within the kidney that contains renal pelvis and major calyces.

Blood Supply to the Kidneys

  • Renal Artery: Supplies blood to kidneys (receives about 1/5th of cardiac output).
  • Branching Arteries:
    • Segmental arteries ➔ Lobar arteries ➔ Interlobular arteries ➔ Arcuate arteries.
  • Renal Vein: Drains filtered blood from the kidneys.

Urinary Bladder Overview

  • Wall Structure:
    • Mucosa: Lined with transitional epithelium that allows for stretching.
    • Submucosa: Contains connective tissue.
    • Detrusor Muscle: Smooth muscle layer crucial for bladder contraction.
    • Rugae: Folds that allow expansion as the bladder fills.
    • Trigone: Smooth triangular area on the bladder floor formed by the openings of the ureters and urethra.

Urethra Anatomy

  • Function: Extends from the bladder to the exterior of the body.
  • Male Urethra:
    • Prostatic urethra: Passes through the prostate gland.
    • Membranous urethra: Short segment between prostate and spongy urethra.
    • Spongy (Penile) urethra: Longest part, runs through the penis.
  • Female Urethra: Shorter and directly opens to the external urethral orifice.

Micturition Process

  • Detrusor muscle contracts involuntarily.
  • Internal urethral sphincter relaxes, allowing urine flow from the bladder.
  • External urethral sphincter allows voluntary control over urination.

Nephron Structure and Function

  • Functional Unit of the Kidney:
    • Approximately 1.2 million nephrons per kidney.
  • Major Components:
    • Renal Corpuscle: Composed of Bowman’s capsule and glomerulus.
    • Renal Tubule:
    • Includes proximal convoluted tubule (PCT), nephron loop (Henle loop), and distal convoluted tubule (DCT).
  • Microscopic Features:
    • PCT: Lined with simple cuboidal cells and microvilli for reabsorption.
    • Loop of Henle: Passes into the medulla; critical for urine concentration.
    • DCT: Further adjusts composition of urine.

Filtration Mechanism

  • The kidneys filter everything in the blood except blood cells and plasma proteins.
  • Approximately 180 liters of fluid is filtered through the glomeruli each day; only about 1.5 liters is excreted as urine.

Tubular Reabsorption and Secretion

  • Reabsorption:
    • Occurs in PCT, Loop of Henle, and DCT.
    • Active and passive transport mechanisms used for reclaiming substances like water, glucose, and ions.
  • Secretion: Additional removal of substances (e.g., potassium, hydrogen ions) from blood into the renal tubule to be expelled in urine.

Control of Filtration Rate

  • Regulated by blood pressure and vessel diameters of afferent and efferent arterioles.
  • Juxtaglomerular apparatus: Senses changes in pressure and adjusts the filtration accordingly.

Hormonal Control

  • Erythropoietin: Stimulates red blood cell production in response to low oxygen levels.
  • Antidiuretic Hormone (ADH): Increases the permeability of the distal tubule to water, promoting water retention.