Urinary System Overview and Functions

Overview of the Urinary System

  • The urinary system consists of organs located in the abdominal and pelvic cavities.
    • Organs include:
    • 2 kidneys
    • 2 ureters
    • 1 urinary bladder
    • 1 urethra

Functions of the Urinary System

  • The kidneys are primarily responsible for the urinary system's overall functions, including:
    • Regulation of blood composition, volume, and pressure
    • Maintenance of electrolyte balance
    • Acid-base balance (pH regulation)
    • Removal of metabolic waste products
    • Hormone production (e.g., Renin, Erythropoietin)
    • Activation of vitamin D (to aid in calcium absorption)

Regulation of Blood Components

  • Key functions:
    • Blood volume regulation: The kidneys control how much water is retained or excreted, affecting blood pressure.
    • Electrolyte balance: Key ions include:
    • Sodium
    • Chloride
    • Potassium
    • Calcium
    • Waste product removal: The kidneys filter waste products and toxins from the blood, such as urea (from protein metabolism) and creatinine.

Acid-Base Balance

  • The kidneys maintain blood pH by regulating hydrogen ion concentration, often referred to as protons.
    • Functionally:
    • Kidney functions can influence pH balance by reabsorbing bicarbonate and secreting hydrogen ions.

Water Balance

  • The kidneys manage the body’s water balance by regulating intake and output:
    • Typical daily fluid intake and production:
    • 1,000 mL from food
    • 1,200 mL from drinks
    • ~300 mL from metabolism
    • Total water input: ~2,500 mL
    • Water loss occurs through:
    • Evaporation from lungs (500 mL)
    • Skin (350 mL)
    • Feces (150 mL)
    • Kidneys excrete around 1,500 mL of urine

Kidneys and Hormone Production

  • The kidneys produce hormones such as Renin (for blood pressure control) and Erythropoietin (to stimulate red blood cell production).
  • They also activate vitamin D, which is crucial for calcium absorption from the gastrointestinal tract.

Structure of the Kidneys

  • Outer kidney layers:
    • Renal capsule (protective outer layer)
    • Cortex and Medulla:
    • The outer cortex and the inner medulla contain nephron structures for filtering blood.
  • Blood enters the kidneys via the renal artery and leaves through the renal vein after filtration.

Nephrons: Functional Units of the Kidneys

  • Approximately 1 million nephrons per kidney perform filtration:
    • Renal corpuscle (where filtration occurs) consisting of:
    • Glomerulus (capillary network)
    • Bowman's capsule (collects filtrate)
    • Renal tubules:
    • Proximal convoluted tubule
    • Loop of Henle
    • Distal convoluted tubule
    • Collecting duct (collects urine from multiple nephrons)

Urinary Bladder and Urethra

  • The urinary bladder stores urine before it is expelled from the body.
    • Male vs. Female Differences:
    • Males have a larger urinary bladder due to the presence of reproductive organs in females.
  • The urethra carries urine out of the body, controlled by two sphincters:
    • Internal urethral sphincter (smooth muscle, autonomic control)
    • External urethral sphincter (skeletal muscle, voluntary control)

Urination Reflex (Micturition)

  • Urination is regulated by the micturition reflex involving:
    • Stretch receptors in the bladder signaling fullness
    • Activation of parasympathetic nervous system to contract the bladder and relax the internal sphincter
    • Brain signals to control external sphincter (voluntary control)

Filtration and Urine Formation Mechanism

  • Filtration occurs in the renal corpuscle:
    • Glomerular filtration allows small molecules (water, ions, etc.) but not large proteins or blood cells through.
  • Reabsorption and Secretion within the renal tubules:
    • Reabsorption: Selective uptake of essential ions and water back into circulation.
    • Secretion: Additional unwanted substances are added to filtrate for excretion.

Acid-Base Regulation

  • The kidneys help maintain blood pH (7.35 to 7.45):
    • Reabsorb bicarbonate
    • Secrete hydrogen ions when necessary

Hormonal Controls of Kidney Function

  • Two key hormones involved:
    • Antidiuretic hormone (ADH): Increases water reabsorption in the kidneys, reduces urine volume.
    • Aldosterone: Enhances sodium (and thus water) reabsorption, influencing blood volume and pressure.

Blood Pressure Regulation

  • Renin-Angiotensin-Aldosterone System (RAAS):
    • Decreased blood pressure leads to Renin release, which generates Angiotensin II, causing vasoconstriction and aldosterone release, thus increasing blood volume and pressure.
  • Atrial Natriuretic Peptide (ANP) from the heart: Released when blood volume is too high, promoting sodium and water excretion to reduce blood pressure.