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How Your Kidneys & Urinary System Works

Understanding the body's filtration system that removes waste and maintains fluid balance

Your Kidneys - Amazing Filters

Your kidneys are like sophisticated water treatment plants that filter your blood 24/7, removing waste products and maintaining the perfect balance of fluids and chemicals in your body.

Kidney Structure

Location and Size

  • Located on either side of your spine, below your ribs
  • Each kidney is about the size of your fist
  • Bean-shaped organs
  • Protected by your lower ribs

Main Parts

  • Renal cortex: Outer layer where filtering happens
  • Renal medulla: Inner layer with collecting ducts
  • Renal pelvis: Collects urine before it leaves kidney
  • Ureter: Tube that carries urine to bladder

How Filtration Works

Step 1: Blood Enters Kidneys

  1. Blood flows into kidneys through renal arteries
  2. About 1.2 liters of blood per minute
  3. Blood contains waste products and excess water

Step 2: Filtration in Nephrons

  1. Each kidney contains about 1 million nephrons
  2. Nephrons are tiny filtering units
  3. Blood pressure forces fluid through filters
  4. Waste products and water are filtered out
  5. Important substances are kept in blood

Step 3: Reabsorption

  1. Useful substances are reabsorbed back into blood
  2. Water, glucose, and minerals are reclaimed
  3. About 99% of filtered fluid is reabsorbed
  4. Only waste products remain

Step 4: Urine Formation

  1. Remaining fluid becomes urine
  2. Urine contains waste products and excess water
  3. Urine flows to bladder through ureters

What Kidneys Filter Out

Waste Products

  • Urea: From protein breakdown
  • Creatinine: From muscle activity
  • Uric acid: From cell breakdown
  • Drugs and toxins: Medications and harmful substances

Excess Substances

  • Water: When you drink too much
  • Salt: Excess sodium
  • Potassium: Excess potassium
  • Other minerals: Excess minerals

What Kidneys Keep

Important substances are reabsorbed back into blood:

  • Glucose: Sugar needed for energy
  • Water: When body needs it
  • Proteins: Needed for body functions
  • Essential minerals: Sodium, potassium, calcium

Other Important Functions

Blood Pressure Control

  • Kidneys regulate blood volume
  • Release hormones that control blood pressure
  • Remove excess salt and water

Red Blood Cell Production

  • Kidneys produce erythropoietin hormone
  • Stimulates bone marrow to make red blood cells
  • Red blood cells carry oxygen throughout body

Bone Health

  • Activate vitamin D for calcium absorption
  • Help maintain strong bones
  • Prevent bone diseases

Acid-Base Balance

  • Remove excess acid from blood
  • Maintain proper pH levels
  • Keep blood slightly alkaline

Urinary System

Ureters

  • Tubes that carry urine from kidneys to bladder
  • About 10-12 inches long
  • Have muscle contractions to move urine

Bladder

  • Muscular sac that stores urine
  • Can hold about 400-600ml of urine
  • Expands as it fills
  • Sends signals when it needs to empty

Urethra

  • Tube that carries urine out of body
  • Shorter in women, longer in men
  • Has sphincter muscles to control flow

Urination Process

  1. Bladder fills with urine
  2. Nerves send signals to brain
  3. Brain decides when to urinate
  4. Sphincter muscles relax
  5. Bladder muscle contracts
  6. Urine flows out through urethra

Detailed Anatomy

Kidney Structure

  • Location: Behind peritoneum (retroperitoneal); right kidney slightly lower than left (due to liver)
  • Size: About 4-5 inches long, 2-3 inches wide; bean-shaped; weighs ~150g each
  • Regions:
    • Renal Cortex: Outer layer; contains glomeruli and convoluted tubules
    • Renal Medulla: Inner region; contains renal pyramids (8-18 per kidney)
    • Renal Pelvis: Funnel-shaped; collects urine; connects to ureter
  • Blood Supply: Renal artery (branches from aorta); ~20% of cardiac output; 1.2 liters per minute

Nephron Structure (Functional Unit)

Each kidney contains ~1 million nephrons:

  • Renal Corpuscle:
    • Glomerulus: Tuft of capillaries; filters blood; high pressure
    • Bowman's Capsule: Surrounds glomerulus; collects filtrate; has two layers (parietal and visceral/podocytes)
  • Renal Tubule:
    • Proximal Convoluted Tubule (PCT): Reabsorbs most water, glucose, amino acids, ions
    • Loop of Henle: Descending limb (water reabsorption), ascending limb (salt reabsorption); creates concentration gradient
    • Distal Convoluted Tubule (DCT): Fine-tunes reabsorption; responds to hormones
    • Collecting Duct: Receives urine from multiple nephrons; final water reabsorption; responds to ADH

Filtration Membrane

  • Three Layers:
    • Fenestrated Endothelium: Capillary wall; has pores (70-100 nm); prevents cells from passing
    • Basement Membrane: Gel-like layer; prevents large proteins from passing
    • Podocytes: Visceral layer of Bowman's capsule; foot processes with filtration slits; final barrier
  • What Passes: Water, ions, glucose, amino acids, small molecules
  • What Doesn't Pass: Blood cells, large proteins, platelets

Urinary Tract Structure

  • Ureters: Two tubes (~10-12 inches); peristalsis moves urine; enter bladder at angles (prevents backflow)
  • Bladder: Hollow, muscular organ; stores urine (300-500ml normal, up to 1 liter); detrusor muscle contracts for urination
  • Urethra: Tube from bladder to outside; shorter in females (~1.5 inches), longer in males (~8 inches)
  • Sphincters: Internal (smooth muscle, involuntary) and external (skeletal muscle, voluntary)

Detailed Physiology

Glomerular Filtration

  • Filtration Rate: ~125 ml/min (180 liters/day); 99% reabsorbed, 1-2 liters excreted as urine
  • Filtration Pressure: Net filtration pressure = Glomerular hydrostatic pressure - (Capsular pressure + Blood colloid osmotic pressure)
  • Glomerular Hydrostatic Pressure: ~55 mmHg; main force pushing fluid out
  • Regulation:
    • Autoregulation: Maintains constant GFR despite blood pressure changes
    • Myogenic mechanism: Smooth muscle responds to pressure
    • Tubuloglomerular feedback: Macula densa detects flow rate

Tubular Reabsorption

  • Proximal Convoluted Tubule: Reabsorbs ~65% of filtrate:
    • Water: Osmosis (follows solutes)
    • Glucose: Active transport (100% reabsorbed normally)
    • Amino acids: Active transport (100% reabsorbed)
    • Sodium: Active transport (creates gradient for other solutes)
    • Chloride, bicarbonate, potassium: Various mechanisms
  • Loop of Henle: Creates concentration gradient:
    • Descending limb: Permeable to water; water leaves by osmosis
    • Ascending limb: Impermeable to water; actively pumps out salt
    • Creates medullary concentration gradient (300 mOsm to 1200 mOsm)
  • Distal Convoluted Tubule: Fine-tuning; responds to hormones (aldosterone, PTH)
  • Collecting Duct: Final water reabsorption; responds to ADH (antidiuretic hormone)

Tubular Secretion

  • Purpose: Removes substances from blood; adds to filtrate
  • Substances Secreted:
    • Hydrogen ions (H+): Regulates blood pH
    • Potassium (K+): Regulates blood potassium levels
    • Creatinine: Waste product from muscle metabolism
    • Drugs and toxins: Many medications eliminated this way
  • Location: Primarily in proximal and distal convoluted tubules

Hormonal Regulation

  • ADH (Antidiuretic Hormone):
    • Released from posterior pituitary
    • Stimulated by: High blood osmolarity, low blood volume
    • Action: Increases water reabsorption in collecting duct
    • Result: Concentrated urine, less water loss
  • Aldosterone:
    • Released from adrenal cortex
    • Stimulated by: Low blood pressure, high potassium
    • Action: Increases sodium reabsorption (and water follows)
    • Result: Increases blood volume and pressure
  • Atrial Natriuretic Peptide (ANP):
    • Released from heart atria
    • Stimulated by: High blood volume/pressure
    • Action: Decreases sodium reabsorption
    • Result: Increases urine output, decreases blood volume

Acid-Base Balance

  • Blood pH: Maintained at 7.35-7.45; kidneys help regulate
  • Mechanisms:
    • Bicarbonate reabsorption: Prevents loss of base
    • Hydrogen ion secretion: Removes acid from blood
    • Ammonia production: Buffers hydrogen ions
    • Phosphate buffering: Excretes hydrogen ions

Micturition (Urination) Reflex

  • Bladder Filling: Stretch receptors detect volume
  • Spinal Reflex: Signals sent to spinal cord; parasympathetic nerves activated
  • Voluntary Control: Cerebral cortex can override; external sphincter under conscious control
  • Urination: Detrusor muscle contracts; sphincters relax; urine expelled

Why It's Critical

Without your kidneys and urinary system, waste products would build up in your blood, leading to serious illness and death. Your kidneys are essential for maintaining the delicate balance your body needs to function properly!

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