Urinorganen del 3 (5)

Scientific Concepts and Processes Presented

• Nephron Anatomy and Function The Nephron is the functional unit of the kidney, consisting of:
  • Glomerulus: A bundle of capillaries surrounded by Bowman's Capsule where blood filtration begins.
  • Bowman's Capsule: Collects the filtrate (primary urine) from the Glomerulus.
  • Proximal Tubule: First segment of the tubule where most reabsorption occurs.
  • Loop of Henle: A U-shaped tubule segment responsible for concentrating urine by selective reabsorption of water and ions.
  • Distal Tubule: Further adjusts filtrate composition, regulating sodium and water reabsorption.
  • Collecting Duct: Final site for water reabsorption and urine concentration before excretion.
• Urine Formation Processes
  1. Filtration:
     • Blood enters the Glomerulus via the afferent arteriole and exits via the efferent arteriole (both arterioles, not veins).
     • Filtration depends on blood pressure pushing fluid through the glomerular capillaries into Bowman's Capsule.
     • Filtration pressure is determined by:
       • Blood hydrostatic pressure (~50 mm Hg pushing fluid out).
       • Fluid pressure in Bowman's Capsule opposing filtration.
       • Colloid osmotic pressure from blood proteins pulling fluid back into capillaries.
     • Net filtration pressure is about 10 mm Hg, allowing approximately 180 liters of primary urine to be filtered daily.
     • The glomerular filtration rate (GFR) is normally about 120 ml/min and must remain fairly constant despite fluctuations in systemic blood pressure.
  2. Autoregulation of GFR:
     • The kidney maintains a stable GFR through two main mechanisms:
       • Myogenic response: The afferent arteriole adjusts its diameter (constricts if pressure rises, dilates if pressure falls) to keep glomerular pressure constant.
       • Tubuloglomerular feedback: The juxtaglomerular apparatus (a specialized structure near Bowman's Capsule) senses flow and pressure, releasing renin when blood pressure drops.
     • Renin activates the Renin-Angiotensin System (RAS), which causes constriction of the efferent arteriole, increasing glomerular pressure and filtration.
  3. Reabsorption:
     • Most filtered substances are reabsorbed back into the blood to prevent loss of essential nutrients and water.
     • Proximal tubule:
       • Reabsorbs ~70% of water and sodium, glucose, amino acids, and other nutrients.
       • Cells have microvilli to increase surface area for absorption.
       • Sodium is actively pumped out, causing water to follow by osmosis.
       • Glucose reabsorption is via specific transporters; excess glucose leads to glucosuria.
     • Loop of Henle:
       • Descending limb is permeable to water (water reabsorbed).
       • Ascending limb is impermeable to water but actively reabsorbs sodium, concentrating salt in the medulla.
     • Distal tubule and collecting duct:
       • Fine-tune sodium and water reabsorption, regulated by hormones.
       • About 20% sodium reabsorbed in distal tubule; about 9% in collecting duct.
       • Remaining sodium and water are excreted in urine.
  4. Hormonal Regulation of Reabsorption:
     • Aldosterone:
       • Secreted by adrenal glands, acts mainly on distal tubule.
       • Increases sodium reabsorption by inserting sodium-potassium pumps in tubular cells.
       • Water follows sodium, reducing urine volume and conserving sodium.
     • Antidiuretic Hormone (ADH):
       • Acts on collecting ducts to increase water reabsorption by making the duct walls more permeable to water.
       • Regulates urine concentration and volume depending on hydration status.
  5. Secretion:
     • Active transport of substances from blood into the tubular fluid.
     • Important for potassium, hydrogen ions, and some drugs.
     • Helps regulate electrolyte and acid-base balance.
     • Potassium is reabsorbed early but secreted later in the distal tubule, regulated by Aldosterone.
     • Hydrogen ions are secreted to acidify urine; bicarbonate ions may be secreted if the body is too alkaline.
• Measurement of Kidney Function:
  • Creatinine clearance test:
    • Creatinine is filtered but not reabsorbed, making it a good marker for GFR.
    • Measurement involves comparing creatinine levels in blood and urine over time to