Summary of "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