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
- 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.
- 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.
- The kidney maintains a stable GFR through two main mechanisms:
- 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.
- 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.
- Aldosterone:
- 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.
- Filtration:
- 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
- Creatinine clearance test:
Category
Science and Nature