Kidneys and Excretion

Kidneys and Excretion: In-Depth Notes

Role in Salt and Water Regulation

  • Osmoregulation: Balancing the concentration of solutes in the body fluids.
  • Ionic Regulation: Maintaining the appropriate levels and balance of ions like Na+, K+, Cl-, etc.
  • Volume Regulation: Controlling the volume of blood and bodily fluids.

Anatomy of the Kidneys

  • Gross Anatomy:

  • Kidneys are bean-shaped organs, located retroperitoneally.

  • Cardiovascular Vessels:

  • Renal artery: Supplies blood to the kidneys.

  • Renal vein: Drains blood from the kidneys.

  • Nephron:

  • Functional unit of the kidney; each kidney has approximately 1 million nephrons.

  • Composed of various segments that facilitate filtration, reabsorption, and secretion.

Function of Nephrons

1. Renal Corpuscle (Glomerulus + Bowman’s Capsule)
  • Filtration:
  • Blood is filtered to produce isosmotic primary urine.
  • Glomerular filtration rate (GFR) is about 120 ml/min in humans.
2. Proximal Convoluted Tubule (PCT)
  • Selective Reabsorption:
  • 100% of glucose, 70% of NaCl, and significant water reabsorbed.
  • Results in a decrease in urine volume while maintaining isosmotic conditions.
3. Loop of Henle
  • Generating Osmotic Gradient:
  • Descending Limb: Permeable to water, creating concentrated urine as water exits.
  • Ascending Limb: Actively pumps NaCl out, impermeable to water, creating a hypoosmotic urine.
4. Distal Convoluted Tubule (DCT)
  • Further Reabsorption:
  • Reabsorbs NaCl from urine to blood, leading to hypoosmotic urine.
5. Collecting Duct
  • Hormonal Control of Permeability:
  • Antidiuresis: Effect of ADH (Vasopressin) increases water reabsorption, producing concentrated urine.
  • Diuresis: Reduced water reabsorption leads to dilute urine.

Environmental Adaptations

  • Length of the Loop of Henle: Longer loops allow for greater concentration of urine.
  • Nitrogenous Waste Products: Varying methods of excretion adapted for specific environments.
  • Marine Mammals: Have adaptations for salt and water regulation, producing urine up to 5x more concentrated than blood.

Kidney Structure Details

  • Renal Corpuscle:
  • Made up of the glomerulus and Bowman’s capsule, where initial filtration occurs.
  • Proximal Convoluted Tubule:
  • Significant reabsorption occurs, where the epithelial cells are lined with microvilli for increased surface area.
  • Transport Mechanisms: Utilizes active and passive transport mechanisms (including cotransporters for Na+ and K+).

Hormonal Regulation of Osmoregulatory Functions

  • Antidiuretic Hormone (ADH):
  • Controls the permeability of the collecting ducts to water.
  • Stimulates the insertion of aquaporin channels into the tubules for water reabsorption.

Concentration of Urine in Various Animals

  • Comparison of U/P Ratios:
  • Terrestrial animals concentrate urine to minimize water loss.
  • Examples:
    • African buffalo: U/P ratio = 4 (osmotic pressure = 1120 mosmol/kg H₂O)
    • Dromedary camel: U/P ratio = ∞ (osmotic pressure = 3200 mosmol/kg H₂O)

Reniculate Kidneys

  • Multi-lobed Structure:
  • Found in certain mammals, increases surface area for filtration and excretion efficiency.

Kidney Stones (Renal Calculus)

  • Types: Calcium phosphate, calcium oxalate, uric acid, magnesium salts.
  • Causes include hypercalcemia, dehydration, frequent UTIs, etc.
  • Treatment: Includes medication, surgery, and lithotripsy (ultrasound to break down stones).

Summary of Key Points

  • Kidneys play a crucial role in homeostasis through complex filtering, reabsorbing, and excreting processes. Understanding nephron function, hormonal regulation, and animal adaptations is essential for grasping kidney physiology.