Lecture Notes: Homeostasis and the Excretory System

Introduction to Homeostasis

• Homeostasis refers to maintaining a stable internal environment.
• Essential for survival despite constant changes (food intake, temperature, activity levels).
• All organ systems contribute, with the excretory system playing a major role.

The Excretory System

• Components: Kidneys, ureters, bladder, and urethra.
• Function: Osmoregulation – maintaining water and dissolved substances balance.
• Aids in removing metabolic byproducts while preventing dehydration.

Animal Excretory Systems

• Vary based on evolutionary history, environment, and behavior.
• Metabolic Waste: Ammonia, a byproduct of protein metabolism, is toxic and needs conversion.
• Conversion:
  • Mammals/amphibians/sharks: Convert ammonia to urea (low toxicity).
  • Birds/insects/reptiles: Convert ammonia to uric acid (excreted as paste).

Human Excretory System Process

• Kidneys: Key organs for filtration and re-absorption.
  • Filter out 180 liters of fluid daily, reabsorb 99%.
  • Only 1.5 liters excreted as urine.

Kidney Structure and Function

• Nephrons: Basic filtering units; 1 million per kidney.
  • Blood enters via renal arteries.
  • Glomerulus: High-pressure capillary network filters blood.
  • Filtrate (water, urea, ions) enters Bowman's capsule.
• Tubules:
  • Proximal convoluted tubule: Reabsorbs organic solutes and some ions.
  • Loop of Henle: Key site for water and salt reabsorption, creating a concentration gradient.
    • Descending limb: Water reabsorption.
    • Ascending limb: Salt reabsorption.
  • Distal convoluted tubule: Further ion regulation via hormonal control.

Final Urine Formation

• Collecting Ducts: Concentration of urine via water reabsorption, regulated by hormones.
• Urine Path: From kidneys through ureters to the bladder, then expelled via urethra.

Variations in Animals

• Kangaroo Rats: Highly concentrated urine, due to long Loop of Henle.
• Beavers: Short Loop of Henle, less focus on water reabsorption.

Conclusion

• Understanding of excretory system processes reveals key biological functions.
• Differences in animal excretory strategies reflect adaptation to environments.

Additional Resources

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