Lecture Notes: Homeostasis and the Excretory System

Introduction to Homeostasis

• Definition: Regulation of a stable internal environment.
• Importance: Allows the body to manage different inputs (food, temperature changes, etc.) without major disruptions.
• Involvement: All organ systems contribute to homeostasis.

Role of the Excretory System

• Components: Kidneys, ureters, bladder, and urethra.
• Function: Maintains water and dissolved substance levels — known as osmoregulation.
• Process: Removes unnecessary byproducts of metabolism while preventing dehydration.

Osmoregulation

• Byproducts: Ammonia from protein metabolism needs conversion.
• Conversion:
  • Urea: Used by mammals, amphibians, some marine animals.
    • Made by combining ammonia and carbon dioxide.
    • Low toxicity; requires water for excretion.
  • Uric Acid: Used by birds, insects, desert lizards.
    • Requires less water, excreted as paste.

Human Excretory System

• Kidneys: Central to water and solute regulation and blood pressure control.
  • Filtration: Filters around 180 liters of fluid daily, with 1.5 liters excreted as urine.
  • Structure: Contains tiny filtering units called nephrons.

Nephrons

• Quantity: About a million per kidney.
• Function: Filtration, reabsorption, and adjustment of the filtrate.

Filtration Process

• Blood Flow: Enters the kidneys via renal arteries.
• Filtrate Formation: Starts in the glomerulus, proceeds to the Bowman's capsule.
• Components of Filtrate: Water, urea, ions, small molecules.

Tubules and Reabsorption

• Proximal Convoluted Tubule: Reabsorbs organic solutes, sodium, potassium, and water.

Loop of Henle

• Structure: Hairpin-shaped, influencing medulla and cortex regions.
  • Descending Limb: Permeable to water.
  • Ascending Limb: Transports ions, impermeable to water.
• Function: Extracts water, salts, and creates a salty medulla for further water reabsorption.

Distal Convoluted Tubule

• Focus: Regulates potassium, sodium, and calcium levels.
• Hormones: Influence reabsorption processes.

Collecting Ducts

• Function: Final filtration adjustments, influenced by hormones.
• Role of Hormones: Control membrane porosity to adjust urine concentration.

Urine Formation and Excretion

• Flow: Urine travels from kidneys to bladder via ureters.
• Storage: Collects in the bladder until excretion through the urethra.

Adaptations in Animals

• Kangaroo Rats: Highly concentrated urine.
• Beavers: Less need for water reabsorption.

Summary

• Homeostasis relies largely on the excretory system.
• The system's efficiency ensures survival and adaptation across different environments.

Additional Resources

• Crash Course Biology: Available on YouTube for further learning.