Chapter 16: Excretory Products and Their Elimination

Overview

• Animals accumulate waste products such as ammonia, urea, uric acid, CO2, water, and ions.
• Ammonia, urea, and uric acid are major nitrogenous wastes.
• Ammonotelism is the process of ammonia excretion; ureotelism and uricotelism are processes for urea and uric acid, respectively.

Types of Excretory Structures

• Invertebrates: Protonephridia, Nephridia, Malpighian tubules, Antennal glands.
• Vertebrates & Humans: Kidneys.

Human Excretory System

• Consists of kidneys, ureters, urinary bladder, and urethra.
• Kidneys: Bean-shaped, located between the last thoracic and third lumbar vertebra.
  • Two zones: Cortex and Medulla.
  • Functional units: Nephrons.
• Nephron Structure: Glomerulus and renal tubule.
  • Glomerulus: Capillary tuft for filtration.
  • Bowman's Capsule: Encloses the glomerulus.
  • Renal Tubule: Includes PCT, Henle’s Loop, DCT, and collecting duct.

Urine Formation

• Involves glomerular filtration, reabsorption, and secretion.
• Glomerular Filtration Rate (GFR): ~125 mL/min.
• Juxta Glomerular Apparatus (JGA): Regulates GFR.

Function of Tubules

• PCT: Reabsorption of nutrients and ions, pH regulation.
• Henle’s Loop: Maintains medullary osmolarity, counter-current mechanism.
• DCT: Adjusts pH and ionic balance, reabsorbs Na+ and water.
• Collecting Duct: Concentrates urine, absorbs urea for osmolarity.

Mechanism of Concentration

• Counter Current Mechanism: Involves Henle’s Loop and Vasa recta.
• Establishes medullary osmolarity gradient.

Regulation of Kidney Function

• Hormonal Regulation: ADH and Renin-Angiotensin mechanism.
• ADH: Facilitates water reabsorption; affects GFR and blood pressure.
• Renin-Angiotensin System: Stimulates aldosterone release, increases blood pressure.

Micturition

• Process of urine release.
• Involves CNS signaling and bladder muscle contraction.

Role of Other Organs

• Lungs: Remove CO2 and water.
• Liver: Secretes bile and other substances.
• Skin: Excretes NaCl, urea.

Disorders

• Uremia: Accumulation of urea in blood.
• Hemodialysis: Removes urea from blood.
• Kidney Transplantation: Corrects acute renal failure.
• Other Conditions: Renal calculi and glomerulonephritis.

Summary

• Excretory processes vary among animals.
• Human excretory system efficiently filters blood and maintains ionic and acid-base balance.

Exercises

• Define and explain GFR, counter current mechanism.
• Describe roles of liver, lungs, skin in excretion.
• Discuss osmoregulation and micturition.