Excretion in Animals

• Excretion is the process of removing waste products such as nitrogenous waste produced by metabolic and biological activities from the body
• Excretion is significant in animal physiology as it removes harmful and unwanted byproducts, maintaining the balance of metabolic and biological activities within the body
• Excretion differs from secretion and elimination:

  • Secretion refers to the production and release of useful substances by glands or cells
  • Elimination refers to the removal of undigested food and waste products out of the body in the form of feces
• Main excretory organs in different animal groups include kidneys, skin, gills, and Malpighian tubules
• Kidneys are responsible for filtration of metabolic wastes and elimination of waste products in the form of urine
• Skin in fishes and amphibians has specialized excretory cells called ionocytes for eliminating waste products like ammonia
• Gills in aquatic animals help eliminate waste products such as ammonia and carbon dioxide
• Malpighian tubules in insects are responsible for removing nitrogenous waste like uric acid
• Waste products produced by animals are ammonia, urea, and uric acid (nitrogenous wastes)
• Ammonia is produced by the metabolism of amino acids and converted to urea in mammals and uric acid in birds, reptiles, and terrestrial arthropods
• Factors influencing the production of waste products include physiological, dietary, environmental, and genetic factors
• Advantages and disadvantages of waste products:

  • Ammonia, urea, and uric acid can be used as soil fertilizer and contribute to plant growth
  • High concentrations of ammonia can lead to acid rain and respiratory problems, affecting soil acidity and causing eutrophication
• Excretory mechanisms involve filtration, reabsorption, and secretion to eliminate metabolic wastes from the body
• The urinary system works with other systems to carry out filtration, reabsorption, and secretion in the excretory system
• Osmoregulation helps maintain internal balance, and excretory systems contribute to it by removing waste products and excess water from the body
• Comparative excretion across animal kingdom:

  • Protozoans have vacuole-like organelles for osmoregulation
  • Annelids have nephridia in pairs on each segment for waste removal
• Segmented worms:

  • Each nephridium is a tiny tubule with one end opening into the bodily cavity and the other opening to the outside world
  • Fluid in the bodily cavity is filtered by the nephridia
  • Cilia near the entrance of the nephridium suck water into a tubule from the coelom
  • Capillaries reabsorb nutrients and other solutes as the filtrate passes through the tubules
  • Filtered fluid, free of nitrogenous waste and impurities, is stored in the bladder and then released via a side orifice
• Molluscs:

  • Possess a renal gland, a large tube that opens at one end to the mantle cavity and at the other to a sac surrounding the heart
  • Mollusks excrete ammonia as the main type of nitrogenous waste
  • Octopuses expel ammonium chloride, while terrestrial snails and slugs release uric acid and ammonia in wet environments
• Arthropods:

  • Coxal glands open at the basal part of each limb
  • Crustaceans have one set of coxal glands
  • Insects have Malpighian tubules that terminate in the body cavity and enter the alimentary canal
  • Uric acid is excreted by birds and reptiles, urea by mammals and amphibians
• Chordates:

  • Mammals excrete nitrogenous waste mostly in the form of urea through the kidney and ducts
  • Uric acid and creatinine are additional nitrogenous substances found in urine
  • Kidneys are compact organs in birds, reptiles, and amphibians, while in fish they are thin strips of tissue
  • Urea is the primary excretory product in mammals and amphibians, uric acid in birds and reptiles
• Regulation of Water and Electrolyte Balance:

  • Excretion is linked to the regulation of water and electrolyte balance in animals
  • Kidneys help maintain the balance of water and electrolytes in the body
  • Hormonal control and feedback mechanisms are involved in maintaining homeostasis
• Environmental Adaptations:

  • Terrestrial animals like mammals and reptiles have evolved kidneys with complex tubular structures to produce concentrated urine and save water
  • Aquatic animals like fish and amphibians expel diluted urine to prevent excessive water loss
  • Desert animals like camels have adaptations to reduce water loss during excretion and save water
• Challenges and solutions associated with excretion in extreme conditions:

  • Animals in arid environments conserve water by producing concentrated urine and reabsorbing water
  • Aquatic animals have specialized mechanisms like chloride cells to maintain electrolyte balance
  • Desert-dwelling species prevent dehydration through adaptations such as minimizing respiratory water loss and conserving water