Excretion 1

• Excretion


  Maintenance of internal environment is vital to survival (e.g. cell integrity)
• Excretion


  Need correct composition of water, solutes (e.g. NaCl) and low levels of waste
• Excretion


  Delicate balance is affected by dietary intake, metabolic products and loss/gain of water and solutes
• Excretion


  Selective excretion is crucial to internal fluid homeostasis and involves several systems
• Primary functions of excretory systems


  • Maintenance of inorganic solutes (e.g. Na+, K+, Cl-)
  • Maintenance of proper plasma water volume
  • Removal of metabolic waste products (e.g. urea)
  • Maintenance of water balance
• Excretion


  Involves both excretion and selective retention
• Evolution of excretory systems

  1. Simple aquatic animals rely on diffusion and membrane transporters

  2. As aquatic animals got larger, required specialized excretory tissues with transport epithelia (e.g. skin, gills)

  3. As animals continued to get bigger and move onto land, developed specialized tubules lined with transport epithelia

  4. In animal kingdom, 4 organ systems evolved to be involved in excretion and retention
• Selective excretion systems


  • Respiratory (e.g. gills, lungs)
  • Digestive (e.g. liver, intestinal epithelia)
  • Skin and glands (e.g. salt glands)
  • Renal (e.g. Malpighian tubules, kidneys)
• Nitrogen metabolism


  Metabolism of food produces CO2; protein and nucleic acid metabolism also produce nitrogenous waste (e.g. Proteins to Amino acids to Ammonia)
• Ammonia (NH3)


  Highly toxic, and can bind to H+ to form NH4+, which is also toxic, particularly to neurons
• NH3 and NH4+ must either be quickly excreted or converted to less toxic form
• Nitrogenous waste products


  • Ammonia
  • Urea
  • Uric Acid
• Ammonia


  Cheap, highly toxic
• Urea


  Moderately costly, toxic
• Uric Acid


  Costly, non-toxic
• Organisms that produce different nitrogenous waste products


  • Aquatic organisms
  • Teleost fish
  • Elasmobranchs, Amphibians
  • Mammals
  • Insects
  • Reptiles
  • Birds
• Epithelium


  A sheet of cells that covers a body surface or organ, or lines a cavity, and forms a boundary between functionally different regions of the body, or between the animal and the external environment
• Epithelium


  • Numerous functional capacities
  • Play major functional roles in animal physiology, particularly excretion
• Types of epithelial tissue


  • Cuboidal epithelium
  • Simple columnar epithelium
  • Simple squamous epithelium
  • Pseudostratified columnar epithelium
  • Stratified squamous epithelium
• Epithelial tissue


  • Sheets
  • Closely packed
  • Barriers
  • Active interfaces
• Epithelial cell structures


  • Sheets or layers (e.g. skin)
  • Tubules (e.g. kidney nephron)
  • Follicles (hollow globes) (e.g. sweat gland)
• Transcellular transport


  Transport through cells
• Paracellular transport


  Transport between cells
• Excretory organ epithelial transport


  1. Typically use Na+/K+ ATPases to set up Na+ or H+ gradients

  2. Can then use these gradients to move others substances (i.e. secondary transport)
• Epithelial salt transport


  1. Step 1: Na/K ATPase pump uses ATP to actively move Na+ to outside basolateral side of cell, causing decrease intracellular Na+

  2. Step 2: Build up of positive Na+ in basolateral fluid draws Cl- ions by charge attraction through either/or: (i) Chloride activated chloride (ClC) channels, (ii) Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) channels
• Epithelial salt and water transport

  1. Step 3: Osmotic pressure builds up in lateral spaces and draws water out of cells, which in turn draws water across apical membrane via AQP1 water channels (i.e. transcellular transport)

  2. Step 4: Sometimes, water and ions can also be drawn through the "tight" junctions between cell as well (i.e. paracellular transport)
• Most animals rely on transport epithelia to move solutes in specific directions - they form a variety of organs, including vertebrate kidneys, that are directly involved with osmoregulation and metabolic waste disposal
• Renal tubules


  Produce urine and heavily rely on transport epithelia
• Processes involved in renal tubules


  • Filtration
  • Secretion
  • Reabsorption
  • Osmoconcentration
• Major renal organs


  • Protonephridia
  • Meso- and Metanephridia
  • Malpighian tubules
• Protonephridia


  • In simple animals (e.g. flatworms, larval stages of fish)
  • Ultrafiltration driven by cilia in ducts ("flame" cells)
  • Ducts project into body cavity
  • Process involves both secretion and absorption
• Mesonephridia and Metanephridia


  • In animals with 2 or more major fluid spaces (e.g. coelom and circulatory system)
  • Basically consist of: (i) filtering capsule/funnel-like opening, (ii) tubule segment for selective secretion + absorption, (iii) canal or ureter leading to bladder
• Vertebrate renal tubules


  Called nephrons
• Invertebrate renal tubules


  Metanephric-type tubules also present in molluscs, annelids and some arthropoda
• Crustacean Antennal Gland


  Invertebrates: Sometimes a single tubule forms a renal organ
• Malpighian tubules


  • Blind-end epithelial ducts, one cell layer thick, and not organized in distinct organ
  • Discovered by Marcelo Malpighi in 17th century Italy
• Malpighian tubule function


  1. Tubules and hindgut modify the lumen fluid by specific secretion and reabsorption

  2. Malpighian tubules in insects initiate excretion by ion secretion, which causes osmosis
• Insect excretion regulation


  Regulated by diuretic and antidiuretic hormones
• Diuretic Hormone


  • e.g. mosquito, must excrete large fluid load
  • Include serotonin and mosquito natriuretic peptide (MNP)
  • Enhance secretion
• Anti-Diuretic Hormone


  • e.g. mealworm beetle, consume dry food and need to retain fluid
  • Tenmo-Adfa reduces secretion and hence conserves water