Excretion

Water Balance and Waste Disposal

• The majority of cells in most animals (all but sponges and cnidarians) are not exposed to the external environment, but are bathed by an extracellular fluid.
• Animals with an open circulatory system have an extracellular compartment containing hemolymph which bathes the cells.
• Animals with a closed circulatory system have two extracellular compartments – interstitial fluid and blood plasma. 

Osmoregulation in Fishes

• Saltwater fish: gain salts and lose water across gills
  • Produce very little urine
  • Drink seawater to replace water lost 
  • Use energy to secrete salt through gill epithelial cells
• Freshwater fish: gain water and lose salt when ventilating gills
  • Kidneys produce copious dilute urine
  • Specialized gill epithelial cells transport Na+ and Cl- from water into fish’s capillaries

Regulate or Conform

• Osmoregulators: maintain constant internal salt concentrations and osmolarities
  • Drink or excrete as needed to maintain 300 mOsm/L
  • All terrestrial animals, freshwater animals, and many marine animals
  • Requires considerable expenditure of energy
• Osmoconformers: match osmolarity of blood and other fluids to seawater at 1000 mOsm/L
  • Most marine invertebrates and some vertebrates
  • Less tendency to gain or lose water
  • Expend less energy
  • Generally limited to marine environment
  • Total amount of salt and organic compounds produce an osmolarity similar to seawater, even though salt concentration is similar to osmoregulators
  • Body fluids less salty than seawater (like osmoregulators) so they tend to gain salt
  • Eliminate excess salt using rectal gland

Excretion

• Excretion: process by which metabolic wastes are eliminated
  • urine, sweat, CO2, nitrogen are primary wastes
• 3 excretory functions:
  • Nitrogen Excretion 
  • Osmotic Regulation
  • Water Balance

Major Fluid Compartments

• Plasma compartment: blood plasma

• Interstitial compartment: bathing tissues & returning to blood

• Intracellular fluid: inside cell

Survey of the Phyla

• Protonephridium: platyhelminthes & nematodes
  • Flame-bulb system
  • Cilia keeps fluid moving through the tubules 
  • Interior fluids are filtered as they pass through the perforations in the flame bulb's cells. As the filtrate passes along the tubule, it is altered. Some materials, such as sugars, are selectively reabsorbed, while other materials, such as nitrogenous wastes, are actively secreted.
  • Wastes either diffuse out of body or are excreted into the gastrovascular cavity
• Metanephridium: annelids
  • Coelom is fluid-filled 
  • Each tubule possesses a nephrostome, collecting tubule, and a nephridiopore
  • Nephrostome drains the metamere just anterior to the one in which the metanephridium is located
  • Cilia keeps fluid moving through the tubules
  • Collect coelomic fluid containing nitrogenous wastes
  • Na+, Cl- and other solutes reabsorbed                                     along tubule
  • Nitrogenous wastes excreted through nephridiopores

Malpighian Tubules

• Uric acid is synthesized by the insect's tissues and released in soluble form 

  into the hemocoel. 

• The uric acid is absorbed by the Malpighian tubules and the low pH in the tubule lumen causes uric acid to precipitate out as dry matter. 

• Then waste is excreted with other solid waste  through the anus of the grasshopper 

• Tubules secrete nitrogenous wastes and salts from the hemolymph; water follows the solutes by

  osmosis.

• Most of the water and salts are reabsorbed across the epithelium in the rectum

• Dry product called frass is eliminated.

Urine Formation

• Filtration: filtrate is forced out of glomerulus & received by Bowman’s capsule.
  • Approximately 180 liters per day or 4.5 x the amount of fluid in the body is forced out into glomerulus.
  • Filters 125 mL per minute 
• Reabsorption: occurs simultaneously with secretion.
  • Mostly salts, H2O, solutes, vitamins are transported back to peritubular capillaries via active transport.
  • 124 mL of the 125 mL filtered out during filtration will be reabsorbed here.
• Secretion: filtrate is passed through the renal tubule
  • Walls of the tubule are a single cellular layer of cubodial epithelium specialized for active transport.
  • Molecules remaining in the plasma are selectively removed (penicillin) from the peritubular capillaries & secreted into the filtrate.
  • Na+ Pump: sodium ions are actively pumped across the membrane and Cl- follow passively by electrostatic attraction. 
• Excretion: remaining fluids leave the nephron and pass into the renal pelvis (funnel of the ureter) and travel to bladder until released through the urethra.