Excretion

• Excretion


  Process that rids the body of nitrogenous metabolites
• Importance of removing metabolic wastes

  • Metabolic wastes must be dissolved in water to be excreted from the body
  • The type and quantity may have a large impact on water balance
• Liver

  • After the skin, the largest organ in the body
  • Average weight of 1.3 kg
  • The only human organ that is capable of natural regeneration
  • Performs many metabolic and homeostatic functions
• Formation of urea in the liver

  1. Breakdown of nitrogenous compounds releases ammonia

  2. Ammonia is rapidly removed by the liver and converted to urea
• Roles of the liver in detoxification

  • Neutralizing xenobiotics - modifying chemical constitution (biotransformation)
  • Inactivation and excretion of hormones such as steroids
• Kidney


  • Central to homeostasis - dispose metabolic waste and control fluid composition
  • Diverse excretory systems are variations on a tubular theme
  • Animals produce fluid waste called urine
• Production of urine


  1. Filtration

  2. Reabsorption

  3. Secretion

  4. Excretion
• Nephron


  • Functional unit of the vertebrate kidney
  • Made up of Glomerulus, Bowman's capsule, Proximal tube, Loop of Henle, Distal tube, Collecting duct
• Adaptations of the glomerulus

  • Larger diameter of the afferent arteriole than the efferent arteriole causes an increase in blood pressure
  • Gaps between epithelial cells allow molecules to leave the blood
  • Filtrate produced contains water, salts, glucose, amino acids, vitamins, nitrogenous waste and other smaller molecules
  • Basement membrane acts as a filter only allowing small molecules through and retains protein and blood cells
• Reabsorption and secretion in the proximal tube

  1. Reabsorption of Na+, Cl-, HCO3-, K+, H2O, glucose, amino acids

  2. Secretion of H+, ammonia, organic acids and bases
• Reabsorption and secretion in the loop of Henle

  1. Reabsorption of H2O, Na+, Cl-, K+, Ca++, HCO3-, Mg++

  2. Secretion of H+
• Reabsorption and secretion in the distal tube and collecting duct


  1. Reabsorption of Na+, Cl-, K+, Ca++, HCO3-, Mg++

  2. Secretion of K+, H+
• Solute gradients

  • Primary solutes affecting osmolarity: NaCl, Urea
  • Deposited by the loop of Henle and collecting duct
• Countercurrent multiplier system


  1. Movement of filtrate in opposite direction through ascending and descending limb of loop of Henle

  2. Transport of NaCl from ascending limb results in passive movement of water out of filtrate in descending limb
• Variations in nephron structure and function

  Equip kidneys for osmoregulation in their various habitats
• Antidiuretic hormone (ADH)


  • Also called vasopressin
  • Produced by hypothalamus and stored in pituitary gland
  • Osmoreceptor cells monitor blood osmolarity and regulate ADH release
  • ADH influences water uptake by increasing aquaporins in collecting duct cells
• ADH release


  Reduces urine volume
• The kidney has one of the highest metabolic rates of any organ
• Osmoregulation


  Process by which animals control solute concentration and balance water gain and loss
• Osmoregulation


  • Maintaining fluid environment of cells, tissues and organs
  • Keeping relative concentrations of water and solutes within fairly narrow limits
  • Ions must be maintained at concentrations that permit normal activity of muscles, neurons and other body cells
  • Based largely on controlled movement of solutes between internal fluids and external environment
• Osmosis & Osmolarity


  1. Selectively permeable membrane

  2. Water enters and leaves cells by osmosis

  3. Osmolarity-unit of measurement of solute concentration (moles solute/L)
• Osmoconformer


  To be isoosmotic with its surroundings
• Osmoregulator


  To regulate internal osmolarity independent of external environment
• Osmoregulation in a marine fish


  1. Gain of water and salt ions from food

  2. Excretion of salt ions from gills

  3. Osmotic water loss through gills and other parts of body surface
• Osmoregulation in a freshwater fish


  1. Gain of water and some ions in food

  2. Uptake of salt ions by gills

  3. Osmotic water gain through gills and other parts of body surface

  4. Gain of water and salt ions from drinking seawater

  5. Excretion of salt ions and large amounts of water in dilute urine from kidneys
• Freshwater animals face the problem of gaining water by osmosis and losing salts by diffusion
• Marine fishes lose water by osmosis and balance water loss by drinking large amounts of seawater
• Energetics of Osmoregulation


  • Maintaining osmolarity requires energy
  • Energy expended to maintain osmotic gradients that cause water to move in or out
  • The energy cost depends on the surroundings, movement of water on the surface, and pumping mechanism
  • The energy cost is minimized by having body fluids that are adapted to the salinity of the animal's habitat
• Ammonia


  • Animals that excrete nitrogenous wastes as ammonia need access to lots of water
  • Ammonia is released across the whole body surface
• Urea


  • Ammonia excretion is less suitable for land animals
  • Produced in the liver, product of the ornithine cycle
  • Urea has very low toxicity and can be transported in the circulatory system
  • Energetically expensive
• Uric acid


  • Birds, insects and many reptiles
  • Relatively non-toxic and not readily dissolvable in water
  • More energetically expensive to produce than urea
• Kidneys filter blood to remove waste products from the body