Excretion

• Metabolism


  The chemical reactions that occur inside living cells
• Metabolic waste products produced by plants

  • Oxygen
  • Carbon dioxide
  • Water/water vapour
• Photosynthesis produces oxygen as a waste product in plants, not water vapour
• Light intensity


  Determines which gas accumulates as a waste product in plants
• At high light intensity more photosynthesis occurs so oxygen accumulates, while at low light intensity more respiration occurs so carbon dioxide accumulates
• During the day, with sufficient light for photosynthesis, oxygen is produced in excess and is the main waste product in plants
• Plants respire at night and during the day
• The majority of water vapour lost from leaves is not a metabolic waste product, but water drawn up from the roots by the transpiration stream
• Excretion


  The removal from the body of the waste products of metabolism, and any substances present in excess, e.g. excess salts
• Waste products produced by human metabolic processes

  • Carbon dioxide from respiration
  • Water from respiration
  • Urea from the breakdown of excess amino acids
• Urea is produced during the breakdown of excess amino acids, not lipids
• Main excretory organs in humans

  • Kidneys
  • Lungs
  • Skin
• Waste products of metabolism excreted by the lungs


  Water and carbon dioxide, both produced during respiration
• A build up of excretory products in the body fluids can cause water to move out of cells
• How waste products are excreted by the skin

  Waste products are excreted by the skin by the sweat glands when they produce sweat
• The kidneys excrete urea, excess water and excess salts, not carbon dioxide (which is excreted via the lungs)
• Main functions of the kidneys

  • Regulating the water content of the blood; this is osmoregulation
  • Excretion of toxic metabolic waste products and excess substances
• Osmoregulation


  The process of maintaining the balance of water and dissolved substances, e.g. salts, within the body fluids
• Osmoregulation is important because changes in the osmotic balance between the cells and the body fluids can lead to cells gaining or losing water by osmosis
• When the solute concentration of body fluids becomes too high, the water concentration is low, so water will move out of the body cells by osmosis down a water concentration gradient
• What happens if the body cells lose water by osmosis

  Losing water by osmosis causes the cells to shrink, or shrivel; this is known as crenation and can lead to inefficiency of cell activity and potentially cell death
• Processes that will cause an increase in the water content of the body fluids


  • Aerobic respiration
  • Consumption of water by eating/drinking
• Organ system responsible for the filtration of blood and the production of urine


  Urinary system
• The liver is not part of the urinary system, which consists of the kidneys, ureters, bladder and urethra
• Main structures of the urinary system

  • Kidneys
  • Ureters
  • Bladder
  • Urethra
• Role of the bladder in the urinary system

  To store urine that is produced by the kidneys. Urine exits the bladder via the urethra which removes it from the body.
• The urethra carries urine from the bladder to the exterior of the body, not the ureter
• Nephrons


  Tiny tubes found in the kidney, the role of which is to produce urine
• Nephrons contain a Bowman's capsule and loop of Henle, but do not contain a renal pelvis (which is the region of the kidney into which the nephrons drain)
• Sections that make up a kidney nephron


  • Bowman's capsule
  • Convoluted tubules (distal and proximal)
  • Loop of Henle
  • Collecting duct
• The Bowman's capsule directly surrounds the glomerulus, a ball of blood vessels from which the blood filters into the nephron
• Ultrafiltration


  The process by which molecules are forced out of the blood and into the Bowman's capsule at high pressure
• Where ultrafiltration occurs in the kidney


  Between the glomerulus and the Bowman's capsule
• Ultrafiltration occurs at high blood pressure, not normal blood pressure
• What determines whether a molecule enters the filtrate or remains in the blood during ultrafiltration


  Whether a molecule enters the filtrate or remains in the blood during ultrafiltration depends on its size; small molecules are filtered and large molecules remain in the blood
• Substances forced out of the capillaries during ultrafiltration

  • Water
  • Glucose
  • Amino acids
  • Urea
  • Salts
• Ultrafiltration


  1. Occurs between the glomerulus and the Bowman's capsule

  2. Occurs at high blood pressure

  3. Small molecules are filtered and large molecules remain in the blood
• Ultrafiltration does not occur at normal blood pressure
• The narrowing capillaries of the glomerulus increase the blood pressure, forcing small molecules into the filtrate
• Substances forced out of the capillaries during ultrafiltration

  • Glucose
  • Water
  • Urea
  • Salts