Paper 2- excretion

• 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
  • Carbon dioxide
• 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


  Sweat glands produce sweat
• The kidneys excrete urea, excess water and excess salts, not carbon dioxide
• 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
• When the solute concentration of body fluids becomes too high, water will move out of the body cells by osmosis
• What happens if the body cells lose water by osmosis


  Cells shrink or shrivel, known as crenation, which 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
• Main structures of the urinary system


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

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


  Tiny tubes found in the kidney, the role of which is to produce urine
• Nephrons do not contain a renal pelvis
• Main 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
• Ultrafiltration


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


  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

  The size of the molecule; small molecules are filtered and large molecules remain in the blood
• Substances forced out of the capillaries during ultrafiltration

  • Small molecules
• 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
• Glomerular filtrate


  The liquid that enters the Bowman's capsule during ultrafiltration. It contains water, glucose, salts and urea.
• Glucose enters the glomerular filtrate due to high blood pressure during ultrafiltration, not active transport
• Proteins do not enter the filtrate during ultrafiltration as they are too large and remain in the blood
• Reabsorption


  Substances are taken back up into the blood after having been filtered by the kidneys
• Parts of the nephron where water reabsorption takes place


  • Loop of Henle
  • Collecting duct
• The volume of water reabsorbed by the kidneys is determined by blood water potential
• Water reabsorption in the kidneys


  1. Salts, e.g. sodium ions, move out of the loop of Henle primarily by active transport, lowering the water concentration of the surrounding tissue

  2. Water moves out of the loop of Henle and the collecting duct by osmosis
• A high salt diet can result in an increase in sodium reabsorption in the kidneys, leading to an increase in blood volume
• Proximal convoluted tubule


  The first part of the kidney nephron after the Bowman's capsule