Gas exchange 4

• Describe the relationship between size and surface area:volume ratio

  Bigger size equals a smaller surface area to volume ratio
• What is meant by metabolic rate?

  Rate at which an organism uses energy over a given time period. Smaller animals have a larger SA:vol so lose heat faster so need a higher metabolic rate so they respire faster, releasing heat and so replaces the lost heat
• What 3 qualities must an efficient gas exchange surface have?

  Large surface area, thin surface, high concentration gradient
• Describe gas exchange in single celled organism?

  Simple diffusion- due to their small size they have a large surface area : volume ratio and a short diffusion pathway so fast rates of diffusion can be achieved

  Continuous respiration maintains conc gradients for O2 and CO2
• Where does gas exchange occur in fish?

  Gills
• How are the gills adapted for efficient gas exchange ?

  Large surface area- each gill consists of many filaments covered in many lamellae

  Short diffusion pathway- many capillaries with thin/single layer epithelium close to thin walled lamellae ensures short diffusion pathway between blood and water

  Concentration gradient- continuous flow of blood through capillaries ensures freshly oxygenated blood is quickly removed from the gills and replaced with deoxygenated blood. Countercurrent mechanism
• Explain the counter current mechanism

  • Water and blood flow in opposite directions
  • Blood always meets water of a higher oxygen concentration
  • So, concentration gradient maintained across the whole lamellae
  • so constant diffusion can occur
• A ventilation mechanism ensured water enters the fish’s mouth and flows over the gills via an operculum thus a constant flow of water over the gills
• How do insects prevent water loss?

  body protected by an exoskeleton made of chitin- waxy and waterproof so minimises water loss across body surface but prevents gas exchange across the surface

  Spiracles are holes in exoskeleton which open and close but not always open

  Spiracles guarded by valves

  hairs trap a layer of moisture around spiracle to prevent evaporation
• Describe gas exchange in insects

  Tracheal system network of air filled tubes called trachea that open to outside through spiracles

  Larger tracheal tubes subdivide into smaller tubes that eventually penetrate to cells

  Finer tubes called tracheoles and are sites of gas exchange, o2 diffuses directly into cells and co2 diffuses out
• What is abdominal pumping?

  Small/inactive insects have a shorter diffusion pathways so can rely on diffusion down a conc gradient

  But active insects use abdominal pumping as flight requires ATP for increased muscle contraction. Muscles of the abdomen contract to force air in and out of spiracles and trachea to maintain a greater air flow and a steeper conc gradient for fast diffusion. They can also remove the fluid from the ends of tracheoles to increase diffusion rates
• How are insects adapted to efficient gas exchange?

  Large surface area- large number of tracheoles branching

  Large conc gradients- cellular respiration, abdominal pumping

  Thin exchange surface- tracheoles have thin walls and are in close proximity to body cells
• Where does gas exchange occur in leaves?

  Spongy mesophyll layer of leaf with its large air spaces and thin walled cells. In close contact with large number of stomata which gases enter and leave leaf via diffusion down steep conc gradients
• Why will there be steep conc gradients for gases? (Leaves)

  CO2 low in leaf in day as it’s used in photosynthesis. O2 low in leaf during night as it’s used in respiration.
• The cells of the spongy mesophyll layer are loosely packed creating air spaces that provide a large surface area for gas exchange
• A short diffusion pathway Is ensured by spongy cells having thin cell walls and being direct contact with the air as well as thinness of leaf itself
• How does leaf minimise water loss?

  Waxy cuticle waterproof on top

  Stomata open and close (to reduce evaporation) by guard cells- located on underside of leaf
• Describe the gross structure of human exchange system

  Trachea branch into bronchi which branches into bronchioles and then Alveoli
• Air enters airway through nose or mouth. Mucus membranes line much of airway, these contain goblet cells which secrete mucus and are lined with a ciliated epithelium
• What is the role of the cilia and mucus ?

  Mucus traps microorganisms and debris keeping airways clear

  Cilia- beat regularly to move microorganisms and dust particles along with the mucus
• Describe the gas exchange surface in humans

  Takes place between the alveoli and the blood capillaries only

  O2 diffuses through alveolar epithelium and into endothelium of capillaries into blood
• How is the alveoli adapted to efficient gas exchange?

  Made of elastic tissue

  Many Alveoli in a cluster increase surface area, blood capillary network

  ventilation maintains conc gradient as air with high o2 breathed in and air with low o2 breathed out. also constant blood supply circulates

  thin walled alveoli and capillaries
• What does breathing involve?

  1. Inspiration- taking air into the thorax
  2. Expiration- moving air out thorax
• Explain in terms of pressure why air is forced out of the lungs

  Pressure needs to be reduced within the thorax. The pressure in the thorax is greater than pressure in the atmosphere so air moves down a pressure gradient
• Describe inspiration 


  • external intercostal muscles contract
  • ribcage moves up and down
  • diaphragm contracts and flattens
  • elastic tissue stretches
  • volume increases in thorax
  • pressure decreases
  • air enters down a pressure gradient

  nb- elastic fibres stretch to increase size of alveoli
• Describe expiration (at rest)

  • external intercostal muscles relax
  • ribcage moves down and in
  • diaphragm relaxes and returns to dome shape
  • elastic tissue recoils
  • volume of thorax decreases
  • pressure increases
  • air forced out down a pressure gradient nb: passive process
• What happens in forced expiration?


  Internal intercostal muscles contract
• Explain the term antagonistic muscles


  A pair of muscles which on contraction produce opposite effects to each other
• Tidal volume


  The volume of air breathed in or out of lungs in a normal resting breath
• Ventilation rate


  Number of breaths in or out per minute
• Pulmonary ventilation 


  Total volume of air that is moved into lungs in one minute
• formula for calculating pulmonary ventilation

  Tidal volume (dm3) x ventilation rate (min-1)
• Define term casually linked


  One variable causes the other to occur