SA:V and Gas exchange

• The larger the organism, the smaller its surface area to volume ratio.
• Single-celled organisms have large surface area to volume ratios, so substances diffuse directly into or out of cells across the cell-surface membrane.
• Multicellular organisms have small surface area to volume ratios and long diffusion paths, so specialised exchange surfaces are needed.
• Small animals have large surface area to volume ratios so lose heat more easily. This is why a high metabolic rate is needed to generate enough heat to maintain a constant body temperature.
• Adaptations in gas exchange:

  • large surface area
  • short diffusion path
  • steep concentration gradient
• Gills of fish:

  • each gill is made of lots of gill filaments, covered in lamellae
  • lamallae consists of a single layer of flattened cells that cover a vast network of blood capillaries
  • counter-current system: blood flow and water flow are in opposite directions, so ensures a concentration gradient is maintained along the length of lamellae/filaments (blood is always passing water with a higher O2 concentration)
• Insect tracheal system:

  • rigid exoskeleton: waxy coating impermeable to gases
  • spiracle: allows air to enter/exit, closed to reduce water loss
  • tracheae leading to tracheoles: walls have reinforcements to prevent collapsing. tracheal is highly branched with thin walls.
  • during flight, lactate production in respiring muscle lowers the water potential so water at the end of tracheoles move in -> larger SA
  • abdominal muscles contract to facilitate air movement -> maintains concentration gradient
• Leaves of dicotyledonous plants:

  • air spaces in spongy mesophyll layer: large SA
  • guard cells control opening and closing of stomata: open when turgid (high water/sunlight availability), close when flaccid (low availability)
  • thin and flat, with lots of stomata in lower epidermis
• Xerophytic plants are plants that have adapted to dry environments.
• Xerophytic plants:

  • sunken stomata/hairs surrounding stomata: traps moist air to reduce water potential gradient between leaf and air -> reduce water loss
  • rolled up leaves: reduce exposure of surfaces to wind
  • reduced number and size of stomata/leaves
  • thick waxy cuticle: waterproof to reduce water loss
  • shallow, extensive roots: rapid absorption of light rainfall
• Humans:

  • trachea -> bronchi -> bronchiole -> alveoli
  • alveoli -> alveolar epithelium -> capillary endothelium -> haemoglobin
  • lots of alveoli, each made of one cell thick walls
  • constant blood flow in capillaries + ventilation: maintains concentration gradient
• Inhalation: active process

  Exhalation: passive process
• Inhalation:

  1. external intercostal muscles contract, diaphragm contracts and flattens, causing the ribcage to expand upwards and outwards
  2. volume of thoracic cavity increases, air pressure decreases
  3. air is drawn into lungs, down a pressure gradient
• Exhalation:

  1. external intercostal muscles relax, diaphragm relaxes, causing the ribcage to move inwards and downwards. elastic fibres between alveoli recoil.
  2. volume of thoracic cavity decreases, air pressure increases
  3. air is forced out of lungs, down a pressure gradient
• During forced expiration, internal intercostal muscles contract, pulling the ribcage further inwards and downwards.
• The movement of intercostal muscles during forced expiration is said to be antagonistic.