Exchange Surfaces

• Large organisms:

  • made of billions of cells + therefore they usually have high metabolic activity
  • distances are large
  • SA:V ratio is small
  • specialised exchange surfaces with large SA
• Characteristics of exchange surfaces:

  • large SA
  • thin layers
  • good blood supply to maintain conc. gradient
  • ventilation maintains a high conc. gradient
• • animals that live on land have to balance water loss with gas exchange
  • gas exchange surfaces are moist - the oxygen dissolves in the water before diffusing into the cells
  • mammals have developed complex systems that all gas exchange but minimise water loss
• Nasal Cavity:

  • large surface area with good blood supply, which warms the air to body temp.
  • a hairy lining which secretes mucus to trap dust and bacteria, protecting delicate lung tissue from irritation and infection
  • moist surfaces which increases the humidity of the incoming air, reducing evaporating from the exchange surfaces
• Trachea:

  • has C rings of cartilage which support the airway and stop it from collapsing; they also allow food to pass down the oesophagus
  • lined with ciliated epithelial cells and goblet cells
  • goblet cells secrete mucus onto the lining of the trachea, which traps microorganisms and wafts them back up to the throat where they are swallowed
  • smoking can cause paralysis of the cilia
• Bronchus:

  • branch into left and right lung; similar in structure to trachea but smaller
• bronchioles:

  • bronchi divide many times to make bronchioles
  • contain smooth muscle allowing them to constrict
  • smaller bronchioles do not contain cartilage but do contain smooth muscle; they can close up
  • the smallest bronchioles are lined with a thin layer of flattened cuboidal epithelial cells allowing for some gas exchange to occur here
• Alveoli:

  • unique to mammalian lungs
  • diameter: 200-300 micrometers
  • consist of flattened epithelial cells, collagen and elastin
  • the elastin allows the alveoli to stretch when air enters the lung. it then helps to squeeze air out when it returns to its original shape - elastic recoil
• Adaptations of the Alveoli:

  • large surface area: there are 300-500 million alveoli per adult lung so the alveolar gas exchange surface in both lungs equates to 50-75 m2
  • thin layers: the walls are only one epithelial cell thick so the diffusion distances are very short
  • good blood supply: the alveoli are supported by a network of around 280 million capillaries, maintaining a steep concentration gradient
  • good ventilation: moving air into and out of the alveoli maintains a steep diffusion gradient
• Layers of moisture in the lungs: the thin layer of moisture in the lungs consists of water, salts and lung surfactant. The surfactant prevents the lungs from collapsing

  • pulmonary surfactant is a surface-active lipoprotein complex formed by type ll alveolar cells. The proteins and lipids have both hydrophobic and hydrophilic regions.
  • surfactant reduces surface tension within the alveoli and prevent them from collapsing
• Ventilation:

  • air is moved in and out of the lungs due to pressure changes in the thorax
  • the ribcage provides a semi-rigid case in which the pressure can be changed above and below atmospheric pressure
  • diaphragm is a dome shaped sheet of muscle
  • internal + external intercostal muscles are found between the ribs
  • the ribs and lungs are lined with pleural membranes filled with pleural fluid to reduce friction
• Inspiration:

  • diaphragm contracts and flattens
  • external intercostal muscles contract, pulling the ribs up and out
  • the volume in the thorax increases and the pressure decreases
  • air is drawn in from the outside of the lungs to equalise the pressure
• Expiration:

  • diaphragm relaxes and moves up
  • external intercostal muscles relax and the ribs move down and in
  • elastic fibres in the alveoli return to their normal length
  • volume decreases, pressure increases and air is forced out of the thorax
• Measuring lung capacity:

  • peak flow meter: measures the rate at which air can be expelled from the lungs
  • vitalograph: produces a graph of the volume of air breathed out and how quickly
  • Spirometer: measures aspects of lung volume, or can be used to investigate breathing patterns
• Tidal volume:

  • the volume of air that moves in and out of the lungs with each resting breath
  • about 500 cm3
  • it uses about 15% of the lungs vital capacity
• Vital capacity:

  • the volume of air breathed in when the strongest possible exhalation occurs followed by the deepest intake of breath
• Inspiratory reserve volume:

  • the maximum volume of air you can breathe in over and above normal inhalation
• Expiratory reserve volume:

  • the extra volume of air you can force out of your lungs over and above the normal tidal volume you breathe out
• Residual volume:

  • the volume of air that is left in your lungs when you have exhaled as much as possible
  • this cannot be measured directly
• Total lung capacity:

  • sum of the vital capacity and the residual volume
• Breathing rate:

  • the no. of breaths in a minute
• Ventilation rate:

  • total volume of air inhaled in one minute
  • ventilation rate = tidal volume x breathing rate
• How exercise affects breathing:

  • tidal volume can increase from 15% to 50% of the vital capacity
  • breathing rate will increase
  • this results in an increase in the uptake of oxygen
• Gas exchange in fish:

  • Water:
  • 1000 times denser
  • 100 times more viscous
  • less oxygen content: the warmer and saltier the water
• Gills:

  • large SA
  • good blood supply
  • thin layers
  • counter-current system (large conc. gradient)
  • located in the gill cavity, covered by a bony flap called the operculum
• Ventilation in fish:

  • moving water in one direction requires less energy and is simpler than moving it in and out of a lung
  • when fish swim, they open their mouths and operculum and water flows over their gills
  • some sharks and fish rely on movement to ventilate their gills - ram ventilation
• buccal pumping:

  • the mouth is opened, opercular valve is shut and the floor of the buccal cavity is lowered - vol. increases, pressure decreases
  • the floor of the buccal cavity starts to move up, pushing water into the opercular cavity/gills where pressure is lower + vol. is higher
  • the mouth closes, the operculum opens and the sides of the opercular cavity move inwards - vol. decreases, pressure increases
  • water is forced over the gills and out of the operculum
  • the floor of the buccal cavity continues to move up to maintain a flow of water
• Countercurrent system:

  • water flows in the opposing direction to blood across the gills
  • an oxygen gradient between the water and the blood is maintained along the gill
  • oxygen continues to diffuse down the concentration gradient so a much higher level of oxygen saturation of the blood is achieved
• Gill adaptations:

  • lamellae: rich blood supply, large surface area
  • filaments: occur in stacks, flow of water separates them
  • efferent blood vessels: carry blood leaving the gills in the opposite direction to the incoming water
• insects:

  • exoskeleton through which little gas exchange can occur
  • openings in thorax and abdomen which lead to muscle
• Spiracles:

  • openings in the abdomen and thorax
  • when air moves in and out, water is lost
  • can be closed when inactive/low oxygen demand
  • no gases move in and out
  • oxygen diffuses into cells through tracheoles and CO2 moves into the body fluids + is held there (buffering)
  • when CO2 levels in body build up, spiracles open
  • pumping movements of thorax and abdomen promote gas exchange
  • can 'flutter' open and closed quickly
• Tracheae

  • lined with chitin which keeps them open and is impermeable to gas so little gas exchange occurs here
  • branch into tracheoles
  • run along insect body
• Tracheoles:

  • 0.6-0.8 micrometers
  • single elongated cell, no chitin
  • run between individual cells of insects
  • moist walls into which oxygen can diffuse -> cells
  • contain tracheal fluid:
  • limits penetration of air for diffusion
  • lactic acid build-up draws tracheal fluid out of tracheole exposing a larger SA for gas exchange
• mechanical ventilation in insects:

  • movement of thorax and abdomen change the vol and pressure in the tracheae/tracheoles, forcing air in and out
• air sacs in insects:

  • act as reservoirs when spiracles are closed
  • can be inflated and deflated by movement of thorax and abdomen to promote air movement
• open circ. system in insects:

  • blood bathes internal organs directly
  • no distinction between blood and tissue fluid
  • one/more hearts pumps the fluid into a system of sinuses (spaces around organs)