Gas Exchange

• Dissecting animal gaseous exchange systems

  - Requires pins, boards, sharp scissors, scalpels, tweezers and mounted needles

  - Dye specific features and look at them through a microscope
• Gill adaptations for efficient gas exchange

  - Large surface area

  - Rich blood supply

  - The tips of adjacent gill filaments overlap, increasing the resistance to the flow of water over the gill surfaces and slowing the movement of water to give more time for gaseous exchange to take place

  - Countercurrent flow maintains a steep concentration gradient by making it so that blood with less oxygen and water with more run alongside each other, leading to faster diffusion (As a result, bony fish with this system extract 80% of oxygen from water whereas cartilaginous fish with a parallel system extract only 50%)
• Ventilation (fish)

  - The mouth opens and the floor of the buccal cavity is lowered, increasing its volume and lowering the pressure, resulting in an inflow of water

  - The opercular valve is shut and the opercular cavity containing gills expands, lowering the pressure in the opercular cavity containing the gills

  - The floor of the buccal cavity starts to move up, increasing the pressure there so water moves from the buccal cavity over the gills

  - The mouth closes, the operculum opens and the sides of the opercular cavity move inwards, decreasing the volume and increasing the pressure in the opercular cavity to force water over the gills and out of the gills

  - The floor of the buccal cavity is steadily moved up, maintaining a flow of water over the gills
• Countercurrent


  In fish gills, an arrangement whereby water flows away from the head and blood flows toward the head to maintain a steep concentration gradient between the two
• Ram ventilation


  Method of forcing water over gills by swimming with mouth open used by some fish such as sharks and rays
• Buccal cavity


  Mouth of the fish
• Gill filaments

  Large stacks of of gill plates that need a flow of water to keep themselves separate, exposing a large surface area needed for gas exchange
• Gill lamellae


  Main site of gaseous exchange in fish with a rich blood supply

  and a large surface area at right angles to increase efficiency
• Operculum


  A protective flap that covers the gills of fishes
• Gills


  An organ in fish containing thin-walled blood vessels that allow for easy absorption of oxygen from the outside surface
• Methods of increasing gas exchange levels in insects

  - Mechanical ventilation of the tracheal system (air is actively pumped into the system by muscular movements of the thorax + abdomen, changing the volume of the body and the pressure in the trachea + tracheoles to draw in or force out air

  - Collapsible enlarged tracheae or air sacs to act as air reservoirs (used to increase the amount of air moved through the gas exchange)
• Tracheal fluid

  - Fluid found at the ends of the tracheoles in insects

  - It helps control the surface area available for gas exchange and water loss by limiting the penetration of air for penetration, although a build-up in lactic acid will result in water moving out of the tracheoles, exposing more surface area for gas exchange
• Tracheoles

  - Smaller subdivision of tracheae, diameter 0.6-0.8 micrometres, with each tracheole being a single, greatly elongated cell

  - They have no chitin lining and so are permeable to gases and are the main sites of gas exchange
• Trachea (insects)


  - The largest tubes in the insect respiratory system, up to 1mm in diameter, that carry air into and along the body

  - The tubes are lined by impermeable spirals of chitin, preventing gas exchange
• Spiracles


  - Small openings along the thorax and abdomen of an insect that open and close to control the amount of air moving in and out of the gas exchange system and the level of water loss from the exchange surfaces.

  - When the insect is active, they will be open to gather oxygen

  - When the insect is inactive, they will be closed to prevent water loss
• Gas exchange in insects

  - Air enters through spiracles along the thorax and abdomen, but water is also lost through them

  - Air moves into the tracheae

  - Air moves into the tracheoles

  - Air is diffused directly into the cells
• Exoskeleton


  A body covering, typically made of chitin, that provides support and protection in insects but prevents gaseous exchange
• Ventilation rate

  Tidal volume x breathing rate
• Breathing rate

  Number of breaths per minute
• Total lung capacity


  Vital capacity + residual volume
• Residual volume


  The volume of air left in your lungs after a maximum exhalation, which cannot be measured directly
• Expiratory reserve volume


  Maximum amount of air that can be exhaled after a normal tidal exhalation
• Inspiratory volume


  Maximum amount of air that can be forcefully inhaled after a normal tidal volume inhalation
• Vital capacity

  The total volume of air that can be exhaled after maximal inhalation.
• Tidal volume


  Amount of air that moves in and out of the lungs per resting breath (normally about 500cm3)
• Structure of spirometers

  (Nose clip for patient)

  Nozzle for patient to breath into -> air goes through a tube into an airtight chamber filled with oxygen -> this causes a change in water level in an outside chamber -> a trace is drawn on a revolving drum as the lid moves up and down -> the air leaves the chamber and goes through a canister of soda lime to remove produced CO2 -> the patient reinhales the air
• Methods of measuring lung capacity

  - Peak flow meter (a simple device that measures the rate at which air can be expelled from the lungs)

  - Vitalographs (A more sophisticated version of a peak flow meter in which a patient breaks out as quickly as they can through a mouthpiece and the instrument produces a graph of the amount of air they breath out and how quickly. The volume of air is called the forced expiratory volume in one second

  - Spirometer (A device commonly used to measure different aspects of lung volume and breathing patterns)
• Forced expiration

  - Contraction of internal intercostal muscles, pulling the ribs down hard and fast, and the abdominal muscles contract, forcing the diaphragm up to increase the pressure in the lungs rapidly

  - This process uses energy
• Expiration

  - Breathing out

  - The diaphragm relaxes (moves up into a domed shape) and the external intercostal muscles also do so (causing the ribs to move down and inwards)

  - The elastic fibres of the alveoli return to their normal length

  - As a result, the volume of the thorax decreases and the pressure of the thorax rises to above that of atmospheric air, causing air to rush out until the pressure gradient equalises
• Inspiration

  - Breathing in

  - The diaphragm contracts (flattening and lowering) as do the external intercostal muscles, moving the ribs upwards and outwards

  - The volume of the thorax increases and so the pressure is reduced, leaving the pressure now lower than that of atmospheric air. This causes air to rush into the lungs, equalising the pressure gradient.
• Thorax

  Pleural cavity lined by pleural membranes
• Surfactant


  Chemical produced in the lungs to maintain the surface tension of the alveoli and keep them from collapsing
• Main adaptations of alveoli

  - Large surface area (there are 300-500 million alveoli per adult lung and the total alveolar surface area for both lungs is around 50-75metres2.

  - Thin layers (The alveoli and surrounding capillaries have single cell-thick epithelial walls)

  - Good blood supply (Alveoli are supplied by a network of capillaries to maintain a steep concentration gradient of gases)

  - Good ventilation (Breathing moves air in + out of the alveoli, maintaining steep diffusion gradients for oxygen + CO2 between blood and air in the lungs
• Alveoli


  - Tiny air sacs which are the main gas exchange surfaces of the body

  - They have a diameter of 200-00 micrometres each

  - They consist of a single cell-thick layer of flattened epithelial cells, some collagen and elastic fibres

  - Elastic fibres make them capable of elastic recoil
• Bronchioles


  - The smallest branches of the bronchi, branching off in the lungs

  - They have a diameter of 1mm or less and have no cartilage rings, instead having smooth muscle in their walls that allow the bronchioles to constrict and dilate, regulating the amount of air flow

  - They are lined with a thin layer of flattened epithelium, allowing some gaseous exchange
• Bronchus

  - Two smaller trachea divisions, with the left bronchi leading to the left lung and the right bronchi leading to the right

  - They are similar in structure to the trachea with the same supporting rings of cartilage, goblet cells and ciliated epithelium
• Ciliated epithelium


  A layer of cells that have many hair-like extensions called cilia which wave and catch small particles such as dust as well as moving mucus away from the lungs
• Goblet cells


  Secrete mucus
• Trachea


  - The main airway carrying clean, warm, moist air from the nose down into the chest.

  - It is a wide tube supported by incomplete rings of strong and flexible cartilage. The rings are incomplete to allow the trachea to flex and allow the trachea to move when food travels down the oesophagus

  - The trachea is lined with ciliated epithelium and goblet cells
• Nasal cavity


  Hollow space in the nose with a large surface area, good blood supply to warm the air to body temperature, a hairy lining which secretes mucus to trap irritative dust and bacteria and most surfaces to increase air humidity and reduce evaporation from exchange surfaces