3.3.2 gas exchange

• fish have a small surface area to volume ratio for gas exchange, apart from this they also have an impermeable membrane so gases cant diffuse through their skin
• bony fish have 4 pairs of gills, each gill supported by an arch. along each arch there are gill filaments, with lamellae on them which participate in gas exchange
• in bony fish, blood and water flow across the lamellae in a counter current direction. this ensures that a steep diffusion gradient is maintained so the maximum amount of oxygen is diffusing into the deoxygenated blood from water
• ventilation is required to maintain a continuous unidirectional flow
• insects do not possess a transport system therefore oxygen needs to be transported directly to tissues undergoing respiration
• transport of oxygen is achieved by the help of spiracles, small openings or tubes, either bigger trachea or smaller tracheoles which run into the body of an insect and supply it with the required gases
• gases move in and out of insects through diffusion, mass transport as a result of muscle contraction and volume changes in the tracheoles
• plants are adapted to efficient gas exchange through many adaptations in their leaves
• leaves have many small holes call stomata which allow gases to enter and exit the leaves
• the large number of stomata in the leaves means no cell is far from one, reducing the diffusion distance
• leaves also possess air spaces to allow gases to move around the leaf and easily come into contact with photosynthesising mesophyll cells
• the lungs are a pair of lobed structures with a large surface area located in the chest cavity that are able to inflate
• a lubricating substance is secreted to prevent friction between the rib cage and lungs during inflation and deflation
• external and internal intercostal muscles between the ribs contract to raise and lower the ribcage respectively
• the air enters the nose, along the trachea, bronchi and bronchioles which are structures well adapted to their role in enabling the passage of air into the lungs. the gaseous exchange takes place in the wall of alveoli
• the trachea, bronchi and bronchioles are held open by rings of cartilage
• alveoli are very thin (one cell thick) and are surrounded by capillaries which are also only one cell thick. this reduces the diffusion pathway for gases
• the constant blood supply by capillaries means that a steep concentration gradient is constantly maintained
• cartilage - involved in supporting the trachea and bronchi, plays an important role in preventing the lungs from collapsing
• ciliated epithelium - present in bronchi, bronchioles and trachea, involved in moving mucus along to prevent lung infection by moving it towards the throat where it can be swallowed
• goblet cells - cells present in the trachea, bronchi and bronchioles involved in mucus secretion to trap bacteria and dust to reduce the risk of infection with the help of lysozymes which digest bacteria
• smooth muscle - their ability to contract enables them to play a role in constricting the airway, thus controlling its diameter as a result and thus controlling the flow of air to and from the alveoli
• elastic fibres - stretch when we exhale and recoil when we inhale, thus controlling the flow of air
• the flow of air in and out of the alveoli is referred to as ventilation and is composed of inspiration and expiration
• inspiration:

  • the external intercostal muscles contract and internal intercostal muscles relax, causing the ribs to move upwards
  • the diaphragm contracts and flattens
  • this causes the volume in the thorax to increase, thus lowering the pressure
  • the difference between the pressure inside the lungs and atmospheric pressure creates a gradient, causing air to be forced into the lungs
• expiration:

  • the internal intercostal muscles contract whereas the external muscles relax, therefore lowering the rib cage
  • the diaphragm relaxes and raises upwards
  • this action decreases the volume inside the thorax, therefore increasing the pressure forcing the air out of the lungs
• a spirometer is a device used to measure lung volume. a person using a spirometer breathes in and out of an airtight chamber, causing it to move up and down, leaving a trace on a graph
• vital capacity - the maximum volume of air that an be inhaled or exhaled in a single breath. varies depending on gender, age, size and height
• tidal volume - the volume of air we breathe in and out at each breath at rest
• the volume of air which is always present in the lungs is known as residual volume