gas exchange

• Reasons for specialised exchange surfaces

  • As the size of the organism and its surface area to volume ratio increases
  • Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen
• Efficient exchange surface

  • Large surface area (e.g., root hair cells, folded membranes)
  • Short distance for substances to cross
  • Good blood supply/ventilation to maintain a steep gradient (e.g., alveoli)
• Features of an efficient exchange surface

  • Large surface area
  • Short distance for substances to cross
  • Good blood supply/ventilation
• The lungs are a pair of structures with a large surface area located in the chest cavity with the ability to inflate
• The lungs are surrounded by the rib cage which serves to protect them
• A lubricating substance is secreted to prevent the friction between rib cage and lungs during inflation and deflation
• External and internal intercostal muscles contract to raise and lower the ribcage respectively
• The diaphragm separates the lungs from the abdomen area
• Air pathway through the respiratory system

  Nose -> Trachea -> Bronchi -> Bronchioles -> Alveoli
• Gaseous exchange takes place in the walls of the alveoli, tiny sacs filled with air
• The trachea, bronchi, and bronchioles enable the flow of air into and out of the lungs
• The airways are held open with the help of rings of cartilage
• The trachea has incomplete C-shaped rings to allow passage of food down the esophagus behind the trachea
• Trachea and bronchi are similar in structure, with bronchi being narrower
• The wall of bronchioles is made of smooth muscle and elastic fibers
• The smallest bronchioles have alveoli clusters at the ends
• Structures and functions of mammalian gas exchange system

  • Cartilage: involved in supporting the trachea and bronchi, prevents lung collapse during pressure drop in exhalation
  • Ciliated epithelium: present in bronchi, bronchioles, and trachea, moves mucus to prevent lung infection
  • Goblet cells: present in trachea, bronchi, and bronchioles, secrete mucus to trap bacteria and dust, reducing infection risk
  • Smooth muscle: contracts to constrict airways, controlling diameter and airflow
  • Elastic fibers: stretch during exhalation and recoil during inhalation, controlling airflow
• Ventilation

  1. The flow of air in and out of the alveoli, composed of inspiration and expiration stages

  2. Inspiration: external intercostal muscles contract, internal ones relax, ribs raise, diaphragm contracts, volume increases, air enters lungs

  3. Expiration: internal intercostal muscles contract, external ones relax, rib cage lowers, diaphragm relaxes, volume decreases, air is forced out of lungs
• Spirometer is a device used to measure lung volume. A person breathes in and out of the chamber, causing it to move and leave a trace on a graph for interpretation
• Vital capacity - the maximum volume of air that can be inhaled or exhaled in a single breath. Varies depending on gender, age, size as well as height
• Tidal volume - the volume of air we breathe in and out at each breath at rest
• Breathing rate - the number of breaths per minute, can be learned from the spirometer by counting the number of peaks or troughs in a trace
• The volume of air which is always present in the lungs is called the residual volume. The tidal volume can be exceeded, in cases such as during exercise where the inking volume is reached in an attempt to move more air through. Similarly, the expiratory reserve volume is the additional volume of air that can be exhaled on top of the tidal volume
• Fish have a small surface area to volume ratio for gas exchange, as they have impermeable membranes so gases can diffuse through their skin. Therefore, fish have specialised exchange surfaces. Each gill has several pairs of gill arches, each gill arch separated by a septum. Along each arch, there are multiple projections called lamellae, with blood vessels on them which participate in gas exchange. Blood and water flow across the lamellae in a counter-current direction, meaning they flow in opposite directions. The projections are held apart by water flow. Therefore, in the absence of water, they stick together, thus meaning fish cannot survive for very long out of water
• Ventilation is required to maintain a continuous unidirectional flow. Ventilation begins with the fish opening its mouth followed by lowering the floor of the buccal cavity, thus enabling water to flow into it. Afterwards, the fish closes its mouth, closing the buccal cavity. The floor of the buccal cavity raises, thus increasing the pressure. The water is forced over the gill filaments by the difference in pressure between the oral cavity and opercular cavity. The operculum acts as a valve and pump and lets water and pass over the gills
• Insects do not possess a closed circulatory system of blood and do not have lungs. Instead, air is transported directly to tissues undergoing respiration. This is achieved with the help of tracheae, small openings of tubes, either bigger trachea or smaller tracheoles, which reach the body of an insect and supply it with the required oxygen
• Upper epidermis


  A waxy, waterproof layer known as cuticle which prevents excessive evaporation of water, it has no chloroplasts for photosynthesis
• Spongy mesophyll


  Contains air spaces which are coated to outer where gas exchange between stomata and the atmosphere takes place
• Lower epidermis


  Contains guard cells which control the opening and closing of the stomata
• Stomata

  Small openings in the leaf surface that allow for gas exchange between the plant and the atmosphere
• Guard cells

  • Play an important role in gas exchange
  • Control the opening and closing of the stomata
  • Inflate to allow water and gas exchange
  • Deflate to prevent water loss
  • Inflate when turgidity caused by an increase in potassium ions, decreasing the water potential, allowing water to enter the guard cells
  • Close following an excess water loss, usually in response to a drop in light intensity and lower rate of photosynthesis
• Photosynthesis

  Site of photosynthesis is in the mesophyll cells
• Leaves have a daily rhythm of opening and closing and respond to changes in environmental conditions to allow diffusion of carbon dioxide and regulate water loss by transpiration
• Stomatal aperture is regulated in response to the availability of carbon dioxide for photosynthesis and to conserve water