2.1 Properties of Gas Exchange Surfaces

• All organisms need to exchange gases with their environment, e.g.
  • Aerobic respiration requires oxygen and produces carbon dioxide as a waste product
  • Photosynthesis requires carbon dioxide and produces oxygen as a waste product
• The process of gas exchange occurs by diffusion
• The surface over which this gas exchange takes place is known as an exchange surface; exchange surfaces have specific properties that enable efficient exchange to take place

Surface area to volume ratio

• The surface area of an organism refers to the total area of the organism that is exposed to the external environment
• The volume refers to the total internal volume of the organism, or total amount of space inside the organism
• The surface area of an organism in relation to its volume is referred to as an organism's surface area : volume ratio (SA:V ratio)
• As the overall size of the organism increases, the surface area becomes smaller in comparison to the organism's volume, and the organism's surface area : volume ratio decreases
  • This is because volume increases much more rapidly than surface area as size increases

• Single-celled organisms have a high SA:V ratio which allows the exchange of substances to occur by simple diffusion
  • The large surface area allows for maximum absorption of nutrients and gases and removal of waste products
  • The small volume within the cell means the diffusion distance to all organelles is short
• As organisms increase in size their SA:V ratio decreases
  • There is less surface area for the absorption of nutrients and gases and removal of waste products in relation to the volume, and therefore requirements, of the organism
  • The greater volume results in a longer diffusion distance to the cells and tissues of the organism
• Large multicellular organisms have evolved adaptations to facilitate the exchange of substances with their environment
  
  • The gas exchange systems of multicellular organisms are adapted to increase the surface area available for the exchange of gases e.g.
    • Alveoli increase the surface area of mammalian lungs
    • Fish gills have structures called lamellae which provide a very large surface area
    • Leaves have a spongy mesophyll layer within which a large area of leaf cell surface is exposed to the air
• Note that the problem of internal diffusion distance is a separate, though connected, issue solved by the presence of a mass transport system such as a circulatory system

Diffusion pathway

• The diffusion pathway, or distance, across an exchange surface is very short
• The surface often contains only one layer of epithelial cells
  • The cells can also be flattened in shape to further reduce the distance across them
• This means that substances have a very short diffusion pathway

Concentration gradient

• This is the difference in concentration of the exchange substances on either side of the exchange surface, e.g. between the air inside the alveoli and the blood
• A greater difference in concentration means a greater rate of diffusion as the gas molecules move across the exchange surface
• The continued movement of exchange substances away from the exchange surface mean that a concentration gradient is maintained
  • This is achieved by e.g.
    • The alveoli have a good blood supply; this constantly removes oxygen from the capillary side of the exchange surface and supplies carbon dioxide
    • The ventilation system in mammals ensures constant inhalation and exhalation; this supplies oxygen and removes carbon dioxide from the alveoli side of the exchange surface

• Fick's Law relates the rate of diffusion to the concentration gradient, the diffusion distance and the surface area
• This relationship can be represented by the following equation, where ∝ means "proportional to"

rate of diffusion  (surface area x concentration difference)thickness of membrane

• Proportionality means the rate of diffusion will double if
  • The surface area or concentration difference doubles
  • The diffusion pathway halves

• Fick's Law can be written as an equation which can be used to calculate the rate of diffusion

Rate = P x A x ((C₁ - C₂)T)

• Where
  • P = A permeability constant that is a quantitative measure of the rate at which a particular molecule can cross a particular membrane
  • A = surface area
  • C₁ - C₂ = the difference in concentration between two areas
  • T = thickness of the exchange surface

Step 1: Substitute numbers into the equation

Rate = P x A x ((C₁ - C₂) T)

Rate = 0.012 x 3 x ((1.8 x 10^-16 - 7.5 x 10^-17)1.5)

Step 2: Complete the calculation

Rate = 0.012 x 3 x (1.05 x 10^-16 1.5)

Rate = 0.012 x 3 x 7 x 10^-17

Rate = 2.52 x 10^-18 molecules m^-2 s^-1

• The lungs of air-breathing animals provide an ideal exchange surface for the diffusion of gases 
  • The lungs are located in the thorax, or chest cavity
  • Some animals rely entirely on their lungs for gas exchange, while amphibians use lungs alongside gas exchange across the skin
• The role of lungs is to maximise gas exchange while minimising the loss of water across the exchange surface
• Mammals have very efficient gas exchange structures in their lungs

The lungs are located in the thorax, and enable efficient gas exchange

• Trachea
  • The trachea is the tube that allows air to travel to the lungs
  • It contains c-shaped rings of cartilage that ensure that the tube remains open at all times and does not collapse
    • The c-shape prevents any friction from rubbing with the oesophagus located close behind, as well as providing increased flexibility when food is being swallowed
  • There is a layer of mucus covering the lining of the trachea that helps to trap dust and pathogens, preventing them from entering the lungs where they could cause infection
    • Tiny hairs called cilia are also found on the lining of the airways, where they waft mucus towards the top of the trachea, removing any trapped particles and pathogens from the airways

• Bronchi
  • Bronchi (singular bronchus) have a similar structure to the trachea but they have thinner walls and a smaller diameter
  • The cartilage rings in the bronchi are full circles rather than c-shaped

• Bronchioles
  • Bronchioles are narrow, self-supporting tubes with thin walls
  • There is a large number of bronchioles present in the gas exchange system
  • Each one varies in size, getting smaller as they get closer to the alveoli
  • The larger bronchioles possess elastic fibres and smooth muscle that enable adjustment of the size of the airway to increase or decrease airflow
    • The smallest bronchioles do not have any smooth muscle but they do have elastic fibres

• Alveoli
  • Groups of alveoli are located at the ends of the bronchioles
  • The alveolar wall consists of a single layer of flattened, or squamous, epithelium
    • The squamous epithelium forms the alveolar wall and is very thin and permeable for the easy diffusion of gases 
    • The alveoli are surrounded by elastic fibres, allowing them to stretch during inhalation
  • Alveoli are surrounded by an extensive capillary network
    
    • Carbon dioxide diffuses out of the capillaries and into the alveoli to be exhaled, while oxygen diffuses from the alveoli and into the capillaries to be carried around the body
  • A layer of moisture lines the alveoli, facilitating the diffusion of gases
    
    • Oxygen and carbon dioxide are able to dissolve in the layer of moisture, so exchange occurs in solution rather than with the air inside the alveoli

Human alveoli are adapted for efficient gas exchange