subject guide notes

B3.1.1—Gas exchange as a vital function in all organisms

Gas exchange occurs via diffusion

challenges in gas exchange increase as the organism grows in size

  • increase in organism’s size = decrease in SA:V ratio = increase in the distance between the surface of organism & cells in interior of organism

  • organism now has specialized structures to facilitate gas exchange


B3.1.2—Properties of gas-exchange surfaces

Large SA - more membrane surface available for gases to diffuse across

Permeability - exchange surfaces have holes that allow for efficient gas exchange

thin layer - gases need to travel a short distance

moist surface - helps dissolve gases before they diffuse across the exchange surface

concentration gradient - difference in concentration of gases between 2 areas means that diffusion occurs, and therefore gases are exchanged


B3.1.3—Maintenance of concentration gradients at exchange surfaces in animals

*remember, the aim of diffusion is to achieve balance/equilibrium

the bigger the difference in concentration (the steeper the concentration gradient), then the more quicker that diffusion occurs

dense network of blood vessels - provides multiple opportunities for substances to be exchanged between the surface & blood

continuous blood flow - as soon as substances are diffused into blood, they’re carried away by the continuous blood flow. this means there is a low concentration of these substances in areas closest to the exchange surfaces, meaning these substances will continue to diffuse into the blood (they move from an area of high concentration into an area of low concentration)

ventilation - ensures the air (or water) that has a high concentration of the gas, is moved across the exchange surface


mammals also hv a double circulatory system - allows them to separate oxygenated & deoxygenated blood, and maintain a steep concentration gradient


B3.1.4—Adaptations of mammalian lungs for gas exchange

surfactant - is a component in the alveolar fluid that’s released by type 2 pneumocytes

  • the alveolar fluid moistens the alveoli’s surface, allowing gases to be dissolved before they’re diffused into the blood

  • the surfactant also reduces surface tension of the alveoli, preventing the alveoli from collapsing into itself during exhalation


network of bronchioles - each bronchiole branches into multiple alveoli, increasing the SA for gas exchange & increasing the amount of exchange surfaces

  • air is evenly distributed through the lungs

  • the bronchioles hv a smaller diameter compared to the bronchi & trachea = slows down the rate of air flow, allowing for efficient gas exchange


extensive capillary beds - these surround the alveoli, and mean that there’s a short distance that gases need to travel

  • short distance that O2 needs to travel to enter the blood

  • short distance that CO2 needs to travel to exist the blood


high surface area - caused by the branched network of bronchioles

but also caused by the high number of alveoli, which increases the amount of exchange surfaces


B3.1.5—Ventilation of the lungs

the following muscles are involved in ventilation of the lungs

diaphragm - a sheet of muscle that’s located below the ribs

intercostal muscles - group of muscles found & located between the ribs

  • internal intercostal muscles

  • external intercostal muscles

abdominal muscles

ribs - protect the lungs & heart, and expand and contract during exhalation & inhalation


B3.1.6—Measurement of lung volumes

total lung volume - total amount of air in lungs after inhalation

forced vital capacity - volume of air that is forcibly exhaled

residual volume - amount of air that cannot be exhaled

tidal volume - volume of air that moves in & out of your lungs

inspiratory reserve - additional amount of air inhaled after taking normal breath

expiratory reserve - additional volume of air exhaled after taking a normal exhale


B3.1.11—Adaptations of foetal and adult haemoglobin for the transport of oxygen

Haemoglobin has 4 binding sites that allow for cooperative binding

  • as oxygen binds to the haem, the conformation of haemoglobin changes, increasing its affinity for oxygen

  • when the oxygen is released, haemoglobin’s affinity for oxygen decreases


CO2 binds to an allosteric region of haemoglobin

  • causes haemoglobin to form carbaminohaemoglobin

    • has decreased affinity for oxygen

    • allows oxygen to be released in areas of low partial pressure of oxygen


B3.1.12—Bohr shift

  • results in oxygen disassociation curve moving to the right, as haemoglobin has a lower affinity for oxygen

  • caused by decrease in blood’s pH

    • in respiring cells, there is a high amount of CO2 that’s being released

    • this reacts with water in the plasma to form carbonic acid, lowering blood’s pH

      • decreases haemoglobin’s affinity for O2

  • important, cuz haemoglobin will release oxygen in areas of high partial pressure of CO2 (such as respiring muscles)


B3.1.13—Oxygen dissociation curves as a means of representing the affinity of haemoglobin for oxygen at different oxygen concentrations

  • rate at which haemoglobin takes in oxygen, increases as partial pressure of oxygen increases

  • but then eventually levels off as the haemoglobin becomes fully saturated with oxygen in areas of high partial pressures of oxygen, such as lungs