Purpose of Gas Exchange
- All aerobic organisms require a constant supply of oxygen to release energy in the form of ATP during respiration.
- The CO2 produced in the process needs to be removed
- Build up of CO2 is harmful to the body.
- The volume of O2 absorbed and the volume of CO2 removed is large in mammals. This is because:1. They
1. are relatively large organisms with a large volume of living cells.
2. They maintain a high body temperature therefore have high metabolic and respiratory rates.
- Mammals have evolves specialized surfaces, called lungs, to ensure efficient gas-exchange between the air and their blood.

Mammalian Lungs
In this IBDP Biology topic, you have to understand the structure and function of lungs.
The lungs are internal (inside the body). This is because:
- The air is not dense enough to support and protect these delicate structures.
- They would otherwise lose a great deal of water and dry out.
- The lungs are supported and protected by a bony box called the rib cage.
- Movement allows ventilation by a tidal stream of air ensuring that the air within them is constantly replenished.
Trachea

- Flexible airway
- Supported by rings or cartilage
- Prevents collapsing (air pressure falls when breathing)
- Tracheal walls – muscle lined with ciliated epithelium and goblet cells.
- Goblet cells – produce mucus – trap bacteria and dirt.
- Cilia moves mucus up the throat then down oesophagus into the stomach.
Bronchi

- 2 divisions of trachea leading to each lungs.
- Similar structure to the trachea.
- Produce mucus and have cilia.
- Larger bronchi are supported by cartilage – reduced amount of cartilage as it gets smaller.
Bronchioles
- Series of branching subdivisions of bronchi.
- Walls are made up of muscles lined with epithelial cells.
- Muscle allows constriction so air flow can be controlled in and out of the alveoli.
Alveoli
- Minute air sacs; diameter 100 – 300µm
- At the end of the bronchioles.
- Contain collagen , elastic fibres and lined with epithelium.
- Elastic fibres allow alveoli to stretch when breathing in and spring back when breathing out to expel the CO2 rich air.
- Alveolar membrane is the gas-exchange surface.
Alveoli In Gas Exchange
- Red Blood Cells are slowed through pulmonary capillaries so more time for diffusion.
- Distance between alveolar air and RBCs reduced, RBCs flattened against capillary walls.
- Alveoli and capillary walls are very thin so diffusion distance is very short.
- Alveoli and pulmonary capillaries very large total surface area.
- Breathing movements constantly ventilate lungs, and heart constantly circulates blood around alveoli – steep concentration gradient of gasses is maintained.
Mechanisms of Breathing
Breathing = Ventilation
Breathing maintains diffusion of gases (air moves in/out)
- Air Pressure of Atmosphere > Air Pressure in Lungs = Air is forced into the alveoli (Inhalation)
- Air Pressure in Lungs > Air Pressure of Atmosphere = Air is forced out of the Lungs (Exhalation)
Pressure changes within the lungs because of the movement of 2 sets of muscles:
1.Diaphragm (sheet of muscle – separates the thorax from the abdomen)
2.Intercostal Muscles (lie between the ribs – 2 sets)
- Internal Intercostal Muscles – Contraction leading to expiration
- External Intercostal Muscles – Contraction leading to inspiration
Inspiration
- Active Process (uses energy)
- External Intercostal Muscles contact while Internal Intercostal Muscles relax.
- Ribs are pulled upwards and outwards, increasing the volume of the thorax.
- Diaphragm muscles contract and flatten, increasing the volume of the thorax.
- Increased volume of the thorax results in reduction of pressure in the lungs.

Expiration
- Passive Process (little energy)
- Internal Intercostal Muscles contract while External Intercostal Muscles relax
- Ribs move downwards and inwards, decreasing the volume of the thorax�
- Diaphragm muscles relax making it an upward dome position, decreasing the volume of the thorax
- Decreased volume of the thorax results in increasing pressure in the lungs.
During normal quiet breathing the recoil of the elastic lungs is the main cause of air being forced out.
Pulmonary Ventilation
- Measurement of how much air is taken in and out of the lungs at a given time.
- PV = Total volume of air that is moved into the lungs in 1 minute.
- To calculate it we multiply together 2 factors
1. Tidal Volume – volume of air taken in at each breath at rest (approx 0.5 dm³)
2.Ventilation (Breathing) Rate – number of breaths in 1 minute (approx 12-20 in a healthy adult)
PV is expressed as dm³min-¹
Pulmonary ventilation (dm³.min-¹) = tidal volume (dm³) x ventilation rate (min-¹)
Exchange Surfaces
- Efficient transfer of materials by diffusion or active transport needs.
- Large surface area to volume ratio – to speed up the rate of exchange.
- Very thin – diffusion pathway short allowing materials to cross rapidly.
- Partially Permeable – allow selected materials to diffuse easily.
- Movement of environmental medium – E.g. Air, to maintain a diffusion gradient.
- Movement of internal medium – E.g. Blood, to maintain a diffusion gradient.
Fick’s Law
Relationship between factors
Diffusion is proportional to:
Surface Area x Difference in Concentration
Length of diffusion path
Why Inside The Body?
- Alveoli very thin so it is easily damaged.
- Needs protection�
- Organism needs to have some means of moving external medium over the surface. E.g. A means of ventilating lungs in a mammal.
This is the end of the topic

Drafted by Eva (Biology)
Photo references:
- https://www.macmillan.org.uk/cancer-information-and-support/lung-cancer/the-lungs
- https://ib.bioninja.com.au/standard-level/topic-6-human-physiology/64-gas-exchange/lung-structure.html
- https://www.pinterest.com/pin/636907572290165200/
- https://courses.lumenlearning.com/ap2/chapter/the-process-of-breathing-no-content/