1. Louis Holdcroft & Joel Willams
The Alveoli
Louis Holdcroft & Joel Willams

2. Adaptations
The lungs Alveoli have many adaptations to make it a lot more effective for efficient gas exchange:
There are millions of Alveoli
Alveoli have a large surface area to volume ratio
They are very thin (1 cell thick)
They are partially permeable so allow only selected materials through
There is a movement of an environmental medium and a movement of an internal medium.
The enviromental medium is the air and the internal medium is the blood.

3. Reasons for these adaptions:
They have a large surface area to volume ratio – This is because the larger the surface area the more oxygen it picks up and speeds up the rate of gas exhange.
They are very thin – Because they are only 1 cell thick this keeps the diffusion pathway short and allows materials to pass through them rapidly. Aswell as this the red blood cells are slowed down and therefore have more time to pick up oxygen, making the gas exchange more efficient.
They are partially permeable – This allows selected materials to diffuse through easily.
They have a net movement of an environmental and internal medium – There is a constant movement of blood around the capillaries of the alveoli and a constant ventilation of air aswell, this maintains the concentration gradient for efficient gas exchange.

4. Role of environmental and internal mediums in the lungs
In the lungs there are 2 mediums which help to increase gas exchange. Breathing movements constantly ventilate the lungs, this means that there is always a supply of oxygen in the lungs which the red blood cells pick up and get rid of CO2, this supply of oxygen maintains a concentration gradient for efficient gas exchange. The internal medium is blood, the heart constantly pumps blood around the body, and around the capillaries in the alveoli. This blood is constantly arriving at the lungs and picking up the oxygen, together both of these mediums insure that a steep concentration gradient of oxygen is maintained.
Constant blood flow
Constant airflow

5. Facts of the alveoli:
Their total surface area is 70 metres squared, around half the area of a tennis court.
Each alveolus is around 0.05 to 0.3 micro metres, which is very thin.
The blood capillaries supplying them are around 7-10 micrometres in diameter and have a wall of around 0.04 – 0.2 micrometres for efficient gas exchange.
Real alveoli in the lungs.