1. Biology The development of internal gas exchange surfaces in larger organisms to maintain adequate rates of exchange. Mammals (alveoli, bronchioles, bronchi, trachea, lungs), including the ventilation system.

2. To enable efficient gas exchange organisms have to be adapted. In this case the walls of the alveoli are made up of a layer of epithelial cells which are flattened. The capillaries are also made up from this way. This allows gases to diffuse through two membranes only. To keep the alveoli moist water constantly diffuses through it.

3. The respiratory system is the gas exchange organ in mammals. It contains the following: Alveoli bronchioles bronchi Trachea lungs This can be seen on the following diagram:

5. The concentration gradient across the respiratory system is maintained by: 1)Blood flow on one side 2)Air flow on the other side This allows oxygen to diffuse down its own gradient from air to blood, while carbon dioxide can diffuse down its own concentration gradient from blood to air. The flow of air into and out of the alveoli is known as Ventilation, and has two stages:

6. Inspiration: How it works- Diaphragm flattens and contracts Intercostal muscles contract making the ribs come up and out This increases the volume of the the thorax which in turn increases the lung and alveoli volume Pressure of air is decreased and so air flows IN to equalise this. Expiration: How it works: Diaphragm relaxes and curves upwards Ribs fall as intercostal muscles relax This decreases volume of thorax which in turn decreases lung and alveoli volume. This increases the pressure of air and so air flows OUT to equalise this.

7. The following diagram helps explain this:

8. Component Atmospheric Air (%) Expired Air (%) N 2 (plus inert gases)78.6274.9 O2O2 20.8515.3 CO 2 0.033.6 H2OH2O0.56.2 100.0% Table to show what happens to the composition of air when it reaches the alveoli

9. The End!