By skyforbes 
Each bronchus branches into small bronchioles. At the end of each bronchioles are clusters of air sacs – called alveoli.
Underneath the lungs lies the diaphragm.
Breathing in:
- Intercostal muscles contract –> ribs move up and out
- Diaphragm contracts and so flattens
This increases thorax volume and reduces pressure –> air is pulled in.
Breathing out:
- Intercostal muscles relax –> ribs move down and inwards
- Diaphragm relaxes and so becomes dome shaped again.
This reduces the thorax volume and increases the pressure –>air is forced out.
Alveoli – where gas exchange happens
The inner surface is moist – stops cells drying out. The fluid contains a surfactant which lowers surface tension stopping the alveoli sticking together when breathing in and out. The oxygen also dissolves in this layer before diffusing into the capillaries around the alveoli.
How are alveoli adapted for efficient gas exchange?
- large surface area – millions of alveoli in the lungs and each one is folded increasing the SA even more.
- thin permeable surface – epithelium of alveoli is made of very thin flat cells. The wall of the capillary is only one cell thick and the capillary is very close to the alveoli. This all means that oxygen has a very small distance to diffuse across.
- large concentration gradient – breathing in brings in air with high oxygen concentration and low carbon dioxide concentration. The capillaries are bringing deoxygenated blood to the alveoli containing lots of carbon dioxide. This sets up a large concentration gradient for oxygen to diffuse into the blood and carbon dioxide to diffuse into the alveoli and so we breathe out air high in carbon dioxide and low in oxygen.
How are the lungs kept clean?
The trachea and bronchi are lined with an epithelium (outer layer of cells). This has 2 cells in it:
- Ciliated cells – have cilia on the edge (hair like structures that are an extension of the cytoplasm). These bend and beat in time with each other and waft any mucus up the trachea, where it can be swallowed.
- Goblet cells – make and secrete mucus. Mucus is slimy and sticky and traps dust and bacteria that is breathed in.
With the mucus sweeping the mucus away this helps to reduce the chance of respiratory infections.