We’ve now looked at three ways in which substances can be transported across cell membranes: diffusion, osmosis, and active transport. Today’s topic is example of where diffusion takes place in the human body, all with the purpose of taking in oxygen for respiration and removing carbon dioxide. There’s quite a lot of information today so we’ll break it down into some subheadings.
The Lungs
Gas exchange in humans (and other mammals) happens in the lungs, so first of all you need to know a bit of anatomy.
The lungs are protected by the ribcage and are also surrounded by the intercostal muscles. Underneath the lungs is the diaphragm (a sheet of thin muscle). All these are important for moving air in and out of the lungs (see the ventilation section).
When you breath in, the air firstly travels down the trachea (the windpipe) which then splits into the two bronchi leading to each lung. These gradually branch off into smaller and smaller bronchioles which eventually end in the alveoli. The alveoli are very small spherical sacs where the actual diffusion of gasses takes place (see the gas exchange section).
Each part of the airway has a different structure, and there are different types of tissue helping the lungs to function:
- Elastic fibres are found in the walls of all the tubes and the alveoli. They allow the system to stretch to fill with air, and recoil to help force the air back out again.
- Smooth muscle is found in the walls of all the tubes (but not the alveoli or the smallest bronchioles). When you exercise, it relaxes and widens the diameter of the tubes to help get more air flow into the lings. More oxygen is needed for aerobic respiration if you are exercising.
- Cartilage is found in the walls of the trachea and bronchi. This keeps the tubes supported so they don’t collapse when you breath out.
The very inner layer in all the tubes except the very smallest bronchioles is a ciliated epithelium. This is a layer of columnar epithelial cells which have cilia projecting into the lumen of the airway. The cilia push mucus along the airways up to the throat where it can be swallowed. The cells which secrete the mucus are called goblet cells, which are also in the ciliated epithelium and have a goblet sort of shape.
Ventilation
Ventilation is not just opening the windows. It is the technical term for breathing. Breathing in is called inspiration, and breathing out is called expiration. The structures surrounding the lungs are key for making this happen. Let’s summarise the two processes in a table:
| Inspiration | Expiration | |
| External intercostal muscles and diaphragm… | Contract | Relax |
| So ribcage moves… | Upwards and outwards | Downwards and inwards |
| Diaphragm shape… | Flattened | Dome shape |
| Thoracic cavity volume… | Increases | Decreases |
| Pressure in thoracic cavity… | Decreases | Increases |
| Air is drawn… | Inwards | Outwards |
| Energy from ATP required? | Yes (Active) | No (Passive) |
It may be easier to visualise the process in the below diagrams. The key point to remember is that the air moves down a pressure gradient, so it’s the change in pressure in the thoracic cavity (the space where the lungs are contained) relative to the atmospheric pressure that causes the air to move in and out. Inspiration requires energy because the external intercostal muscles and diaphragm are contracting.
Of course sometimes you force yourself to breath out further than you normally would. Forced expiration involves the internal intercostal muscles contracting to pull the ribcage down further, so this does require energy.
Gas Exchange in Humans
Now we get down to the really small scale stuff going on in the lungs. The alveoli are found at the ends of the bronchioles and there are loads of them – hundreds of millions in a pair of human lungs. This provides a large surface area for gas exchange to take place.
The walls of the alveoli (the alveolar epithelium) are only one cell thick and the cells are quite flattened which provides a short diffusion pathway. The capillaries are right next to the alveoli and also have walls that are one cell thick (the capillary endothelium). Because of these adaptations, the rate of diffusion of oxygen and carbon dioxide can be kept high. Oxygen gas diffuses from the air space into the blood to bind to haemoglobin in red blood cells, and carbon dioxide gas diffuses out of the blood into the air space ready to be breathed out.
The gases are both diffusing along their concentration gradients. Because we are constantly breathing in and out, and because the blood is constantly flowing, high concentration gradients of both gases are maintained. Another good adaptation of the alveoli is that they secrete a substance called a surfactant which prevents the alveoli from collapsing due to the surface tension of the water inside. The surfactant disrupts the interactions between the water molecules so they don’t pull together as much.
This is definitely a big topic. As with many topics, try to learn how structure relates to function. It will really help you to answer exam questions. Let’s try and summarise some key points:
- Air travels down the trachea, bronchi, bronchioles and finally reaches the alveoli.
- Inspiration and expiration happen due to changing pressure in the thoracic cavity, and involves contraction and relaxation of the external intercostal muscles and diaphragm.
- Forced expiration involves contraction of the internal intercostal muscles.
- Gas exchange takes place in the alveoli. They are well adapted to maintain a high rate of diffusion.
