9. Gas exchange systems GapFill

Gas exchange in humans

All our body cells need a good supply of   proteinsoxygencarbon dioxidecytoplasmhormonesnitrogen for respiration – if they don’t get it the cells will die.

If enough cells die the person will die, so the first priority for paramedics and first-aiders when treating a casualty is to make sure the airways are clear.

Breathing

We breathe in and out, or inhale and exhale, to move air in and out of our  windpipebloodcellsheartlungsintestines.

This allows the exchange of gases to take place: oxygen is absorbed into the blood, and carbon dioxide is removed from the blood and breathed out.

When we breathe in:

• The muscles between our ribs contract, which moves the ribcage upwards and outwards, and the  diaphragmcell membranebicepsskintricepschest bone contracts, pulling downwards. The result is that...
• the chest increases in volume, lowering the pressure inside it and sucking air into the lungs.  

When we breathe out:

• The rib muscles relax, so the ribcage drops downwards and inwards, and the diaphragm relaxes and moves back up.
• The result is that the chest decreases in volume, increasing the pressure and forcing air out.  

When you breathe in air through the nose or mouth it goes down the trachea, sometimes called the   lungfoodpipegulletchannelbronchiolewindpipe.

The trachea branches off into two bronchi, one going into each lung. The bronchi branch off into smaller tubes called bronchioles, which form a network of air tubes inside the lungs like the branches of a tree. The bronchioles end in air sacs called  tuberscystsarteriesalveolivillipockets; the air sacs are surrounded by lots of tiny blood vessels. This is where gas exchange takes place − oxygen diffuses out of the air into the blood, and carbon dioxide diffuses out of the blood into the air in the alveoli.

The oxygen bonds with haemoglobin in the  white blood cellsplasmaplateletsred blood cellsepithelial cellsxylem, and is transported in the bloodstream to the body cells, where the oxygen is needed for respiration.

Carbon dioxide diffuses out of the body cells into the blood plasma, and is carried to the lungs. It diffuses into the alveoli and you breathe it out.  

The alveoli are well adapted for efficient gas exchange because they have:

• A large   volumewidthsurface areacellmasslength for diffusion to occur.
• A good blood supply, to transport gases.
• Thin walls, so that the gases don’t have far to travel.

The air you breathe in contains dirt, dust and pathogens (harmful microorganisms) which can damage your   braindiaphragmintestinesnosebloodlungs and cause infection.

To prevent these things from getting in, your breathing passages are lined with mucous membranes, which contain special cells that clean the air as it comes in. It works like this:

• The lining of the trachea and bronchi has some cells which produce a sticky substance called  plasmaslimebilecytoplasmhaemoglobinmucus.
• The other cells are ciliated epithelial cells; that means they have tiny hairs called cilia on the outside of the cell.
• As you breathe in, dirt and pathogens get trapped in the mucus. The cilia beat to move the mucus back up to the throat and away from your lungs.
• The mucus and cilia work together to clean the air you breathe in, and to protect your lungs.  

Smoking stops the   mucusciliaribsvillimembranesdiaphragm from working properly, and that’s why smokers are more likely to get chest infections than non-smokers.