Angharad "Rad" Yeo:
Take a deep breath in …and out. Feels good, right? And it's also necessary for staying alive. But what exactly is breathing and why do we need it?
The parts of our body involved in breathing are called the respiratory system. This includes the lungs, the airways, and the muscles used for breathing. These specialised structures of the respiratory system work together to achieve the function of the system: moving air into our lungs so we can absorb oxygen into our blood and releasing carbon dioxide out of our blood. There's a few different types of respiration, but here with our respiratory system, we're referring to breathing air in and out, and exchanging gases in and out of the blood.
First up, let's find out why we need to breathe to begin with. It all starts with oxygen. Every part of our body needs oxygen to release energy from food, and this oxygen gets delivered around the body in our blood. As we use up the oxygen to release energy, carbon dioxide is produced as a waste product, which then gets released into the blood.
The blood then travels to the lungs where it's up to the respiratory system to provide new oxygen and get rid of the carbon dioxide. This process is called gas exchange. When we breathe that in, it has 20% oxygen and less than 1% carbon dioxide. The lung's job is to get the air close to the blood, which will absorb oxygen and release carbon dioxide. The air we breathe out has about 16% oxygen and 4% carbon dioxide. They swap or exchange. Gas exchange!
The respiratory system's job can be broken down into three specific tasks or functions. Bring in a constant supply of air, make that air warm and damp, and bring the air and blood close enough together for gas exchange to happen. The need for a constant supply of fresh air means we need to keep breathing in and out. This makes sure there's always plenty of oxygen to absorb and stops the carbon dioxide from building up.
We take air in, which is called inhalation and let air out — exhalation. This is essential for us to absorb the oxygen we need and release the carbon dioxide that we don't. How much we need to breathe does depend on how hard our body is working. But roughly speaking, the volume of air we breathe each day would fill up a standard backyard swimming pool.
Lungs can't move air in and out on their own. They're a bit like floppy balloons — they don't have muscles to move by themselves. It's actually the muscles of the chest that do this job. Muscles in the ribs called intercostal muscles and a big flat muscle called the diaphragm that sits under the lungs, kind of like a trampoline. To breathe in, we use these muscles to make the space inside our chest bigger. Put your hands on your ribs and take a deep breath. Do you feel your ribs moving up and apart? You can't usually feel your diaphragm, but a good deep breath will push your tummy out as your diaphragm moves downwards. When the muscles involved in breathing relax, the chest cavity goes back to its normal size, and the air rushes out again. In and out. In and out. We breathe up to 22,000 times every day. It's a good thing that we can breathe without even thinking about it.
So, bringing air in and out is the job of the muscles of breathing. The next function to check off is the need for air to be warm and damp. It does sound kind of gross, but you know how really dry air can leave you with a sore throat and prickly nose? Well, it can dry out your lungs too, so on its way to the lungs, the air passes through the nose or over the mouth and tongue. It also passes through the throat called the pharynx and the windpipe, which is called the trachea. As the air passes through the damp airways, it picks up warmth and moisture.
Okay, so we've covered the muscles of breathing and the upper airways and how they do their work. But what about the lungs themselves? This is where the final function, gas exchange, happens. Remember, gas exchange is the body swapping oxygen and carbon dioxide as needed, and the key to efficient gas exchange is surface area. To make as much surface area as possible our trachea branches. First into bronchi and then further into bronchials. They keep branching up to 25 times, ending in little bunches of sacks called alveoli that you need a microscope to see. There are blood vessels around each of the alveoli that bring our blood very close to the air. By the time all this branching is done, we have up to half a billion alveoli and more surface area for gas exchange than the floor in an average classroom.
So, now we know how the structures of the human respiratory system achieve the function of getting oxygen in and carbon dioxide out of the blood. But what about other animals? What about animals that live in water?
Fish don't breathe air, but they still need oxygen. So, it's a good thing the water they live in has oxygen dissolved in it. Fish have a slightly different kind of respiratory system, but it still has the same function. Water passes through their mouth and over their gills, which are branched and folded and full of blood vessels. This too creates a large surface area for gas exchange. So, whether lungs or gills, air or water, the parts of the respiratory system work together to do the vital function that all animals need for life, of getting rid of carbon dioxide and supplying plenty of oxygen to help us keep swimming.