Inside the cells the oxygen reacts with glucose (from digested carbohydrates) and the energy stored in the food is gradually released. This process is called respiration. The energy keeps your cells alive and working. During respiration, a waste gas called carbon dioxide, and water, are formed in your cells. You get rid of these when you breathe out.
The breathing process
1. Your lungs and windpipe are known as your respiratory system. When you breathe in, air is sucked through your nose or mouth, down your windpipe (trachea) and into two passages called bronchi. One bronchus goes to the left lung, the other goes to the right. The bronchi gradually divide to form smaller and smaller passages, rather like tree branches.
2. At the ends of the smallest passages are bunches of air sacs called alveoli. These are like tiny balloons and fill with air when you breathe in. All together there are about 300 million alveoli in your lungs. Their total surface area is about 70 square meters, which is about 40 times the area of your skin.
3. The walls of the alveoli are only one cell thick. The oxygen from the air is able to pass through the walls and into the network of blood which surrounds them. Your red blood cells carry the oxygen round your body. Your blood also carries back to the alveoli the carbon dioxide produced in your cells during respiration so that it can be removed from your body when you breathe out.
How you breathe
Your breathing is controlled by the movements of muscles in your chest, in particular your diaphragm muscle and the muscles between your ribs. To breathe in, your diaphragm contracts downwards while your rib muscles pull your ribs up and out. This expands the space in your lungs, making the air pressure lower inside your lungs than outside your body. Air rushes in to fill the space. When your diaphragm relaxes upwards and your ribs move down and in, the space in your lungs is reduced again and air is squeezed out. Most people usually have about three liters of air in their lungs and exchange only half a liter of it on each breath. During exercise, your body needs more energy, so you breathe faster and deeper to take in more air.
Your voice box (larynx) is at the top of your trachea. When you breathe out, air passes between your “vocal cords.” If there is enough air, the cords vibrate and this produce sounds. The muscles of your larynx can alter the shape of the cords. This produces different pitched sounds. By using the muscles of your pharynx (throat), mouth and lips, you form the sounds into words.
Coughing and sneezing
A slippery liquid called mucus is produced in your nose and air passages. This warms and moistens the air you breathe so it can travel along the passages more easily. It also helps to trap dust particles. Tiny hairs called cilia gently waft the mucus away from your lungs towards your nose and throat. If particles irritate your nose, you sneeze them out. If they get into your lower air passages, you cough. When you have a cold, more mucus is produced. This also makes you cough and sneeze, and makes your nose run.
These are caused by your diaphragm contracting more violently than usual so that your in-breaths come in short gasps. The strange noise is caused by your vocal cords suddenly closing. Nobody knows why hiccups start but even unborn babies get them.
Everybody’s lungs gradually get blackened by breathing in dirty and polluted air but smokers are particularly likely to develop serious, and often fatal, diseases as a result of inhaling the dangerous chemicals in tobacco smoke. The chemicals irritate the air passages and increase the amount of mucus produced in them. This is one of the causes of “smoker’s cough.”
The chemicals also make the cilia less efficient at clearing the mucus away and so it builds up, making the lungs more prone to infection. Smoking is one of the main causes of bronchitis (inflammation of the air passages) and 90 per cent of lung cancer is caused by smoking. Someone who smokes only five cigarettes a day is eight times more likely to die of lung cancer than a non-smoker. Smoking does not only affect the lungs. The chemicals also get into the blood, reducing its ability to carry oxygen and damaging the heart and blood vessels. Statistics show that if 1,000 children born today all take up smoking, 250 of them will be killed by smoking.