Copyright © 2007 Dorling Kindersley
The respiratory system provides your body cells with the oxygen they need and rids them of carbon dioxide, a waste product. Playing a central part in this process are the lungs, two organs in the chest that work closely with the blood circulation. BREATHING is the cycle of moving air into and out of the lungs. Structures in the respiratory system are also vital to SPEECH.
Blood is carried from the heart to the lungs by the pulmonary arteries. They are the thick red vessels at the centre of this photograph of a resin cast of two lungs. The pulmonary arteries split into many branches, forming an intricate network of vessels that carry blood to the lungs’ alveoli. There, oxygen enters the blood, and carbon dioxide leaves it.
When air is breathed in, it passes down the trachea. This divides into two airways called main bronchi, which go to the two lungs. Each splits into smaller bronchi, which then split into bronchioles. These terminate in groups of tiny air sacs called alveoli.
Each lung contains millions of alveoli (air sacs), which are the site of gas exchange. Groups of alveoli, looking like clusters of grapes, are found at the ends of bronchioles, the tiniest of the lungs’ air passages. The walls of the alveoli are surrounded by a dense network of capillaries carrying blood.
As oxygen-depleted blood passes close to the wall of an alveolus, carbon dioxide passes from the blood into the alveolus. At the same time, oxygen passes from the alveolus into the blood, where it binds with haemoglobin in red blood cells. The swapping of carbon dioxide for oxygen in the lungs is called gas exchange.
In 1905, the scientist J S Haldane made the important discovery that the urge to breathe is caused by a build-up of carbon dioxide in the blood. As the blood level of carbon dioxide rises, this is detected by a small region in the brain, which triggers quicker breathing.
Our ability to speak relies on the presence of two folds of tissue called vocal cords in the larynx (voice box) at the top of the trachea. As air passes between the cords when we breathe out, they vibrate. During speech, a centre in the brain sends signals to tiny muscles that alter the position and length of the cords, producing different sounds. These are modified into meaningful speech by movements of the lips, cheeks, and tongue.
Breathing is the process of drawing air into the lungs and then expelling it again. Adults breathe at a rate of around 12-15 times per minute at rest but at a faster rate during exercise. With each breath, the lungs take in around 0.5 litres (1 pint) of air.
If dust or germs enter the respiratory system, they can irritate the larynx, trachea, or bronchi. This may trigger coughing. When you cough, muscles in the chest and abdomen contract suddenly, increasing air pressure within the lungs. As a result, a spray of liquid drops containing dust and other unwanted material is forced out. Germs can pass from one person to another in this way.
During inhalation, the intercostal muscles between the ribs contract. So does the diaphragm, a muscular sheet at the base of the chest cavity. The ribcage expands, and the diaphragm flattens, which increases the size of the chest cavity. This increase in chest volume causes the pressure of air in the lungs to be lower than the pressure of the air outside the body. As a result, air is drawn down the trachea into the lungs.
During exhalation, the intercostal muscles relax, and so does the diaphragm. The ribs move downwards and inwards, causing the ribcage to contract, and the diaphragm moves up. As the volume of the chest cavity decreases, the pressure of air within the lungs becomes higher than the pressure in the air outside the body. As a result, air moves back up the trachea and is expelled to the outside through the nose and mouth.