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.
BIOGRAPHY: JOHN SCOTT HALDANE Scottish, 1860-1936
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.
The larynx lies between the back of the pharynx (throat) and the
top of the trachea. The vocal cords stretch across the larynx. When air from
the lungs passes through them, they vibrate to produce sounds.
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.
