Page 65 - Science Class 07

Nasal cavity there is a cluster of
In human beings, air containing oxygen enter
the body through nose. There are two holes in
our nose called nostrils. The nostrils opens
behind in the nasal cavity. The nasal cavity lies
above the buccal cavity. The func on of nasal
cavity is to moisten, warm and trap dust
par cles present in the air coming in it through
nostrils. The nasal cavity opens behind in the
pharynx.
Pharynx
Pharynx is a common passage for both the
respiratory and diges ve system. It leads into
the larynx (voice box).
Larynx
It is also known as voice box. It lies at the top
of trachea. The pharynx opens into the larynx
through a slit like opening called glo s. The
glo s is protected by a leaf shaped flap of
car lage called epiglo s. Epiglo s covers the
mouth of glo s when we swallow food, so that
food may not enter the trachea.
There is a pair of vocal cords present inside the
larynx. When air passes through the larynx,
vibra ons are produced in the vocal cords and
it results in produc on of sound.
Trachea
It is also known as wind pipe. It is 10-12 cm.
long tube with a diameter of 2-3 cm. Trachea is
supported by 15-20 car laginous rings. These
rings prevent trachea from collapsing when no
air is present in it and also provides flexibility
and strength.
At its lower end, the trachea divides into two
smaller tubes called bronchi (singular
bronchus) which enters the right and le lungs.
The walls of bronchi are also supported by
car laginous rings. In the lungs, each bronchus
further divides into s ll smaller tubes called
bronchioles. At the ends of each bronchiole,
Science-7
ny and thin walled sacs
called alveoli. Each alveoli is surrounded by a
network of very fine blood capillaries.
Lungs
In humans, a pair of lungs is present in the
thoracic cavity, one on each side of the heart.
These are called right lung and le lung
respec vely. The right lung is larger than the
le lung. The right lung has three lobes and two
fissures. While, the le lung has two lobes and
only one fissure. Both the lungs are covered by
two membranes called pleura. The fluid
present in between these two membranes is
called pleural fluid. It protects the lungs from
mechanical injuries.
Why do We Sneeze?
When any foreign par cle enters into the nose
cavity, the sensi ve mucus membranes get
irrita on. This irrita on triggers sneezing so as
to remove the par cles.
What is Choking?
When we eat and swallow food rapidly while
talking, a few par cles of food enter in our
wind pipe. The sensi ve membranes of wind
pipe try to throw out the food par cles which
results in choking and you become restless.
PROCESS OF RESPIRATION
You already know that respira on consists of
two phases, i.e., external respira on (breathings)
and internal respira on (breaking down of food).
Now, you will study the mechanism of breathing
as well as internal respira on.
Mechanism of Breathing
The air which we breathe in and out of the
lungs varies in its pressure. So, basically when
there is a fall in air pressure the alveolar spaces
falls and the air enters the lungs (inspira on)
and when the air pressure within the alveoli
exceeds atmospheric pressure, the air is blown
65