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BREATHING
This is the inhalation (taking in) and exhalation (giving out) of air.
The proportions of gases in inhaled and exhaled air are different as shown by the table below.
Gas
Nitrogen
Oxygen
Noble gases
Carbon dioxide
Water vapour
Inhaled air (%)
78
21
1
0.03
0/ variable
Exhaled air (%)
78
16
1
4
1/saturated
The body of a mammal is differentiated into two parts, an upper thorax and a lower abdomen. The
thorax and abdomen are separated by a dome-shaped muscular wall known as the diaphragm.
The thorax is supported by a bony frame-work and consists of the thoracic cavity. It is made up of the
breast bone (sterum) infront, the backbone (vertebral column) at the back and twelve pairs of ribs at the
sides. The ribs are attached to the back bone and the breast bone, hence forming a bony cage. They are
connected to one another by muscles known as the intercostal muscles.
The respiraroty system
The inside of the thoracic cavity is lined with a thin membrane known as the outer pleural membrane.
This membrane folds over so that part of it covers the lungs. This part is known as the inner pleural
membrane. The space between the two membranes is known as the pleural cavity and is filled with a
fluid which allows the lungs to move smoothly and prevent them from sticking to the chest wall.
Oxygen is used by the body cells and carbon dioxide is produced. Oxygen and carbon dioxide are
exchanged between the inhaled air and the blood across the inner surface of the lungs in structures
called alveoli (air sacs).
Beyond the pharynx, in the region of the throat lies the larynx (voice box). This structure leads into a
long tube called the trachea (windpipe). At its base, it branches into two bronchi. Each one enters a lung
and in turn branches into several bronchioles. These bronchioles and their smaller branches terminate
into groups of tiny air sacs called alveoli.
There are two lungs (left and right). Each lung is a soft, pink, spongy structure. It consists of many lobes,
each lobe is made up of several millions of alveoli.
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The membrane which lines the upper respiratory tract (trachea, bronchus and bronchiole) is well
supplied with blood, which warms the air to body temperature. Hairs in the nasal passage filter out large
particles of dust. A sheet of mucus lines the respiratory tract and traps bacteria, viruses and dust
particles. The mucus comes from cells in the membrane lining called goblet cells. Cilia, rows of fine hairs,
sway to and fro and sweep the mucus with the trapped substances into the pharynx, where it is either
swallowed, sneezed out or coughed up. The air which then passes to the lungs is cleaned and freed from
germs.
Roles of the ribs, intercostals muscles and diaphragm in breathing
1. Inhalation (breathing in)
a.
b.
c.
d.
e.
f.
g.
h.
External intercostals muscles contract
Internal intercostals muscles relax
Ribcage moves upwards and outwards
Diaphragm muscles contract
Diaphragm flattens
Volume of thoracic cavity increases (lungs expand)
Pressure in the lungs decreases
Air rushes into the lungs
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2. Exhalation (breathing out)
a.
b.
c.
d.
e.
f.
g.
h.
External intercostals muscles relax
Internal intercostals muscles contract
Ribcage moves downwards and inwards
Diaphragm muscles relax
Diaphragm assumes a dome-shape
Volume of thoracic cavity decreases (lung volume decreases)
Pressure in the lungs increase
Air is forced out of the lungs
Exchange of gases at the alveoli
In inhaled air, there is higher concentration of oxygen in the alveoli than in the blood. This allows oxygen
to diffuse from the alveolus into the blood. There is more carbon dioxide in the blood coming towards
the alveolus than in the inhaled air in the alveolus, therefore carbon dioxide diffuses from the blood into
the alveolus.
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Features of the alveoli which allow gaseous exchange.
1. An alveolus is one cell thick which allows gases diffuse across its walls
2. It has film of moisture which dissolves gases to allow rapid diffusion from the alveolus into the
blood
3. Close to a network of blood capillaries which allows gases to travel shorter distances between it
and the blood
4. Numerous in number which increases surface area for exchange of gases.
Effects of tobacco smoke and its major components on health
Major components of tobacco smoke
i.tar
ii. nicotine
iii. carbon monoxide
EFFECTS OF TOBACCO SMOKE
1. Lung cancer
Tar contains chemical substances called carcinogens which trigger the abnormal multiplication
of cells in the lungs leading to cancer of the lungs.
2. Bronchitis
Some chemicals in tar lead to overproduction of mucus and also stop cilia from beating/moving.
This result in the mucus and trapped substances eg. Bacteria being clogged in the air passages,
leading to infection. Infection result in the inflammation of the air passages and difficulty in
breathing leading to a condition called bronchitis.
3. Emphysema
Some chemicals in tobacco smoke weaken the alveoli and as a smoker coughs due to irritant
substances in tobacco smoke, the weakened alveoli break. This reduces the surface area for
gaseous exchange which may result in a person experiencing breathing problems. This condition
is called emphysema. The person may become breathless and exhausted.
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4. Heart problem
The carbon monoxide increases the rate at which fatty material is deposited in the arteries.
The nicotine and carbon monoxide from tobacco smoke promote formation of blot clots in the
blood vessels which may lead to heart attack.
Effect on pregnant woman
Carbon monoxide in tobacco smoke diffuses into the blood preventing supply of oxygen to the
foetus . This may result in miscarriages, still births or babies born underweight.