1.1 Human Breathing
1) Human Respiratory System
Living organisms must be able to take
oxygen from the air and get rid of carbon
dioxide to the air.
Swapping oxygen for carbon dioxide in this
way is called gas exchange.
Gas exchange takes place through a gas
exchange surface also known as a
For gas exchange we need a respiratory
Breathing is an external respiration.
Breathing consist of two stage:
1. Inhalation: air is taken into the lungs
2. Exhalation: air is passes out of the lungs
The breathing system or the human respiratory
system consists of the following structures:
1. The nasal cavity
6. Rib cage
8. The intercostals muscles
Structure of the human respiratory system:
Structure of the lung
The lungs are the respiratory organs.
We have a pair of lungs in the thoracic
They are protected by the rib cage.
The lungs are connected to the trachea by
two tubes. These tubes are right bronchus
and left bronchus.
The bronchus divide into smaller tubes called
The bronchioles end in clusters of tiny air
sacs known as alveoli, in the lungs.
Flow of air into the lungs:
Air is breathed in through the nose and enters the
The nostrils leads to the nasal cavity. While passing
through the nasal cavity the air is warmed up and
Hairs and sticky mucus trap particles inside the nasal
cavity. The clean air then enters to the trachea.
The trachea branches into two bronchi.Eachs bronchus
leads directly into a lung.
The bronchus branches into bronchioles.
Each bronchiole end in a cluster of air sacs called
The wall of the alveolus is only one-cell thick. It is thin,
moist and surrounded by a network of capillaries.
The exchange of respiratory gases occurs between the
alveolus and capillaries.
The path of air into the lungs is summarized:
Nasal cavity Trachea
The breathing mechanism
Breathing mechanism involves:
1. Inhalation (breathing in)
2. Exhalation (breathing out)
1. The intercostals muscles and the diaphragm
2. The diaphragm moves down wards, increasing the
volume of the thoracic cavity.
3. The intercostals muscles pull the ribs up, expanding
the ribs and further increasing the volume of the
4. These actions lower the air pressure in the alveoli.
5. Air from the outside then rushes in through the nasal
cavities, trachea and lungs. The lungs expand.
1. The intercostals muscles relax causing the rib
cage to move downwards and inwards.
2. At the same time, the muscles of the diaphragm
relax. The diaphragm curves upwards and returns
to its original dome shape.
3. These action return the thoracic cavity to its
original volume. The air pressure inside the lungs
is now higher than the atmospheric pressure
4. The lungs contract and the air is forced out
through the respiratory tract.
Comparison between inhalation and
Inhalation Aspect Exhalation
Movement of rib
cage and the
wall of the
and is pushed
Relaxes and is
Inhalation Aspect Exhalation
Air pressure in
From the lungs to
1. Name all parts in the human respiratory
2. Trace the flow of inhaled air through the
nostrils to the lungs
cage,diaphragm and the intercostals muscles.
Nasal cavity Trachea
3. Explain how the diaphragm and the
intercostals muscles force air out from the
The diaphragm muscles relax and move up
and at the same time the intercostals muscles
pull the ribs downwards and inwards. These
actions cause the air to flow out of the lungs.
Transport of Oxygen in the
1. Diffusion of oxygen from the
alveolus to the capillaries.
The exchange of gases occurs by
diffusion in the alveoli.
The characteristics enable oxygen
to diffuse through the walls of the
alveoli easily and efficiently
1. Have very large surface areas
2. Thin walls (only one- cell thick).
3. The inner surfaces of the alveoli are always
4. The outer surfaces of the alveoli are
surrounded by a network of blood capillaries.
Inhaled air is rich in oxygen.
The oxygen concentration in
the alveolus higher than the oxygen
concentration in the deoxygenated blood
in the capillaries.
The difference in oxygen concentration
makes the oxygen diffuse easily into
the blood capillaries.
Oxygen in the alveolus diffuses through
the walls into the blood.
Carbon dioxide and water vapour diffuse
from the blood into the alveolus.
Diffusion of oxygen from the
alveolus to the capillaries
2. Oxygen transport
Haemoglobin in the red blood cell
needs to carry oxygen from the lungs
to all parts of the body.
Haemoglobin is a blood pigment.
As the oxygen concentration
is high in the alveolus, oxygen
diffuses into the capillaries.
Oxygen then combines with
haemoglobin in the red blood
cells and forms oxyhaemoglobin.
3. Diffusion of oxygen from the capillaries
to the body cells
When blood reaches the body cells,
oxyhaemoglobin is broken down into
haemoglobin and oxygen.
The oxygen then diffuses through
the walls of the capillaries into the cells.
Oxygen is then used to oxidise
food to release carbon dioxide during
At the same time,carbon dioxide from the cells
diffuses into the capillaries to be carried back
to the lungs.
1. What process is involved in the exchange of
gases in the alveolus?
2. Which gases are exchanged in the
The movement of gases through the wall of
the alveolus is known as diffusion
Oxygen and carbon dioxide
3. What are the two characteristics of the
alveolus which enable it to function
4. Why are the alveoli in the lungs surrounded
by blood capillaries?
i. Has a very large surface area.
ii. Has a thin wall which is one-cell thick.
This is to allow oxygen in the alveolus to enter the blood
in the capillaries and carbon dioxide from the blood in the
capillaries to diffuse into the alveolus.
The importance of a healthy
1. Effects of harmful substances:
i. Nicotine in cigarettes cause
the following effects:
a) Stimulates the production of cells
in the trachea and lungs
and leads to lung cancer.
a) Narrows and hardens the
blood vessels. This affects blood flow
and causes heart attacks.
ii. Tar in tobacco is carcinogenic and can cause lung cancer.
iii. Nitrogen dioxide in cigarette smoke forms an acid which can
destroy lung tissues.
iv. Sulphur dioxide is very soluble in the alveoli and it forms an
acid which destroys the lungs.
v. Carbon monoxide is a toxic gas,
combines with haemoglobin in the
red blood cells and prevents oxygen
from combining with haemoglobin.
ii. Dust and dirt can hinders the
exchange of gases.
2. Diseases of the respiratory system:
Air pollution, the presence of harmful
microbes in air, smoking and crowded cities
cause respiratory diseases. Examples
include asthma, influenza, pneumonia,
tuberculosis, bronchitis, emphysema and
Asthma: condition in which the tubes of the
lungs become inflamed. This is because the
air tubes are narrower and partially blocked.
Diseases of the respiratory system
Influenza: cause by viruses which attack the
mucus membranes in the respiratory system.
Symptoms: Blocked nose, teary eyes,
headache, aches in the limbs, cough and
Pneumonia: caused by bacteria, viruses and
chemical substances in polluted air. The
lungs are filled with pus and fluid.
Symptoms: chest pain, fever and cough.
Tuberculosis: caused by bacterial infection
which are carried by water droplets and dust
in the air.
Symptoms: prolonged cough and spit out
blood in the end stages.
Bronchitis: caused by viral infections.
Symptoms: cough, produce a lot of mucus,
colds and phlegm. Makes respiration and
gaseous exchange difficult.
Emphysema: This disease is linked to
smokers and people who work in dust areas.
The patient experiences breathing difficulties.
Lung cancer: Cause by smoking cigarettes.
Air polluted with carcinogen from factories
and vehicle emissions, dust and asbestos
also cause lung cancer.
3. Improving Air Quality
Pollution is mainly caused by irresponsible
human behavior and improperly planned
We must be responsible for preserving and
conserving air quality.
Air pollution interferes with the respiratory
It can cause respiratory diseases which can be
To preserve and conserve the quality
of air we must:
1. Support and campaign against open burning
2. Give up the habit of smoking.
3. Reduce the number of vehicles
on the road.
4. Avoid the burning of forest
for agriculture, housing and industries.
1. Name 4 respiratory diseases caused by
pollutants in the air.
2. Why does a person feel sick and dizzy after
inhaling carbon monoxide?
Asthma, bronchitis, emphysema and lung
Carbon monoxide combines with haemoglobin in
the blood.This prevent oxygen from combining
with haemoglobin.Oxygen cannot be transported
to the body cells that need it.This makes a
person feels sick and dizzy.
3. Name four dangerous chemical substances
4. State two human activities which can help to
reduce air pollution.
Nicotine,tar,carbon monoxide and hydrogen
Stop open burning of rubbish and use public
transport to help reduce pollutants and keep
the air clean.