Chapter 35: The human
breathing system
Leaving Certificate Biology
Higher Level
Macrostructure of the Human
Breathing System
• Nasal and buccal cavities:
– Warm and moisten air entering lungs
– Mucus and small hairs filter the air and then
transport the dirt-loaded mucus to the pharynx
where it is swallowed
• Pharynx (throat):
– Junction between oesophagus and windpipe
(trachea)
– Pharynx has a sphincter (epiglottis) that closes
over the opening to the trachea (glottis) that
prevents food travelling into the trachea
Macrostructure of the Human
Breathing System
• Glottis:
– Opening to the trachea
• Epiglottis:
– Sphincter that closes over the glottis to prevent
food getting into the trachea during swallowing
– Swallowing causes the vocal cords to pull on the
glottis and the larynx to be pulled upwards
thereby closing the epiglottis over the glottis
– Put your finger on your larynx and swallow
Macrostructure of the Human
Breathing System
• Larynx (voice box):
– Made of cartilage and sits on top of the trachea
– Three functions:
• Produces sound
• Controls air flowing into and out of the trachea
• Directs food into oesophagus
• Trachea (windpipe):
– Directs inhaled air into the lungs
– Contains c-shaped rings of cartilage that keeps
the trachea open
– Cilia of trachea carry dirt-laden mucus up to the
pharynx
Macrostructure of the Human
Breathing System
• Bronchi:
– Two divisions of the trachea
– Directs air into respective lung
– Supported by cartilage
• Bronchioles:
– Tiny divisions of the bronchi
– Air passages that are less then 1 mm in diameter
– Not supported by cartilage
Macrostructure of the Human
Breathing System
• Alveoli:
– Tiny air sacs at the end of the bronchioles where
gas exchange occurs
– Walls of alveoli are only 1 cell thick to maximise
diffusion
– Each alveolus has rich blood supply as many
capillaries surround each
– There are ~700 million alveoli with a total surface
are of 90 m2 (surface area is increased in people
who take part in high intensity sports)
Essential Features of Alveoli and
Capillaries
•
•
•
•
•
•
Alveoli are extremely numerous
Alveoli have rich blood supply
Alveoli have walls only one-cell thick
Alveoli surface is moist
Alveoli walls are elastic
Capillaries that surround each alveolus are
only one-cell thick
Gaseous
Exchange
O2
Exchange of
gases in lungs
Alveolus
Exchange of
gases in alveolus
• Gaseous
exchange occurs
by diffusion.
Heart
Exchange of
gases between
Energy
Cellular
blood and cells
CO
+
2
respiration
Glucose
+ H2O
Transport of Gases
• Inhaled O2:
• Exhaled O2:
21% (atmospheric oxygen)
16%
• Inhaled CO2:
• Exhaled CO2:
0.04%
4%
• Inhaled H2O(g):
• Exhaled H2O(g):
variable
100% humidity
Transport of Gases
• Oxygen is transported mostly (97%) by
haemoglobin as oxyhaemoglobin
• Remaining oxygen (3%) is carried dissolved in
solution by the plasma
• Carbon dioxide is transported mostly (80%) by the
plasma as either hydrogen carbonate ions, HCO3–
(70%) or as dissolved carbon dioxide (10%)
• Remaining carbon dioxide (20%) is carried by the
haemoglobin in red blood cells
Macrostructure of the Human
Breathing System
• Lungs:
– Composed of spongy, elastic tissue that expands
easily during inhalation and recoils rapidly as
exhalation occurs
• Pleural membranes:
– Thin pair of membranes covering and separating
the lungs from other organs, such as the heart
– The lungs are stuck to the rib cage and
diaphragm by the pleural fluid (think of a layer of
water between a table and a piece of glass and
how difficult it is to lift it off the table)
Macrostructure of the Human
Breathing System
• Rib cage:
– Composed of 12 thoracic vertebrae, 12 ribs, and
the sternum
– Muscles are located between each rib – called
intercostal muscles that contract causing the rib
cage to move upwards and outwards, drawing
air into the lungs
Mechanism of Breathing
• Inhalation:
– Active process where the brain sends signal to
the inspiratory muscles (intercostals and
diaphragm) to contract
– Rib cage expands upwards and outwards and
the diaphragm pulls downwards
– The movements of the rib cage and the
diaphragm reduce the pressure within the
thoracic cavity and air rushes in
– Inhalation can be consciously and subconsciously (during sleeping) controlled
Mechanism of Breathing
• Exhalation:
– Passive process where there is normally no signal sent
to the inspiratory muscles
– Can be an active process during strenuous activity when
the brain send signal to the abdominals to contract
forcibly expelling air from the thoracic cavity
– During exhalation intercostals and diaphragm relax
– Rib cage moves down and inwards and the diaphragm
pushed upwards
– The movements of the rib cage and the diaphragm
during exhalation increase the pressure within the
thoracic cavity and air rushes out
Carbon Dioxide and Breathing
Control (Higher Level Ext.)
• Carbon dioxide dissolved in the blood is the
most powerful stimulant for an increase in
the rate of breathing
• Receptors in the brain sense the levels of
carbon dioxide in the blood and respond by
increasing or decreasing the rate and depth
of breathing
Effect of Exercise on Breathing
Rate
• Exercise stimulates increased respiration
which produces more carbon dioxide which
diffuses into the bloodstream
• The brain is extremely sensitive to changes
in the carbon dioxide concentration within the
bloodstream and acts on this by increasing
breathing rate and heart rate to excrete the
excess carbon dioxide
Breathing Disorders
• Asthma:
– One possible cause: immune reaction to an external
allergen (e.g. pollen)
– Symptoms: difficulty breathing (‘asthma attack’) due to
constriction of the airways
– One possible preventative measure: avoid the allergen
(e.g. pollen) by avoiding area where the allergen is
present in high quantities
– One possible treatment: most common treatment for
asthma is the inhaler that has drugs in it that stimulate
the airways (bronchi and bronchioles) to widen and dilate
Breathing Disorders
• Bronchitis:
– One possible cause: smoking, air pollution, dust, viral
infection, bacterial infection
– Symptoms: laboured breathing, episodes of constant
coughing, excessive production of mucus and inflamed
airways
– One possible preventative measure: do not smoke,
avoid second-hand smoke, pollutants and dust
– One possible treatment: stop smoking, avoid polluted
air, use of bronchiodilating drugs or antibiotics if
pathogenic bacteria are the cause