3.4 BREATHING SYSTEM AND EXCRETION
3.4.4 The Breathing System in the Human
Macrostructure and basic function of the breathing tract in humans.
The function of the breathing system (lungs) is to allow gas exchange – to allow oxygen uptake and to
excrete carbon dioxide
Part
Nasal Cavity
Buccal Cavity & Pharynx
Epiglottis
Glottis
Larynx (Voice box)
Trachea (Windpipe)
Bronchus (Plural = Bronchi)
Bronchiole
Alveolus (Air sac)
Rings of cartilage
Pleural membranes
Ribs
Intercostal muscles
Diaphragm
Function
Clean, warm and moisten the air
Allow air to pass in and out of breathing system
Prevent food entering the trachea (windpipe) during swallowing
Let air in and out of the trachea (windpipe)
Produce sound
Carry air in and out of lungs
Carry air from trachea to bronchiole
Carry air from bronchus to alveolus
Allow gas exchange with blood
Keep trachea open
Protect and lubricate the lungs (allow friction free movement)
Protect the lungs
Move rib cage up and outwards during breathing
Take part in inhalation
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.4 BREATHING SYSTEM AND EXCRETION
Essential features of the alveoli and capillaries as surfaces over which gas exchange takes
place.
Gas exchange occurs by diffusion.
Carbon dioxide (in the plasma) and water vapour diffuse out from a region of high concentration to a
region of low concentration in the alveolus and are exhaled.
Inhaled oxygen diffuses form a region of high concentration in the alveolus to a region of low
concentration in the red blood cells.
Adaptations of the alveoli for gas exchange
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Large surface area – the alveoli are numerous allowing maximum diffusion of gases
Thin walls – the walls are one cell thick so gases diffuse quickly
Good blood supply – the alveoli are covered in capillaries which maximises the amount of
oxygen uptake and carbon dioxide excretion
Elastic walls – this allows expansion to breathe in gases and recoil allows exhalation
Layer of moisture – moisture allows the gases to dissolve and diffuse into or out of blood
quickly
Adaptations of capillaries for gas exchange
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Large surface area – the capillaries are numerous allowing maximum diffusion of gases
Thin walls –the walls are one cell thick so gases diffuse quickly
Slow blood flow – allows time for oxygenation of the blood and carbon dioxide excretion
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.4 BREATHING SYSTEM AND EXCRETION
Role of haemoglobin in oxygen transport
Oxygen is carried by haemoglobin in the red blood cell. The oxygen combines with the iron containing
haemoglobin (dull red) to form oxy-haemoglobin (bright red).
Haemoglobin + O2

Oxyhaemoglobin
Source of carbon dioxide from the plasma and water vapour exhalation.
Carbon dioxide is carried dissolved in the plasma of the blood. Plasma is the liquid
part of the blood and contains water. The carbon dioxide and water vapour diffuse
from the plasma into the alveolus and are breathed out.
Description of the mechanism of the breathing system in the exchange of gases in humans.
Respiratory centres in the medulla oblongata of the brain control the rate of breathing. A message is
sent via nerves to the intercostal muscles and diaphragm.
Inhalation (Inspiration)
 The intercostal muscles contract
 The ribs move up and outwards
 The diaphragm contracts and moves down
 The air cavity (thoracic cavity) gets larger
 Air rushes in because it is at a higher pressure outside than inside
 It is an active process, requiring energy and involving muscles
Exhalation (Expiration)
 The intercostal muscles relax
 The ribs move down and inwards
 The diaphragm relaxes and moves up
 The air cavity (thoracic cavity) gets smaller
 Air rushes out because it is at a lower pressure outside than inside
 It is a passive process, requiring no energy and not involving muscles.
Breathing disorder: Asthma - one possible cause, prevention, and treatment.
Asthma is a breathing disorder where the symptoms are noisy wheezy breathing
Cause
The airways narrow (especially the bronchioles) when allergens like animal dander, pollen, house dust
and dust mites are inhaled.
Prevention
Clean air and identify and avoid the allergens that cause attacks
Treatment
Use inhalers with bronchodilator drugs
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
3.4 BREATHING SYSTEM AND EXCRETION
H.3.4.7 Carbon Dioxide: A Controlling Factor in Gaseous Exchange
Respiratory centres in the medulla oblongata of the brain control the rate of breathing. These cells are
sensitive to the levels of carbon dioxide in the blood. When carbon dioxide levels are high and the
blood is getting acidic, it sends a message to the intercostal muscles and the diaphragm to contract and
fresh air is breathed in. Vigorous exercise causes the carbon dioxide to rise faster than normal so the
brain stimulates the breathing to become faster and deeper. This means that carbon dioxide levels in the
blood control the rate of breathing.
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Recovery Time
This is the length of time it takes for the rate of breathing to return to normal after exercise. It can be a
sign of cardiovascular fitness. The breathing rate returns to normal faster in fitter individuals. After
exercise the rate of breathing often falls below the resting rate. This is due to deeper breaths.
It is difficult to accurately measure the effect of exercise on breathing as there is a certain degree of
voluntary control involved.
Comparing Inhaled Air and Exhaled Air
Gas
Inhaled
Exhaled
Nitrogen
79%
79%
Oxygen
21%
16%
Carbon Dioxide
0.03%
4.0%
Water vapour
Varies
Saturated
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork