Human Breathing System

Structure and function of organs of the Human Breathing System

Structure of the human breathing system

Nasal and buccal cavities:

  • Mouth and internal areas of the nose
  • Function in warming and moistening 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):

  • Area 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


  • Opening to the trachea


  • 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

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 the 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 the pharynx


  • Two divisions of the trachea
  • Directs air into each lung
  • Supported by cartilage


  • Tiny divisions of the bronchi
  • Air passages that are less then 1 mm in diameter
  • Not supported by cartilage


  • 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)

Rib cage:

  • Composed of 12 thoracic vertebrae, 12 ribs, and the sternum
    • First 7 pairs are called ‘true’ ribs (because they attach directly to the sternum)
    • Next 3 pairs are called ‘false’ ribs (because they are only attached to the sternum by cartilage)
    • Final 2 pairs are called ‘floating’ ribs (because they do not attach to the sternum at all)
  • 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


  • 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 capillary network surrounding it
  • 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)
Structure of alveoli

Essential Features of Alveoli and Capillaries

  • Alveoli are numerous
  • Alveoli have rich blood capillary network nearby
  • Alveoli have walls only one-cell thick
  • Alveoli surface is moist
  • Alveoli walls are elastic
  • Capillaries that surround each alveolus have walls that are only one-cell thick

Gas exchange:

Occurs by diffusion. Water and carbon dioxide generally move outwards (from the blood to the alveolar space). Oxygen generally moves inwards (from the alveolar space into the blood) – see diagram below:

Gas exchange in the alveolus

Transport of Gases

  • Inhaled O2: 21% (atmospheric oxygen)
  • Exhaled O2: 16%
  • Inhaled CO2: 0.04%
  • Exhaled CO2: 4%
  • Inhaled H2O(g): variable
  • Exhaled H2O(g): 100% humidity
  • 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

Mechanism of breathing:


  • 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 moves 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 sub-consciously (during sleeping) controlled


  • Passive process where there is normally no signal sent to the inspiratory muscles
  • Can be an active process during strenuous activity (e.g. intense exercise, coughing, sneezing, etc.) 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 moves upwards
  • The movements of the rib cage and the diaphragm during exhalation increase the pressure within the thoracic cavity and air rushes out
The processes of inhalation and exhalation

Carbon Dioxide and Breathing Control

  • 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 via the lungs

Breathing Disorders


  • 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
A child using an inhaler


  • 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

Practical activity: to investigate the effect of exercise on breathing rate.

  • Breathing rate is measured by counting the number of breaths in one minute. One breath is counted as a combination of one inhalation and one exhalation.
  • Measure breathing rate at rest three times and average. Average breathing rate at rest is between 12 – 20 breaths per minute.
  • Walk slowly or exercise very gently for three minutes and count the number of breaths per minute either during the exercise or at the end.
  • Walk briskly/jog slowly/exercise moderately for three minutes and count the number of breaths per minute.
  • Run/exercise vigorously for a three minutes and once again measure the number of breaths per minute.
  • Record results in a table – see below.
  • Conclusion: exercise increases breathing rate.
Breathing rate (breaths per minute)
Walking/exercising gently
Walking/exercising moderately
Running/exercising vigourously