Breathing

Structure of the human gas exchange system

Lungs

  • Pair of lobed structures made of

    • highly branched tubules (bronchioles)

    • Tiny air sacs (alveoli)

  • Site of gas exchange

  • supported and protected by the rib cage

  • ventilated by a tidal stream of air

    • ensures air within lungs is constantly replenished

Trachea

  • The tube that carry’s air from mouth/nose to lungs

  • made of

    • flexible cartilage rings

      • stops the trachea from collapsing under the air pressure

    • muscle

      • lined with cilliated epithelium and goblet cells

Bronchi

  • 2 divisions of the trachea

    • each going to one lung

  • produce mucus to trap dirt and debris that have entered with the air

  • have cilia to move the dirt-laden mucus up to the throat

  • the larger bronchi are supported by cartilage

    • amount of cartilage decreases as bronchi gets smaller

Bronchioles

  • series of branching subdivisions of the bronchi

  • made of muscle lined with epithelial cells

    • allows them to constrict to control airflow in and out of alveoli

Alveoli

  • minute air sacs at the end of bronchioles

  • site of gas exchange

    • alveolar membrane= gas exchange surface

  • between the alveoli there are collagen and elastic fibres which allow the alveoli to stretch

    • expand during inhalation

    • constrict during exhalation

  • lined with epithelium

  • Easily damaged

    • stored within the organism to protect them

Diaphragm

  • sheet of muscle

  • separates thorax from abdomen

Intercostal muscles

  • muscles between ribs

  • internal intercostal muscle (IIM)

    • contraction leads to expiration

  • External intercostal muscle (EIM)

    • contraction leads to inhalation



Inspiration (Breathing in)

  • Active process (requires energy)

  • Breathing in occurs as follow:

    • The external intercostal muscles contract, while the internal intercostal muscles relax.

    • The ribs are pulled upwards and outwards, increasing the volume of the thorax.

    • The diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax.

    • The increased volume of the thorax results in reduction of pressure in the lungs.

    • Atmospheric pressure is now greater than pulmonary pressure, and so air is forced into the lungs .



Expiration (breathing out)

  • Largely passive process (does not require much energy)

  • Breathing out occurs as follow:

    • The internal intercostal muscles contract, while the external intercostal muscles relax.

    • The ribs move downwards and inwards, decreasing the volume of the thorax.

    • The diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration. The volume of the thorax is therefore further decreased.

    • The decreased volume of the thorax increases the pressure in the lungs.

    • The pulmonary pressure is now greater than that of the atmosphere, and so air is forced out of the lungs.