• The?lungs?are the?gas exchange surface?in humans
• All gas exchange surfaces have features in common which?allow the maximum amount of gases to be exchanged across the surface in the smallest amount of time
• They include:
  • Large surface area?to allow faster diffusion of gases across the surface
  • Thin walls?to ensure diffusion distances remain short
  • Good ventilation with air?so that diffusion gradients can be maintained
  • Good blood supply?to maintain a high concentration gradient so diffusion occurs faster

   

Structures in the human breathing system

Breathing Structures Table

Cilia and mucus

• The passages down to the lungs are lined with?ciliated epithelial cells
• Cilia cells have tiny?hairs?on the end of them that?beat?and?push mucus up the passages towards the nose and throat?where it can be removed
• The?mucus?is made by special mucus-producing cells called?goblet cells?because they are shaped like a goblet, or cup
• The mucus traps?particles, pathogens like bacteria or viruses, and dust?and?prevents them?from getting into the?lungs?and damaging the cells there

Mucus traps particles, dust and pathogens and cilia beat and push it up and away from the lungs

• The alveoli are highly specialised for gas exchange
  • There are many rounded alveolar sacs which give a very?large?surface area to volume ratio
  • Alveoli (and the capillaries around them) have?thin, single layers of cells to minimise diffusion distance
  • Ventilation?maintains high levels of?oxygen?and low levels of?carbon dioxide?in the alveolar air space
  • A?good blood supply?ensures constant supply of blood high in?carbon dioxide?and low in?oxygen
  • A?layer of moisture?on the surface of the alveoli helps diffusion as gases dissolve

   

Alveoli are specifically adapted to maximise gas exchange

• Muscles are only able to pull on bones, not push on them
• This means that there must be?two sets?of intercostal muscles to work antagonistically to facilitate breathing
  • External?intercostal muscles, pull the rib cage up
  • Internal?intercostal muscles?pull the ribcage down

   

• The?diaphragm?is a thin sheet of muscle that separates the chest cavity from the abdomen

There are 2 sets of intercostal muscles: the external, on the outside of the rib cage, and the internal, on the inside of the rib cage

Ventilation

• During inhalation
  • The diaphragm?contracts?and flattens
  • The?external set of intercostal muscles contract?to pull the ribs?up and out:
  • This?increases the volume?of the chest cavity (thorax)
  • Leading to a?decrease in air pressure?inside the lungs relative to outside the body
  • Air is drawn in

   

• During exhalation
  • The diaphragm?relaxes?it moves upwards back into its domed shape
  • The external?set of intercostal muscles relax?so the ribs?drop?down and in
  • This?decreases the volume?of the chest cavity (thorax)
  • Leading to an?increase in air pressure?inside the lungs relative to outside the body
  • Air is forced out

   

Changes in the thorax during ventilation

Forced Exhalation

• The external and internal intercostal muscles work as?antagonistic?pairs (meaning they work in different directions to each other)
• When we need to increase the rate of gas exchange (for example during strenuous activity) the internal intercostal muscles will also work to?pull the ribs down and in?to decrease the volume of the thorax more, forcing air out more forcefully and quickly – this is called?forced exhalation
  • There is a greater need to rid the body of increased levels of carbon dioxide produced during strenuous activity

   

• This allows a?greater volume of gases to be exchanged

The differences between inspired air and expired air during ventilation