human gas exchange

• the volume of oxygen that has to be absorbed and the volume of carbon dioxide that must be removed are large in animals because:

  • they are relatively large organisms with a large volume of living cells
  • they maintain a high body temperature which is related to them having high metabolic and respiratory rates
• why are lungs located inside of the body

  • air is not dense enough to support and protect their delicate structure
  • the body as a whole would otherwise lose a large amount of water and dry out
• main parts of the human gas exchange system

  • lungs
  • trachea
  • bronchi
  • bronchioles
  • alveoli
• structure and functions of the lungs 


  they are a pair of lobed structures made up of a series of highly branched tubules, called bronchioles, which end in tiny air sacs called alveoli
• structure and functions of the trachea


  this is a flexible airway supported by rings of cartilage that prevents the trachea collapsing as the air pressure inside falls when breathing in. the tracheal walls are made up of muscle, lined with ciliated epithelium and goblet cells
• structure and functions of the bronchi


  these are two divisions of the trachea, each leading to one lung. they are similar in structure to the trachea and also produce mucus to trap dirt particles and have cilia that move the dirt-laden mucus to the throat. the large bronchi are supported by cartilage, although the amount of cartilage is reduced as the bronchi get smaller
• structure and functions of the bronchioles


  a series of branching subdivisions of the bronchi. their walls are made of muscle lined with epithelial cells. this muscle allows them to constrict so that they can control the flow of air in and out of the alveoli
• structure and functions of the alveoli 


  these are minute air sacs at the end of the bronchioles, and lined with epithelium. between them there are some collagen and elastic fibres. the elastic fibres allow the alveoli to stretch as they fill with air when breathing in. they then spring back when breathing out to expel the carbo dioxide rich air. the alveolar membrane is the gas-exchange surface
• the pressure changes in the lungs are bought about by the movement of 3 sets of muscles:

  • the diaphragm, which is a sheet of muscle that separates the thorax from the abdomen
  • the internal intercostal muscles muscles between the ribs, whose contraction leads to expiration
  • the external intercostal muscles between the ribs, whose contraction leads to inspirations
• inspiration 


  breathing in is an active process and occurs as follows:

  • external intercostal muscles contract - internal intercostal muscles relax
  • ribs are pulled upwards and outwards - increased thorax volume
  • diaphragm contracts - flattens, - increased thorax volume
  • increased thorax volume - reduced pressure in the lungs
  • greater atmospheric pressure than pulmonary pressure - air forced into lungs
• expiration 


  breathing out is a largely passive process and occurs as follows:

  • internal intercostal muscles contract - external intercostal muscles relax
  • ribs move downwards and inwards - decreased thorax volume
  • diaphragm relaxes - pushed up against contents of the abdomen - thorax volume further decreased
  • decreased thorax volume - increased pressure in lungs
  • pulmonary pressure now greater than atmospheric pressure - air forced out of lungs
• diffusion of gases between the alveoli and the blood will be very rapid because

  • red blood cells are slowed in pulmonary capillaries, so more time for diffusion
  • short distance between alveolar air and blood as red blood cells are flat against capillary walls
  • alveoli and capillary walls are thin so short diffusion pathway
  • alveoli and pulmonary capillaries have large surface area
  • breathing constantly ventilates lungs, heart constantly circulates blood around alveoli. ensures concentration gadient is maintained
  • blood flow through pulmonary capillaries maintains a concentration gradient