3.1

• • All cells need a supply of ​oxygen and nutrients to ​survive.​
  • They also need to remove ​waste products, so they do ​not build up and become toxic.​
  • For some organisms, these substances simply diffuse over the surface of the body.​
  • Other organisms require specialised exchange surfaces.​
• Exchange surface = a specialised area that is adapted to make it easier for molecules to cross from one side of the surface to another.​
• There are 3 main factors that affect the need for an exchange system:​

  1. Size​
  2. Surface area to volume ratio​
  3. Level of activity​
• • In very small organisms (e.g. single celled), all the cytoplasm is very close to their environment.​
  • Simple diffusion is enough.​
  • Multicellular organisms may have several layers of cells. The bigger the organism, the greater the distance substances need to travel to reach the centre.​
  • This makes the diffusion distance too long.​
  • Diffusion alone would be insufficient to supply the innermost cells.​
• • Small organisms have a small surface area, but they also have a very small volume.​
  • Compared with their volume, their surface area is relatively large. ​
  • They have a large surface area to volume ratio.​
  • This means that they have a large enough surface to supply the organism with the resources it needs, and for waste products to diffuse out.​
• • Although large organisms have a larger surface area, their volume is significantly greater.​
  • As size increases, the volume rises more quickly than the surface area.​
  • Therefore, large organisms have a small surface area to volume ratio.​
  • This makes it even harder to absorb enough oxygen for the needs of the body and remove waste products.​
• Metabolic rate = the amount of energy transferred by that organism within a given period of time.​
• Metabolic activity requires oxygen to release energy from food in aerobic respiration.
• • Larger organisms tend to have a higher metabolic rate, so need to exchange lots of materials fast.​
  • Organisms that are very active need to supply their cells with lots of nutrients and oxygen to supply the energy for movement.​
  • This need for energy is increased in animals, such as mammals, that keep themselves warm.​
• To increase the rate of diffusion, good exchange surfaces have:​

  • A large surface area to overcomes the limitations of the SA:V ratio of larger organisms. This provides more space for molecules to pass through. It is often achieved by folding the walls and membranes involved.​
  • A thin barrier to provide a short diffusion distance. The barrier must be permeable to the substances being exchanged.​
  • A good blood supply and/or ventilation to maintain a steep concentration gradient.​
• Alveoli


  • Large numbers of small, spherical
  • Increase the surface area
• Alveolar walls

  • Made of squamous epithelium, which consists of flattened cells
  • One cell thick, which reduces the diffusion distance
• Alveoli

  • Have a good blood supply to maintain the concentration gradient
• Alveoli ventilation

  1. Constantly supply oxygen

  2. Remove carbon dioxide

  3. Maintains the concentration gradient
• Alveoli


  • Warm, so the rate of diffusion stays high
• Alveoli

  • Contain elastic fibres
  • Allow the alveoli to stretch - increasing the surface area
  • Allow the alveoli to recoil - helping to force air out, thereby maintaining the concentration gradients
• Inner surface of alveoli

  • Coated in lung surfactant
• Lung surfactant

  • A phospholipid that coats the surfaces of the lungs
  • Reduces cohesion between water molecules, preventing the alveoli from collapsing
  • Further increases the surface area
• Elastic recoil = the ability to return to original shape and size following stretching.​
• • Air can pass into the lungs through the nose and along the trachea (windpipe), bronchi, and bronchioles.​
  • Finally, it reaches tiny air-filled sacs called alveoli.​
  • These are the surfaces where the exchange of gases takes place – oxygen diffuses into the blood and carbon dioxide diffuses out.​
• The gaseous exchange system in mammals consists of the lungs and associated airways that carry air into and out of the lungs.
• The lungs are a pair of inflatable sacs lying in the chest cavity.​
• Each of the lungs is enclosed in a double membrane known as the pleural membrane.​
• The space between the two membranes is called the pleural cavity and is filled with a small amount of pleural fluid.
• • Pleural fluid lubricates the lungs.​
  • It also adheres to the outer walls of the lungs to the thoracic (chest) cavity by water cohesion, so that the lungs expand with the chest while breathing.​
• • Land animals face a constant battle between gas exchange and retaining water.​
  • The inner surface of the alveoli is covered in a thin layer of a solution of water, salts, and lung surfactant. This surfactant reduces the cohesive forces between the water molecules, making it possible for the alveoli to remain inflated.​
  • Oxygen dissolves in the water, then diffuses into the blood.​
  • However, water can also evaporate into the air of the alveoli and is lost as we breathe out.​
• Nasal Cavity

  • Large surface area and good blood supply – this warms the air as it passes into the body.​
  • Hairy lining – hairs trap dust and bacteria in mucus and prevent them from reaching the lungs, which could cause infection.​
  • Moist surfaces – increases the humidity of the incoming air, this reduces the evaporation of water in the lungs.​

  After passing through the nasal cavity, the air entering the lungs is a similar temperature and humidity to the air already there.
• Trachea

  • The trachea is the airway that leads from the mouth and nose to the bronchi.​
  • It supported by a layer of cartilage ​that holds the trachea open and ​prevents it from collapsing.
  • The rings are incomplete ​to allow it the bend when ​food is swallowed down the ​oesophagus behind.​Gaps between the cartilage filled by smooth muscle and elastic fibres.​
  • The cartilage, smooth muscle, and elastic fibres hold the trachea open but allow flexibility during inspiration and expiration.​
• The trachea is lined with:​

  • Goblet cells, which secrete mucus.​
  • Ciliated epithelial ​cells, which move ​the mucus (along ​with trapped dust ​and bacteria) away from the lungs.​
• The bronchi are extensions of the trachea that split into two for ​the left and right lung
• Bronchus

  • about 20mm in ​diameter.​
  • Bronchi have a very similar structure to the trachea but smaller.​
  • Cartilage rings hold the pipe open
• • The bronchus split into much smaller tubes called bronchioles.​
  • These are about 1 mm or less in diameter.​
• • All bronchioles contain smooth muscle, which holds them open.​
  • When this smooth muscle contracts, the bronchioles constrict. This reduces the air flow to the lungs.​
• • Larger bronchioles contain plates of cartilage.​
  • They also have goblet cells and ciliated epithelium to trap dust and microorganisms.
• • The smaller bronchioles do not have cartilage, goblet cells, or ciliated epithelium.​
  • They have clusters of alveoli at the end.​
  • The bronchioles are lined with a thin layer of epithelial tissue, making some gas exchange possible.​
• Active inhalation

  • Diaphragm contracts and flattens.​
  • External intercostal muscles contract, moving the ribcage up and out.​
  • Thoracic volume increases.​
  • Thoracic pressure decreases.​
  • Air flows into lungs (to equalise the pressure difference).​
• Passive Exhalation

  • Diaphragm relaxes and curves up.​
  • External intercostal muscles relax, moving the ribcage down and in.​
  • Thoracic volume decreases.​
  • Thoracic pressure increases.​
  • Air flow – out the lungs (to equalise the pressure difference) ​
• Active Exhalation

  • Internal intercostal muscles contract, pulling the ribcage down and in.​
  • Abdominal muscles contract: moving the diaphragm up.​
  • Thoracic volume decreases hard and fast.​
  • Thoracic pressure increases rapidly.​
  • Air flows out of the lungs.​
• • A peak flow meter is a simple device that measures the rate at which air can be expelled from the lungs – forced exhalation.​
  • These are often used by asthmatics to monitor how well their lungs are working.​
• • A vitalograph is a more sophisticated versions of a peak flow meter.​
  • The patient breathes out as quickly as they can, and a graph is produced.​
  • This volume of air is called the forced expiratory volume in 1 second.​