Gas exchange

• Lungs

  Function: Get the oxygen that we need from the air all around us into our bloodstream where it can be transported to the rest of the body
• Air flow through the lungs

  1. Mouth/nose

  2. Trachea

  3. Bronchi

  4. Bronchioles

  5. Alveoli
• Alveoli


  • Small sacks arranged like bunches of grapes
  • Site of gas exchange
• Alveolus

  • One layer of very thin cells
  • Large surface area
  • Moist walls
• Oxygen concentration in alveoli

  Higher than in blood capillaries
• Carbon dioxide concentration in blood

  Higher than in alveoli
• Carbon dioxide is dissolved in blood plasma, not carried by red blood cells
• Gas exchange happens continually with a constant supply of deoxygenated blood entering capillaries and oxygenated blood leaving
• Breathing rate

  Calculated as number of breaths taken divided by time in minutes
• Breathing rate changes during exercise
• Topics covered in the lesson

  • Structures of the respiratory system
  • Features of gas exchange surfaces
  • Differences between inspired and expired air
  • Effects of physical activity on rate and depth of breathing
  • Role of respiratory muscles in breathing
  • Effects of physical activity on rate and depth of breathing
  • Ways the respiratory system protects itself from pathogens and particles
• Role of the breathing system

  Move air into and out of the lungs so that oxygen needed for respiration can diffuse into the bloodstream and carbon dioxide can diffuse outwards
• Main structures of the respiratory system

  • Lungs
  • Diaphragm
  • Intercostal muscles
  • Mouth/nasal passages
  • Larynx
  • Trachea
  • Bronchi
  • Bronchials
  • Alveoli
• Alveoli

  • Vast number and specialized shape maximizes the area of the exchange surface
  • Excellent blood supply as each alveolus is surrounded by a network of capillaries
  • Walls of the alveoli and the capillaries that surround them are only one epithelial cell thick, reducing the distance over which gases must diffuse
  • Well ventilated by pulmonary ventilation (breathing)
• Breathing in

  1. Air passes through mouth/nasal passages

  2. Through larynx

  3. Into trachea

  4. Trachea divides into bronchi

  5. Bronchi divide into bronchials

  6. Air reaches alveoli
• Lime water in tube A (inspired air)

  Remains clear
• Lime water in tube B (expired air)

  Turns milky, indicating presence of carbon dioxide
• Differences between inspired and expired air

  • Expired air contains more carbon dioxide and less oxygen than inspired air
  • Expired air is saturated with water vapor, while inspired air varies depending on atmospheric humidity
• Investigating effects of physical activity on breathing

  1. Subject breathes in and out of spirometer

  2. At rest

  3. During exercise
• Depth of breathing (tidal volume)

  Indicated by amplitude/height of spirometer trace
• Breathing rate

  Indicated by frequency of spirometer trace
• During exercise

  Breathing rate and tidal volume increase to meet higher demand for oxygen in working muscles
• Breathing in

  1. Diaphragm contracts and flattens

  2. External intercostals contract to expand ribcage

  3. Increases volume of thorax and lungs

  4. Causes air pressure within to drop below atmospheric pressure

  5. Air rushes in through nose and trachea
• Breathing out forcibly (e.g. during exercise)

  1. Diaphragm relaxes and domes upwards

  2. Internal intercostal muscles contract to pull ribcage downwards and inwards

  3. Decreases volume of thorax and lungs

  4. Increases air pressure within lungs

  5. Air forced outwards
• Trachea and bronchi lined with rings of cartilage to prevent airways from collapsing
• Inspired air

  Contains around 21% oxygen and 0.04% carbon dioxide
• Expired air

  • Contains less oxygen and significantly more carbon dioxide than inspired air
  • Saturated with water vapor
• During exercise

  • Rate of reaction increases
  • Carbon dioxide accumulates in muscle cells and diffuses into blood plasma more rapidly
  • Brain detects rising carbon dioxide in blood
  • Stimulates breathing muscles to contract more rapidly and forcefully
  • Increases rate of carbon dioxide expiration and oxygen inspiration
• Alveoli

  • Extremely delicate and vulnerable to physical damage and infection
• Respiratory system protection from pathogens and particles

  1. Goblet cells in trachea, bronchi and larger bronchials secrete mucus to trap pathogens and dust

  2. Ciliated cells in epithelium carry mucus up towards top of trachea to be swallowed
• Lungs


  • Highly adapted organs that allow mammals to exchange gases between the environment and their blood
• Gas exchange in the lungs


  1. Trachea carries air from environment to lungs

  2. Trachea splits into left and right bronchi

  3. Bronchi split into bronchioles

  4. Bronchioles end in alveoli

  5. Millions of alveoli in each lung
• Thorax


  • Contains the two lungs
  • Ribs protect the lungs
  • Intercostal muscles involved in breathing
  • Diaphragm separates thorax from abdomen and involved in breathing
• Pleural membranes


  • Surround the thorax cavity and provide an airtight seal and lubrication
• Major lung structures


  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli
  • Ribs
  • Intercostal muscles
  • Diaphragm
  • Pleural membranes
• Ventilation


  The process of breathing in (inhalation) and breathing out (exhalation)
• Inhalation


  1. Intercostal muscles contract

  2. Ribs move up and out

  3. Diaphragm contracts and flattens

  4. Chest volume increases

  5. Air pressure decreases, drawing air in
• Exhalation


  1. Intercostal muscles relax

  2. Ribs move down and in

  3. Diaphragm relaxes and moves up

  4. Chest volume decreases

  5. Air pressure increases, forcing air out