Human Gas Exchange

• Why is air moved constantly in and out of the lungs?

  To maintain diffusion of gases across the alveolar epithelium
• What is ventilation?

  Breathing, which involves air being moved constantly in and out of the lungs
• What occurs during inspiration?

  Inhalation occurs when the air pressure of the atmosphere is greater than the air pressure inside the lungs
• What happens during expiration?

  Exhalation occurs when the air pressure in the lungs is greater than that of the atmosphere
• What happens during inspiration in terms of diaphragm, intercostal muscles, ribs, volume of thorax, and pressure?

  • Diaphragm muscles contract, flattening the diaphragm
  • External intercostal muscles contract, moving ribs upwards and outwards
  • Volume of thorax increases, reducing lung pressure
  • Pressure of thorax becomes less than outside, forcing air into the lungs
• What happens during expiration in terms of diaphragm, intercostal muscles, ribs, volume of thorax, and pressure?

  • Diaphragm muscles relax and dome up
  • External intercostal muscles relax, ribs move downwards and inwards
  • Volume of thorax decreases, increasing lung pressure
  • Pressure of thorax becomes more than outside, forcing air out of the lungs
• What is the main cause of air being forced out of the lungs?

  The recoil of the elastic tissue in the lungs
• Which process is active and which is passive in respiration?

  Inspiration is active, while expiration is passive
• Why do mammals need large volumes of oxygen and CO2 removal?


  • Mammals are relatively large organisms with a large volume of living cells
  • They maintain a high body temperature related to high metabolic and respiratory rates
• Describe the structure and adaptations of the trachea.

  • Flexible airway supported by rings of cartilage
  • Prevents collapsing as air pressure falls
  • Walls made of muscle lined with ciliated epithelium and goblet cells
  • Goblet cells produce mucus to trap dirt particles
  • C-shaped cartilage provides flexibility
• What is the bronchi and its structure and adaptation?

  • Two divisions of the trachea
  • Produce mucus to trap dirt particles
  • Cilia move dirt-laden mucus towards the throat
  • Supported by cartilage to prevent collapsing
• What is the bronchioles and its structure and adaptation?

  • Series of branching subdivisions of the bronchi
  • Walls made of muscle lined with epithelial cells
  • Muscle allows constriction to control air flow in and out of alveoli
• What are the alveoli and their structure and adaptation?

  • Small air sacs at the end of the bronchioles
  • Collagen prevents collapsing; elastic fibres allow stretching
  • Lined with epithelium
  • Spring back during exhalation to expel carbon dioxide-rich air
• What are the adaptations of the alveoli for gas exchange?

  • Red blood cells are slowed in pulmonary capillaries for more diffusion time
  • Distance between alveolar air and red blood cells is reduced
  • Very thin walls of alveoli and capillaries shorten diffusion distance
  • Large total surface area for gas exchange
  • Breathing movements and heart action maintain steep concentration gradient
  • Blood flow through pulmonary capillaries maintains concentration gradient
• Describe the path by which oxygen goes from an alveolus to the blood.

  Oxygen diffuses from a high concentration in the alveolus through the alveolar epithelium and capillary endothelium to the blood where the concentration is low
• How does ventilation help maintain the difference in oxygen concentration between the alveoli and lung capillaries?

  • Ventilation removes gas with low oxygen and high carbon dioxide from alveoli
  • Replaces it with fresh air that has high oxygen and low carbon dioxide