Gas exchange in humans

• Gas exchange surfaces


  • Large surface area to allow faster diffusion of gases
  • 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
• The surfaces where gas exchange occurs in an organism are very different and different organisms have evolved different mechanisms for getting the gases to the gas exchange surface depending on size, where they live etc.
• All gas exchange surfaces have features in common that allow the maximum amount of gases to be exchanged across the surface in the smallest amount of time
• Alveolus


  The gas exchange surface in humans
• Several features of alveoli that make them suited to their function are the same as those that make villi or root hair cells suited to their function, as they are all involved in transporting substances across their surfaces
• Structures in the human breathing system

  • Ribs
  • Intercostal muscle
  • Diaphragm
  • Trachea
  • Larynx
  • Bronchi
  • Bronchioles
  • Alveoli
• Ribs


  Bone structure that protects internal organs such as the lungs
• Intercostal muscle


  Muscles between the ribs which control their movement causing inhalation and exhalation
• Diaphragm


  Sheet of connective tissue and muscle at the bottom of the thorax that helps change the volume of the thorax to allow inhalation and exhalation
• Trachea


  Windpipe that connects the mouth and nose to the lungs
• Larynx


  Also known as the voice box, when air passes across here we are able to make sounds
• Bronchi


  Large tubes branching off the trachea with one bronchus for each lung
• Bronchioles


  Bronchi split to form smaller tubes called bronchioles in the lungs connected to alveoli
• Alveoli


  Tiny air sacs where gas exchange takes place
• Investigating the differences in inspired and expired air


  1. Air drawn through boiling tube A when breathing in

  2. Air blown into boiling tube B when breathing out

  3. Limewater becomes cloudy when carbon dioxide is bubbled through it
• The limewater in boiling tube A will remain clear, but the limewater in boiling tube B will become cloudy, showing the percentage of carbon dioxide in exhaled air is higher than in inhaled air
• Inspired air


  Air that is breathed in
• Expired air


  Air that is breathed out
• Air that is breathed in and air that is breathed out has different amounts of gases in it due to exchanges that take place in the alveoli
• Composition of Inspired Air

  • Oxygen: 21%
  • Carbon dioxide: 0.04%
  • Nitrogen: 78%
• Composition of Expired Air

  • Oxygen: 16%
  • Carbon dioxide: 4%
  • Nitrogen: 78%
• Exhaled air contains more water vapour and is at a higher temperature than inhaled air
• Exercise


  • Increases the frequency and depth of breathing
• Investigating effects of physical activity on breathing

  1. Count breaths per minute at rest

  2. Measure average chest expansion over 5 breaths

  3. Exercise for set time

  4. Immediately after, count breaths per minute and measure average chest expansion over 5 breaths
• Following exercise, the number of breaths per minute will have increased and the chest expansion will also have increased
• Intercostal muscles


  Muscles found between the ribs
• Types of intercostal muscles

  • External intercostal muscles (on outside of rib cage)
  • Internal intercostal muscles (on inside of rib cage)
• Function of cartilage in trachea

  Supports the airways and keeps them open during breathing
• Diaphragm


  Thin sheet of muscle that separates the chest cavity from the abdomen, ultimately responsible for controlling ventilation in the lungs
• Inhalation


  1. Diaphragm contracts, flattening and increasing volume of chest cavity

  2. This decreases air pressure inside lungs, drawing air in
• Exhalation


  1. Diaphragm relaxes, moving upwards and decreasing volume of chest cavity

  2. This increases air pressure inside lungs, forcing air out
• Role of external intercostal muscles

  1. Contract during inhalation to pull ribs up and out, increasing chest volume and decreasing pressure to draw air in

  2. Relax during exhalation to allow ribs to drop down and in, decreasing chest volume and increasing pressure to force air out
• Role of internal intercostal muscles


  Contract during forced exhalation to pull ribs down and in, decreasing chest volume and increasing pressure to force air out more forcefully
• Increased need for gas exchange during strenuous activity


  Internal intercostal muscles contract to decrease chest volume and force air out more quickly
• This allows a greater volume of gases to be exchanged
• Oxygen is removed from blood by respiring cells, so blood returning to lungs has lower oxygen concentration than air in alveoli
• Carbon dioxide is produced by respiration and diffuses into blood from respiring cells, then transported to lungs where it diffuses into alveoli
• Water evaporates from moist lining of alveoli into expired air due to body warmth
• Nitrogen concentration does not change between inspired and expired air as it is very stable and cannot be used by the body
• Aerobic respiration


  Chemical reactions that release energy from food, using oxygen