B11 | Gas Exchange in Humans

• Characteristics of respiratory surfaces

  Large surface area - Allows faster diffusion of gases across the surface

  Thin walls - Help ensure diffusion distances remain short

  A good ventilation with air maintains the diffusion gradients

  A good blood supply maintains a high concentration gradient so diffusion occurs faster
• Alveoli is where gas exchange occurs in the body because

  • Alveoli have:
  • A large surface area
  • 100m² in lungs
  • Thin walls
  • 1 cell thick
  • Good ventilation
  • Lungs provide an oxygen supply
  • Good blood supply
  • Surrounded by capillaries
• Alveoli

  Tiny air sacs where gas exchange takes place.
• Bronchioles

  Bronchi split to form small tubes called bronchioles in the lungs connected to alveoli.
• Bronchi

  Large tubes branching off the trachea with one bronchus to each lung.
• Larynx

  Voice box - Contains vocal cords that can be tightened to form different pitches.
• Trachea

  Windpipe that connects mouth and nose to lungs.

  Surrounded by rings of cartilage.

  Functions to support the airways and keep them open during breathing →  Prevents inwards collapse when the air pressure drops in the tubes.
• Ribs

  Bone structure that protects internal organs (lungs, bronchioles, etc.)
• Intercostal Muscles

  Muscles between the ribs which control 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.
• Intercostal Muscles are only able to pull on bones, NOT push on them
• Humans require two sets of intercostal muscles; one to pull the rib cage up and another set to pull it down
• External Intercostal Muscles

  Intercostal muscles found on the outside of the ribcage
• Internal Intercostal Muscles

  Intercostal muscles found on the inside of the ribcage
• When inhaling, the Diaphragm contracts to flatten, which increases the volume of the thorax. This leads to a decrease in air pressure inside the lungs compared to outside the body. This draws air in.
• When exhaling, the Diaphragm relaxes and moves upwards back into its domed shape. This decreases the volume of the thorax, which leads to an increase in air pressure inside the lungs relative to outside the body. This forces air out.
• The external and internal intercostal muscles work as antagonistic pairs
• When inhaling, the external set of intercostal muscles contract to pull the ribs up and out → increases the volume of the chest cavity (decreasing air pressure) → drawing air in
• When exhaling, the external set of intercostal muscles relax so the ribs drop down and in → decreases the volume of the chest cavity (increasing air pressure) → forcing air out
• During strenuous activity, the internal intercostal muscles contract more forcefully and quickly, further decreasing the volume of the thorax → forced exhalation. This helps get rid of more CO2
• Air that’s breathed in and air that’s breathed out have different amounts of gases due to the exchanges that happen in the alveoli.
• The oxygen content in inspired air is 21%, while in expired air, it decreases to about 16%. This is because O₂ is removed from the blood and used by respiring cells, so when the blood returns to the lungs, there is less O₂.
• The carbon dioxide content in inspired air is 0.04%, while in expired air, it increases to about 4%. This is because CO₂ is produced by respiration and diffuses into the blood from respiring cells. Blood transports the CO₂ to the lungs where it diffuses into the alveoli (because the alveoli has a lower concentration than blood) and exchanges with oxygen.
• The nitrogen content in inspired air and expired air is 78%. It is the same for both inspired and expired air because nitrogen gas is very stable and cannot be used by the body in the gas form.
• The water vapor content in inspired air is lower than in expired air. This is because water evaporates from the moist lining of the alveoli into the expired air.
• • As we breathe in, air is drawn from boiling tube A
  • As we breath out, air is blown into boiling tube B
  • Lime water is normally clear, but becomes cloudy/milky when carbon dioxide is bubbled through it.
  • The lime water in boiling tube A will remain clear while the lime water in boiling tube B will become cloudy.

  A) Boiling Tubes

  B) Rubber Tubings

  C) Lime Water

  D) Mouth

  
• Frequency and depth of breathing increase when exercising
• When exercising:

  • Muscles are working harder and aerobic respiration increases → cells need more oxygen to keep up with the energy demand
  • If muscles cannot meet energy demands they will also respire anaerobically (producing lactic acid)
• • After exercise, the lactic acid built up in muscles needs to be removed → lowers the pH of cells and can denature enzymes catalyzing cell reactions
  • Can only be removed by combining it with oxygen – this is known as ‘repaying the oxygen debt’
  • The longer it takes for breathing to normalize, the more lactic acid produced during exercise and the greater the oxygen debt to be repaid