Gas Exchange

Why is it needed?

• Respiration requires oxygen; CO2 must be released
• Photosynthesis requires CO2, oxygen must be released
• Single-celled organisms have large surface-area to volume ratio
  • Gas exchange not a problem
• Multi-celled organisms have much smaller surface area to volume ratio
  • Complex mechanisms needed to support gas exchange
    • Plants -> inside leaf
    • Insects -> Tracheal system (internal tubes)
    • Fish -> Gills
    • Many vertebrates -> Lungs

How does it happen?

• In one word: diffusion
• In plant cells, through stomata
• In humans, across the thin walls of alveoli

Gas exchange in flowering plants

• Respiration happens all the time (this is true for all organisms)
  • Oxygen used up; carbon-dioxide produced
• Photosynthesis happens when sufficient sunlight is present
  • Oxygen produced; carbon dioxide used up
  • The rate depends directly on the light intensity
• Night-time
  • Respiration only; oxygen taken in, CO2 released
• Daytime (with sufficient light)
  • Photosynthesis rate > Respiration rate
    • More oxygen produced than used up
• Compensation points:
  • Time(s) of day during which oxygen and carbon-dioxide are produced at the same rate they are used up

How plant structure helps gas exchange

• Leaf adaptations:
  • large surface area
  • thinness
    • gases have to travel short distances
  • spongy mesophyll
  • stomata

Gas exchange in humans

• Breathing in through nose and mouth
  • Air moistened and warmed in nose
• Air travels down trachea into lungs
• Air enters lungs through bronchi
  • Cilia (tiny hairs) filter out dirt & microorganisms
• Bronchi divide into a network of bronchioles
• Air sacs present at end of bronchioles
• Bulges on air sacs called alveoli
  • Alveoli covered in tiny capillaries
  • This is where diffusion of gases happens

Breathing

• Pleural membranes
  • Inner: attached to lung surface
  • Outer: attached to thoracic wall and diaphragm
  • They enclose a fluid
• Intercostal muscles
  • Join together ribs
  • Two sets (internal and external)
• Diaphragm
  • Below the lungs
  • Tough sheet of tendon

Ventilation = breathing

Inhalation/inspiration = breathing in

Exhalation/expiration = breathing out

Thorax = chest cavity

Breathing is a mechanical process

It is aided by the coordinated action of the following:

Breathing (contd.)

• Pleural membranes
  • Inner: attached to lung surface
  • Outer: attached to thoracic wall and diaphragm
  • They enclose a fluid
• Intercostal muscles
  • Join together ribs
  • Two sets (internal and external)
• Diaphragm
  • Below the lungs
  • Tough sheet of tendon

Ventilation = breathing

Inhalation = breathing in

Exhalation = breathing out

Thorax = chest cavity

Breathing is a mechanical process

It is aided by the coordinated action of the following:

Inhalation and Exhalation

• It's all about air pressure
• Inhalation:
  • Pressure inside the lungs needs to be decreased
• Exhalation:
  • Pressure inside the lungs needs to be increased
• This is achieved by changing the volume of the thorax
  • The diaphragm and intercostal muscles work together for this
• This is an active process
  • Muscles contract => energy has to be expended

Inhalation

1. Bow-shaped diaphragm contracts and flattens
2. External intercostal muscles contract
   • Ribcage moves up and out
3. Volume of thorax increases
   • Air pressure in thorax decreases
   • This draws air into the lungs

Exhalation

1. Diaphragm relaxes
2. External intercostal muscles relax
   • Ribcage moves down and in
3. Volume of thorax decreases
   • Air pressure in thorax increases
   • Air expelled out of the lungs

Alveoli

• Alveoli are the respiratory surface where diffusion takes place
  • = site for gas exchange in the lungs
• Adaptations of the alveoli
  • thin permeable walls
    • diffusion distance reduced
  • moist lining in which gases dissolve
  • large surface area
  • high concentration gradient
    • capillaries carrying blood around it
      • Normally, the blood approaching the alveolus through the capillary has higher CO2 and lower O2 concentration

The harms of smoking

• Smoking harms the respiratory system
• Smoking harms the circulatory system
• Can cause cancer
• Many harmful chemicals in the smoke are small enough to diffuse into the blood

Respiratory problems caused by smoking

• Bronchitis: inflammation of bronchial tube linings
  • Tar in smoke coats the cilia and reduces their effectiveness
    • Sticky mucus builds up in the lungs
      • Causes lung infections, thick cough
• Emphysema: a disease that makes the patient unable to get sufficient oxygen
  • How it's caused: Continued coughing damages the alveoli, breaking down the divisions between them and reducing their surface area

The harms of smoking (contd.)

• Addiction
  • Makes it difficult to give up
• Cancer (uncontrolled cell division)
  • Carcinogenic substances present in tobacco tar
  • Can cause cancer of mouth, throat and lung
• Circulatory system diseases
  • Nicotine in tobacco is stimulant
    • Can increase blood pressure (B.P.)
  • Carbon monoxide in smoke
    • Decreases blood's oxygen carrying capacity
    • Can also increase B.P.
    • In pregnant women, can slow down fetus growth rate
      • Baby born with low birth weight
      • More susceptible to heart disease

The harms of smoking (contd.)

• Circulatory system diseases
  • Nicotine in tobacco is stimulant
    • Can increase blood pressure (B.P.)
  • Carbon monoxide in smoke
    • Decreases blood's oxygen carrying capacity
    • Can also increase B.P.
    • In pregnant women, can slow down fetus growth rate
      • Baby born with low birth weight
      • More susceptible to heart disease
        
  • Some smoke chemicals cause cholesterol release into blood
    
    • This thickens blood vessel linings
      
      • Blood flow becomes difficult/blocked
        
      • Can lead to stroke or heart disease