The Lungs as an Organ of Exchange

Air can pass into lungs through nose, along tranchea, bronchi & bronchioles

• Reaches Alveoli (air sacs) Ribs + diaphragm = produce breathing movements (ventilation)
  • Walls = exchange of gases take place
  • O2 from air in alveoli to blood in capillaries
    
  • CO2 from blood to alveoli

Large SA

• More space for molecules to pass through
• 100-300µm across
  • BUT so many = more SA than our skin
• Total SA = 70m² = 1/2 tennis court

Permeable Barrier

• Plasma membranes surrounding cell's thin cytoplasm = barrier to exchange
  • Allow diffucion of O2 and CO2

Thin Barrier

• Walls = both 1 cell thick
• Squamous cells = flattened / very thin
• Capillaries = close contact with alveolus walls

For diffusion to be rapid, a steep diffusion gradient is needed. 

This means having a HIGH CONCENTRATION of molecules on the SUPPLY side of the exchange surface and a LOW CONCENTRATION on the DEMAND side.

To maintain this, a fresh supply on one side is needed to keep the concentration high, whilst removing molecules on the other side maintains low concentration there.

• This is achieved by the action of the blood transport system and the ventilation (breathing) movements.

• The blood brings CO2 from tissues -> lungs = concentration of CO2 in blood is HIGHER than air of alveoli
• Carries O away from lungs = concentration of O in blood LOWER than air in alveoli
• Heart pumps blood along pulmonary artery to lungs.
  • In lungs = artery divides to finer vessels
  • Carry blood to capillaries on surface of alveoli 

Breathing movements of lungs ventilate the lungs, they replace the used air with fresh air

Brings more O into lungs and ensures concentration of O in air of alveolus = higher than concentration of blood

Constant supply of gas to one side of the exchange surface and its removal from the other side ensures that diffusion, and therefore exchange, can continue.