Topic 5.5: Gas Exchange
Homepage
Topic Index
Human Physiology Index
5.5.1 Alveoli Adaptations (increase the rate of gas exchange) 
alveoli
  
  • Large surface area due to the combined spherical shape (600 million alveoli = 80 m2
  • Flattened epithelial cells of alveoli and close association with capillaries
  • Short diffusion distance from alveoli to blood (0.5-1.0 um)
  • Dense capillary network
  • Moist surface for the solution of gases
 
5.5.2 Technical terms

 

Ventilation:

 

  • The flow of air in and out of the alveoli is called ventilation and has two stages: inspiration (or inhalation) and expiration (or exhalation).

  • Lungs are not muscular and cannot ventilate themselves, but instead the whole thorax moves and changes size, due to the action of two sets of muscles: the intercostal muscles and the diaphragm.

 
Gas Exchange: gas exchange  

This is the diffusion of gases (oxygen and carbon dioxide)

There are two sites for gas exchange

  1. (a)Alveoli: Oxygen diffuses into the blood from the alveoli and carbon dioxide diffuses from the blood into the alveoli

  1. (b)Tissues: Oxygen diffuses from blood into the cells and carbon dioxide diffuses from cells to the blood

 
Cell Respiration:

This is the synthesis of ATP

  • Aerobic respiration uses oxygen in the mitochondria and produces carbon dioxide
  • Anaerobic respiration does not use oxygen but still produces carbon dioxide
 
5.5.3 The function of the ventilation system
  • A ventilation system is needed to maintain concentration gradients in the alveoli
  • The steep concentration gradient across the respiratory surface is maintained in two ways: by blood flow on one side and by air flow on the other side. The ventilation system replaces diffuses oxygen (keeping the concentration high) and removes carbon dioxide (keeping the concentration low).
  • This means oxygen can always diffuse down its concentration gradient from the air to the blood, while at the same time carbon dioxide can diffuse down its concentration gradient from the blood to the air.
 
5.5.4 The Structure of the ventilation system
lung structure  

(a) Trachea

(b) Cartilage ring support

(c) Bronchi (plural) Bronchus (single)

(d) Lung

(e) Heart

(f) Sternum

(g) Rib cage

 

 

 

 

(h) Bronchioles

(j) Alveoli

(k) Diaphragm

 
5.5.5 Mechanism of ventilation
inspire  
  • The diaphragm contracts and flattens downwards.
  • The external intercostal muscles contract, pulling the ribs up and out
  • this increases the volume of the thorax this increases the lung and alveoli volume
  • this decreases the pressure of air in the alveoli below atmospheric (Boyle's law)air flows in to equalise the pressure
 
exhale  
  • The diaphragm relaxes and curves upwards
  • the external intercostal muscles relax, allowing the ribs to fall
  • this decreases the volume of the thorax
  • this decreases the lung and alveoli volume
  • this increases the pressure of air in the alveoli above atmospheric (Boyle's law)air flows out to equalise the pressure

  • The abdominal muscles contract, pushing the diaphragm upwards
  • The internal intercostal muscles contract, pulling the ribs downward
  • This gives a larger and faster expiration, used in exercise
back to index