Objective: Describe gas exchange between air in the alveoli and blood in the capillaries.
The efficient exchange of oxygen and carbon dioxide is crucial for cellular respiration. This process occurs within the lungs, specifically in the alveoli, where gas exchange takes place between the air and the blood.
Structure of the Respiratory System Relevant to Gas Exchange
Understanding the structure of the lungs is essential for comprehending gas exchange. Key components include:
Trachea: The windpipe, which carries air to the bronchi.
Bronchi: Two tubes that branch off from the trachea, leading to each lung.
Bronchioles: Smaller branches of the bronchi.
Alveoli: Tiny air sacs clustered at the ends of the bronchioles. These are the primary sites of gas exchange.
Suggested diagram: A diagram illustrating the trachea branching into bronchi, then bronchi into bronchioles, and finally bronchioles leading to clusters of alveoli.
Alveolar Structure and Gas Exchange
Alveoli are remarkably adapted for gas exchange:
Thin Walls: Alveolar walls are only one cell thick (Type I pneumocytes), minimizing the diffusion distance.
Large Surface Area: There are approximately 300 million alveoli in each lung, providing a vast surface area for gas exchange.
Rich Capillary Network: Each alveolus is surrounded by a dense network of capillaries.
Moist Surface: The alveolar surface is kept moist, allowing gases to dissolve and diffuse.
Type II Pneumocytes: These cells secrete surfactant, which reduces surface tension within the alveoli, preventing them from collapsing.
The Process of Gas Exchange
Gas exchange occurs through diffusion, the movement of gases from an area of high concentration to an area of low concentration.
Oxygen Exchange
The concentration of oxygen in the alveoli is higher than in the blood arriving at the capillaries. Therefore, oxygen diffuses from the alveoli into the blood.
The partial pressure of oxygen in the alveoli (PO2) is typically around 100 kPa. The partial pressure of oxygen in arterial blood (PaO2) is around 100 kPa. This high partial pressure gradient drives oxygen diffusion.
Carbon Dioxide Exchange
The concentration of carbon dioxide in the blood arriving at the capillaries is higher than in the alveoli. Therefore, carbon dioxide diffuses from the blood into the alveoli.
The partial pressure of carbon dioxide in venous blood (PvO2) is typically around 40 kPa. The partial pressure of carbon dioxide in the alveoli (PO2) is typically around 40 kPa. This high partial pressure gradient drives carbon dioxide diffusion.
Table Summarizing Gas Exchange
Gas
Location of Higher Concentration
Direction of Diffusion
Driving Force
Oxygen (O2)
Alveoli
Into Blood
High PO2 in Alveoli, Low PO2 in Blood
Carbon Dioxide (CO2)
Blood
Into Alveoli
High PCO2 in Blood, Low PCO2 in Alveoli
Factors Affecting Gas Exchange
Several factors can influence the efficiency of gas exchange:
Surface Area: A larger surface area (e.g., due to more alveoli) increases the rate of diffusion.
Diffusion Distance: A shorter diffusion distance facilitates faster diffusion.