Week 16-26: Canalicular Phase

• Occurs at approximately 25 weeks in of the gestational period

• An air-blood barrier is created that allows oxygen to enter the blood in respiratory capillaries and carbon dioxide to flow out of respiratory capillaries and into the lungs.
• Air transports tissues, where the tissues become discernible from other gas-exchanged tissues
  .

In this diagram, the type I pneumocytes differentiate out of the type II pneumocytes. The capillaries are also approaching the walls of the acini during this phase.

http://www.embryology.ch/anglais/rrespiratory/phasen04.html ttp://www.livestrong.com/article/163935-fetal-lung-development-stages/