It is important to understand the anatomy of the horses’ respiratory system to fully appreciate the toll respiratory diseases can have. Oxygen is essential to the survival of all mammals, the horses’ cardiovascular system accomplishes this, even at peak respiratory rate the spleen can dump its erythrocytes into the bloodstream to provide more oxygen to the body. Signs and symptoms will vary between respiratory conditions so it will be important to understand the causative factors of the different disease processes.
(please see this link for a review of the anatomy with relationships to disease)
http://www.thehorse.com/articles/10488/common-respiratory-problems