Changes

*[[Ventilation - Anatomy & Physiology|Ventilation]] in birds is strikingly different to that of mammals in that air flows through the lungs in the same direction during both [[Ventilation - Anatomy & Physiology#Inspiration|inspiration]] and [[Ventilation - Anatomy & Physiology#Expiration|expiration]]. In addition, both the intaking of air and the expelling of air are active processes, requiring [[Muscles - Anatomy & Physiology|muscle]] contraction.

*[[Ventilation - Anatomy & Physiology|Ventilation]] in birds is strikingly different to that of mammals in that air flows through the lungs in the same direction during both [[Ventilation - Anatomy & Physiology#Inspiration|inspiration]] and [[Ventilation - Anatomy & Physiology#Expiration|expiration]]. In addition, both the intaking of air and the expelling of air are active processes, requiring [[Muscles - Anatomy & Physiology|muscle]] contraction.

*During Inspiration, the [[Ribs and Sternum - Anatomy & Physiology|ribs]] are drawn forwards and the [[Ribs and Sternum - Anatomy & Physiology|sternum]] lowered, the Caudal air sacs receiving fresh air.   

*During Inspiration, the [[Ribs and Sternum - Anatomy & Physiology|ribs]] are drawn forwards and the [[Ribs and Sternum - Anatomy & Physiology|sternum]] lowered, the Caudal air sacs receiving fresh air.   

*Simultaneously, the Cranial air sacs recieve air which was inhaled at the previous inhalation which is drawn from the lungs, this air has lost much of it's oxygen content.

*Simultaneously, the Cranial air sacs receive air which was inhaled at the previous inhalation which is drawn from the lungs, this air has lost much of it's oxygen content.

*During Expiration, the [[Ribs and Sternum - Anatomy & Physiology|sternum]] is drawn caudal and dorsal, the air sacs are compressed, air from the caudal air sac passes through the [[Lungs - Anatomy & Physiology|lungs]], while the air in the cranial air sac leaves via the [[Trachea - Anatomy & Physiology|trachea]].

*During Expiration, the [[Ribs and Sternum - Anatomy & Physiology|sternum]] is drawn caudal and dorsal, the air sacs are compressed, air from the caudal air sac passes through the [[Lungs - Anatomy & Physiology|lungs]], while the air in the cranial air sac leaves via the [[Trachea - Anatomy & Physiology|trachea]].

*Thus oxygenated air passes through the lungs on both [[Ventilation - Anatomy & Physiology#Inspiration|inspiration]] and [[Ventilation - Anatomy & Physiology#Expiration|expiration]].

*Thus oxygenated air passes through the lungs on both [[Ventilation - Anatomy & Physiology#Inspiration|inspiration]] and [[Ventilation - Anatomy & Physiology#Expiration|expiration]].