Avian Respiration - Anatomy & Physiology

Introduction

The avian respiratory system contains some fundamental differences to the mammalian system.

The nostrils of the bird, which lead into the nasal cavity, may have a flap of horn to protect them, known as the Operculum.
The Oral Cavity and the Nasal Cavity of the bird are interconnecting via a slit in the hard palate called the Choana. Birds lack a soft palate.

©Nottingham 2008 Saggital Section of an Avian skull showing the conchae.

There are Rostral, Middle and Caudal Conchae arising from the lateral wall, filling part of the nasal cavity.
- Rostral Conchae - Vestibular Region - lined with Stratified Squamous Epithelium
- Middle Conchae - Respiratory Region - lined with Respiratory Epithelium.
- Caudal Conchae - Olfactory Region - lined with olfactory epithelium.
The Infraorbital Sinus is a triangular cavity under the skin, rostroventral to the eye.
Some marine birds have a Salt Gland (nasal gland) which excretes sodium.

The larynx is on the floor of the oropharynx supported by Cricoid and paried Arytenoid Cartilages which are different in structure to those in mammals.
There is no Epiglottis.
There are no vocal folds - birds vocalise using a syrinx.

The avian trachea is composed of tightly stacked cartilages which are shaped similarly to signet rings. These are complete with no dorsal space as in the mammalian trachea. They overlap considerably.
The trachea can be palpated on the right side of the neck; it runs alongside the oesophagus.
The trachea is lined with Respiratory Epithelium.
The trachea bifurcates into two main bronchi as in mammals. The syrinx is formed by this terminal part of the trachea.

Avian lungs are relatively small, with a bird's lungs being approximately 50% as large as the lung of a mammal of a similar size. The lungs are unlobed and do not have the capacity to expand.
The lungs are positioned in the craniodorsal region of the body, and are deeply indented by both the thoracic vertebrae and ribs.
Birds do not have a pleural cavity as the lungs do not expand, thus it is not necessary.

Birds lack a diaphragm, and their thoracic and abdominal cavities are continuous.
The bird has a number of thin walled, easily distensible Air Sacs which can extend to approximatley 10x the volume of the lungs. These are present within body cavities, and extend into bones, to take the place of marrow. This has the added function of reducing the weight.
The chicken has 8 air sacs:
- Cervical - this extends within the cervical and thoracic vertebrae.
- Clavicular - lies within the thoracic inlet, surrounding the heart, and within the humerus in the forelimb.
- Cranial Thoracic (x2) - these are ventral to the lungs.
- Caudal Thoracic (x2) - located between the body wall and the thoracic air sacs.
- Abdominal (x2) - these are the largest air sacs and fill the caudodorsal region of the abdomen, in contact with intestines, kidneys and genital organs. In addition these airsacs utilise space within the acetabulum and synsacrum.
The Cervical, Clavicular and Cranial Thoracic air sacs form one functional group - Cranial, with the Caudal Thoracic and Abdominal Air sacs forming another, Caudal functional group.
The air sacs have a vital role in ventilation, but do not have the capacity for gaseous exchange.

Ventilation is birds is strikingly different to that of mammals in that air flows through the lungs in the same direction during both inspiration and expiration. In addition, both the intaking of air and the expelling of air are active processes, requiring muscle contraction.
During Inspiration, the ribs are drawn forwards and the sternum lowered, the Caudal Sacs recieving 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.
During Expiration, the sternum is drawn caudal and dorsal, the air sacs are compressed, air from the caudal sac passes through the lungs, while the air in the Cranial Sac leaves via the trachea.
Thus oxygenated air passes through the lungs on both inspiration and expiration.

One Primary Bronchi from the trachea enters each lung, narrowing as it travels through, and communicates with the Abdominal Air Sac. This bronchi gives off branches as it travels through the lung, known as Secondary Bronchi. Each of these gives off a further 400-500 Parabronchi in the walls of which, gaseous exchange takes place.
Avian Gas exchange takes place not in alveoli, as in mammals, but within Air Capillaries which are extensions of the parabronchial lumen. They are an interconnecting network of loops, and closely intertwine with Blood Capillaries.
The Air Capillaries and Blood Capillaries are arranged so that flow is crosscurrent. This makes the gaseous exchange which occurs from one to the other extremely efficient.

Dyce, K.M., Sack, W.O. and Wensing, C.J.G. (2002) Textbook of Veterinary Anatomy. 3rd ed. Philadelphia: Saunders.
Sjaastad, O.V., Hove, K. and Sand, O. (2004) Physiology of Domestic Animals. Oslo: Scandinavian Veterinary Press.