Introduction
As animals evolved from single-celled organisms to multilayered organisms, it became impossible for oxygen to diffuse effectively to every cell of the body. Therefore, animals evolved systems to deliver nutrients to every cell.
The respiratory and circulatory system became the means by which nutrients are delivered to each cell and wastes are removed. The functions of the respiratory system include gas exchange, regulation of blood pH, olfaction, filtration of inspired air, production of sounds, and elimination of some water and heat via expired air.The primary function of the respiratory system is the intake of oxygen and the elimination of carbon dioxide from the body. The respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and lungs. Structurally, the respiratory system can be divided into the upper respiratory system, including the nose, pharynx, and associated structures, and the lower respiratory system, including the larynx, trachea, bronchi, and lungs. Functionally, it can also be divided into two sections: (1) The conducting portion consists of a series of connected tubes, both outside and within the lungs, that filters, warms, moistens, and conducts air to and from the lungs. This portion includes the nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles; and (2) the respiratory portion is the site of gas exchange between the air and blood and consists of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
Anatomy and Physiology of Domestic Animals, Second Edition. R. Michael Akers and D. Michael Denbow. © 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.