The respiratory system works in conjunction with the circulatory system to provide oxygen to and to remove carbon dioxide from the body tissues. Oxygen is the driving force behind all chemical reactions that occur internally. Obviously, life could not exist without it.
As a result, the function of all body systems, including the respiratory system itself, depends first on the ability of this system to deliver its product. In addition to this vital function, the respiratory system also serves as a means of thermoregulation.
Anatomy and Physiology
The respiratory system begins with the mouth and nose, which, under the influence of the breathing mechanism, facilitate the passage of air into the trachea. The wall of this cylindrical structure is lined with rings of tough cartilage which prevent it from collapsing during normal breathing activity.
The trachea enters the thorax, or chest cavity, and eventually branches into bronchi and smaller bronchioles within the lungs themselves. Thin membranes called pleura line the lungs and inner wall of the thorax.
Pleuritis is the term used to describe inflammation of these membranes, which can make normal respiration difficult and painful. The smallest unit of the respiratory system is the alveolus, located at the terminus of the bronchioles.
It is within these alveoli that gas exchange occurs between the lungs and the circulatory system. Surfactant (surface-active agent) is a special substance found lining the insides of normal alveoli. It is responsible for preventing alveolar collapse during the breathing cycle.
The major blood supply to the lungs and alveoli comes from the pulmonary artery originating from the right ventricle of the heart. In dogs and cats, heartworms reside in the right side of the heart and can effectively clog this artery and its branches supplying the lungs.
The resulting disruption of blood flow and increase in pulmonary blood pressure can have devastating consequences on respiratory function. The only air within the thorax, or chest cavity, is contained within the lungs.
As a result, a negative pressure system exists that facilitates normal breathing. Intake or inspiration of air occurs as the diaphragm, the large muscular band separating the thorax from the abdominal cavity, flattens and lowers itself, and the ribcage expands.
The resulting negative pressure caused by the increased thoracic size actively draws air through the trachea and into the lungs. On exhalation, or expiration, the diaphragm and ribcage are returned to their normal size, forcing air out of the lungs.
Pneumothorax is a life-threatening condition in which air is allowed into the thoracic cavity, either through a penetrating wound through the skin and ribcage or through a tear in the lung tissue.
Either way, the loss of negative pressure within the thorax quickly collapses the lungs, and renders the normal breathing mechanisms inoperable. Because of its direct exposure to a hostile environment, the respiratory system contains several defense mechanisms to help keep foreign invaders and particulate matter out of the lungs.
The sticky substance called mucus, produced by cells lining the trachea and bronchi, serves to trap contaminants and foreign debris that might gain external access to the respiratory system.
In addition, tiny, movable, fingerlike projections called cilia line the surface of airways and function to mechanically maneuver trapped contaminants in a direction away from the lungs. Any significant buildup of respiratory mucus or irritation to the respiratory lining results in a cough, and (hopefully) the forceful expulsion of any offending substance.
The airways are also lined with surface antibodies that provide a first line of defense against infectious organisms.