The canine respiratory includes the nose, airways (pharynx, larynx, trachea) and lungs. It serves two main purposes: Respiration and the regulation of body temperature. Respiration allows the blood to be enriched with oxygen which is circulated into all parts of the body, and for the release of carbon dioxide - a waste product of metabolism, is transported from the cells to the lungs and removed through the air. The other important function of the canine respiratory tract is the regulation of body temperature. In contrast to the heart, respiration does not have an average rate, but is controlled by the respiratory center contained in the central nervous system. The respiratory rate of a dog at rest is between 15 and 40 breaths per minute. Note that the respiratory rate is influenced by a variety of factors. Dogs will normally breath faster after exercise or in hot conditions. Rapid breathing can also be caused by psychological factors i.e. stress, fear or excitement. To determine your dog’s respiratory rate, place your hands along the animal’s thorax and count how many times it is lifted within 30 seconds. If you double the figure and can calculate the rate of respiration per minute. Nose: Along with respiration, the canine nose serves as the exceptionally well-developed smelling organ. About 220 million olfactory neurons (appx. 5 mil in humans) allow dogs to sense the slightest smells or to pick up even the smallest trace of a scent. A dog’s sense of smell is also important in finding a partner, and helps the dog to fulfill the role of rescuer in human society by searching for lost people. As far as appearances are concerned, a dog’s nose is normally cold and moist. This is owed to the fact that the inside of the nasal cavity is lined by mucosa, which requires fluid at all times. A dog’s sense of smell begins when fragrances enter the nose through the nostrils. Tiny odor molecules then disperse in the nasal fluid thus making contact with olfactory receptor neurons. These neurons then relate the “smell” information to the olfactory center of the brain where the information is processed. Furthermore, a dog’s nose is for more than smelling, as they normally use nasal breathing for oxygen intake. Air that passes through the nose is warmed and moisturized inside the nasal cavity before reaching the lower airways. Only in very hot conditions, after exercise or in pathological conditions, is nasal respiration alone is not sufficient, and dogs will additionally make use of the mouth for breathing. Airways: The pharynx is the space behind oral and nasal cavity. It represents the junction of both air- and nutritional passage and ends at the trachea or esophagus respectively. The larynx consists of cartilage, muscle and connective tissue. It serves as the closing mechanism of the trachea preventing swallowed ingesta from entering the airways. Vocal chords are also part of the larynx, and allow the dog to create sound by causing passing air to vibrate. The trachea connects the pharynx to the lower airways (bhonchi and lungs). The long extend of the organ allows inhaled air to be warmed during passage. The trachea consists of C-shaped cartilage rings that stretch during breathing. Bronchi serve as the lower extension of the trachea. Before the trachea reaches the lungs, it branches. These branches are called bronchi. They have a circular diameter and divide even further like the branches of a tree. In the terminal bed of their division they are called bronchioles. Lungs: Canine lungs consist of two separate segments, which are embedded into the thorax and attached to the thoracic wall by means of a thin mucus layer. This allows the lungs to follow the movement of the thorax during inhalation. The respective segments are divided further into lobes. The terminal bed of the bronchial tree consists of millions of alveoli where the actual respiration is taking place. The alveoli are surrounded by an intricate web of blood vessels. The walls of the alveoli are very thin thus allowing the passage of oxygen into the blood. At the same time, carbon dioxide - a waste product of metabolism - passes from the blood into the outside air. The ribs and muscles of the thorax, as well as the diaphragm (a dome-shaped muscle separating the thoracic from the abdominal cavity), serve as a type of bellows during breathing. The process of inhalation is active: the dog's muscles lift the thorax and thereby extend the lungs. Exhalation is passive, and occurs when the same muscles relax and allow the thorax to distend causing inhaled air to be pressed out of the airways. The frequency of this process is controlled by the respiratory center inside the brain.
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