The respiratory system of the pig commences at the nostrils which lead into two nasal passages. These contain the dorsal and ventral turbinate bones. (Fig.1-8). The ventral turbinates consist of four thin main bones, two on each side separated by a cartilaginous septum. You can imagine these as four hair curlers placed inside the nose. The respiratory tract is lined by a smooth membrane called a mucous membrane because it is bathed in a sticky mucus. It is also covered with minute hair like structures which are able to brush the mucous across the surface by their wavy motion. They move the mucous in the nose, bronchial tree and trachea to the throat where it is swallowed. The air breathed in through the nose is warmed by the turbinate bones which, because of their scroll-like shape, cause turbulence. This throws out the larger of the small particles so that they stick to the mucus and are swept to the throat. The many branches of the bronchi as they decrease in diameter have a similar effect on more minute particles. The mucus elevator then carries them to the throat. Only the vary smallest particles reach the alveoli where the alveolar macrophages engulf and remove them. Internally, the nasal passages open into the pharynx (throat) which is a common passage for food and air. The food is swallowed down the oesophagus and the air is sucked into the larynx at the back of the throat. The larynx (voice box) controls inspiration and expiration. It opens into the trachea which passes down into the chest where it divides into two bronchi. The bronchi branch into smaller bronchi and continue to branch gradually reducing in size to become bronchioles which terminate in very tiny air sacs called alveoli. Oxygen is passed from the alveoli into the blood stream and carbon dioxide is passed out. The lungs are divided into seven lobes as shown in Fig.1-8.

Terminology
Abscess - Area of pneumonia containing pus where the infection has been sealed off from the remainder of the lung tissue by a fibrous capsule.
Actinobacillus pleuropneumoniae - Originally called haemophilus. A bacterium that produces a severe haemorrhagic and necrotising pneumonia with pleurisy.
Alveolar macrophages - These cells which are located in the alveoli engulf bacteria and viruses. They are destroyed by some viruses e.g. the porcine respiratory reproductive syndrome (PRRS) virus.
Atrophic rhinitis - Rhinitis caused by toxigenic (toxin producing) strains of Pasteurella multocida, in which the turbinates loses their tissues (atrophy) irreversibly. This is now called progressive atrophic rhinitis to distinguish it from non-progressive atrophic rhinitis caused by Bordetella bronchiseptica (with the addition of other organisms) and/or environmental contaminants, which is less severe and heals when the infection is stopped by the immune response.
Bronchitis - Inflammation of the bronchi or bronchioles in the lung.
Consolidating pneumonia - The lung tissue has collapsed and become solid. A common example is infection by Mycoplasma hyopneumoniae (enzootic pneumonia) which causes inflammation of the anterior lobes of the lungs.
Enzootic pneumonia - Also called mycoplasmal pneumonia. Caused by Mycoplasma hyopneumoniae, which produces a consolidating pneumonia of the lower parts of the anterior lobes of the lungs.
Glässers disease - Caused by Haemophilus parasuis. It can produce a severe pneumonia and consolidation with fibrinous pleurisy.
Lung worms - Small thread-like worms causing a parasitic pneumonia.
Necrotising pneumonia - Necrosis means death of tissue within the living animal. Necrotising pneumonia occurs where the organism or its toxins kill lung tissue. An abscess may result. A common example is pneumonia caused by Actinobacillus pleuropneumoniae.
Pasteurella - Bacteria found as normal inhabitants of the upper respiratory tract. They often cause secondary infections, for example, following Mycoplasma hyopneumoniae infection. There are two common species P. haemolytica and P. multocidia the latter being the common one in pigs.
Pleurisy - Also called pleuritis. The shiny membranes that covers the surface of the lungs and the inside of the chest wall are called the pleura. Infection or inflammation of these surfaces is called pleurisy. This together with pericarditis is very common in the pig and accounts for considerable loss through condemnation at slaughter. Viruses such as flu, PRRS, swine fever and the bacteria Actinobacillus pleuropneumoniae, Haemophilus parasuis and Pasteurella multocidia can cause pleurisy.
Pneumonia - Inflammation in any part of the lung tissue. There are different types of pneumonia.
Pyaemic pneumonia - Multiple small abscesses scattered through the lungs that have been carried there via the blood stream. A common example is pyaemia from tail biting. The carcase is condemned at slaughter.
Respiratory rate - This varies from 20-40 breaths per minute in piglets and growing pigs and 15-20 per minute in sows.
Rhinitis - Describes any form of inflammation to the delicate mucous lining of the nose. Some agents such as dust and gases may cause it but there is no long-term damage to the nose structure. Sneezing always occurs with rhinitis.
Salmonella choleraesuis - A bacterium specific to the pig causing generalised salmonellosis and pneumonia.
Swine influenza (SI) - A virus infection which produces clearly demarcated dark purple red lesions in the lungs.
Tracheitis - Inflammation of the trachea (windpipe). Influenza may cause a very heavy "barking" cough.
Turbinate bones - Dorsal and ventral. Scrolls of bone inside the nasal passages. They warm and filter air as it passes through the nose.