Nostril and Nasal Cavity
Cotain folds and shelves of skin, which make the opening less vulnerable to the entry of foreign bodies. The nasal passages are separated into two halves compsing the left and right nostril by the nasal septum. The nasal passage is also separated from the oral cavity by the hard palate which continues cadually towards the throat before tuning into the soft palate. The nasal passage is also lined with turbinates which are thin bones lined with nasal epithelium. These turbinates increase the surface area inside the nasal passages to therefore increase the sense of smell, provide a tortourous pathway and produce more mucus from goblet cells to stop bacteria reaching the throat. The lining of the nasal passage is composed of puesdostratified columnar epithelium with cilia. The cilia beat in a sychronised pattern to move the mucus containing the bacteria to the pharynx where is can be swallowed and destroyed by the hydrochloric acid in the stomach,.
Nasal Blood Supply
The inhaled air is warmed by the heat of the blood just under the surface and picks up moisture from the mucus to prevent the mucous membranes of the deeper aiways becoming chilled or dried by the air flow. This is known as warming and humidifying.
The Larynx has three functions which can be remebered as the three P's:
- Protection - protects the airway when swallowing
- Passage of air - the laryngeal cartilages are able to open wider during exercise to maximise air intake.
- Phonation - Flow rate and volume of exhaled air can be modified to cause the vocal chords to vibrate and produce a sound.
The trachea divides into the left and right bronchi. This splitting is known as the bifurcation of the trachea and occurs roughly level with the heart. The trachea is composed of fibrous tissue and smooth muscle and is held open by c shaped rings of hyaline cartilage which provides structural support during air pressure changes and can dilate and constrict. Similar to the nasal passages the trachea is lined by pseudostratified ciliated epithelium which traps pathogens and moves in a synchornised pattern to move up towards the pharynx where it can be swallowed. This is known as the muco-ciliary escalator!
The Bronchi divide into lobular bronchi which enter each of the lung lobules (7 lung lobules in cats and dogs) before further dividing into tiny bronchioles. Each terminal bronchus divides into two or three alveolar ducts which turns into an alveoli. Alveoli are closed sacs lined with simple squamous epithelium which creates a short diffusion pathway for gaseous exchange.
The Alveoli are surrounded by a dense network of capillaries which allows oxygen to diffuse from the capillary into the alveoli and vice versa for carbon dioxide to be exhaled.Once the oxygen has entered the venous system it is dissolved into the haemoglobin producing oxyhaemoglobin which carries the oxygen around the body.
Any part of the respiratory system not contributing to gasesous exchange is known as dead space. Bits which never contribute to gasesous exchange e.g. trachea is known as anatomical dead space.
Mechanical dead space e.g. endotracheal tubing.
Physiological dead space e.g. lungs filled with fluid so are unable to contribute to gasesous exchange.