Align-ju STUDENT
Epiglottis Larynx
Thyroid cartilage Cricoid cartilage Trachea
Left bronchus Bronchial tree
Left lung
The bronchi divide as they enter a lung lobe , where they continue to divide into smaller bronchioles until they reach their terminal bronchiole . This subdivision of the bronchi is called the bronchial tree . As the bronchi divide , the cartilage becomes sparser and eventually disappears altogether [ 1 ] .
The area from the bronchi to the alveoli looks like upside-down trees or broccoli . At the end of each terminal bronchiole is an alveolar duct , which terminates at an alveolus . Alveoli are vascularised air sacs where the gaseous exchange in the lungs takes place [ 3 ] .
Gaseous exchange is a process by which oxygen enters the blood and carbon dioxide leaves the blood . Molecules move by diffusion down a gradient , from a high concentration to a lower concentration . Oxygen diffuses from the alveoli into the blood , and carbon dioxide diffuses out of the blood through the alveoli into the lungs to be breathed out ( Figure 3 ).
Pleural covering |
Right lung
Alveolus
Alveolar sac
|
Alveolar duct |
Alveolus |
Air in and out |
Figure 2 . The lower respiratory tract . Adapted from Colville [ 1 ] .
The respiratory system starts at the nose , which is divided into left and right chambers , partly filled with nasal turbinates . The turbinates are lined with a ciliated mucous membrane that moistens and warms the air entering the respiratory tract . The membrane also filters and traps particles in the air entering the nasal cavity , preventing the particles from moving further into the respiratory tract .
The pharynx is the area at the back of the mouth , often referred to as the throat . The nose , mouth , eustachian tubes from the middle ear , oesophagus and larynx all connect to this area [ 2 ] . The pharynx divides caudally to the oesophagus ( as part of the digestive tract ) and ventrally to the larynx .
The larynx is a rigid hollow structure made of cartilage , which forms the opening to the lower respiratory tract . The opening to the larynx is called the epiglottis , and it is this structure that is visualised and gently pushed down like a drawbridge when placing an endotracheal tube during intubation . The larynx is also known as the voice box ; it contains structures that enable the animal to make sounds .
The trachea is a tube made of incomplete ( C-shaped ) rings of hyaline cartilage . Between the cartilage rings are bands of fibrous connective tissue and smooth muscle lined by ciliated epithelium . The trachea enters the thorax at the thoracic inlet and branches into the left and right bronchi , just above the heart .
Blood vessel ( capillaries )
Figure 3 . Gaseous exchange . Adapted from Colville [ 1 ] .
The lungs are two spongy organs in the thorax , one either side of the mediastinum . They are connected to the trachea by the left and right bronchi . Both are divided into separate lobes . The right lung has four lobes : the cranial lobe , middle lobe , accessory lobe and caudal lobe . The left lung is divided into three lobes : the cranial lobe , middle lobe and caudal lobe . The lobes are divided by fissures , which help to increase the surface area and maximise gaseous exchange , and can enable the body to ‘ wall off ’ a problem area of the lung , if necessary , and divert blood to healthy regions .
Each lobe has its own arterial and venous supply . The pulmonary artery branches many times as it follows the bronchioles , before becoming pulmonary capillaries near the alveoli . Once the blood has been reoxygenated , it moves from the alveoli via capillaries to the pulmonary veins , before returning to the heart to be circulated around the body ( Figure 4 , page 56 ).
Volume 39 ( 3 ) • June 2024
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