Page 415 - WSAVA2018
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J. Hatcher1, C. Harvey Stevenson1, M. Oleary1
1Provet AIRC, Animal Industries Resource Centre, Brisbane, Australia
Jo Hatcher Cert IV VN, TAE, Dip VN, AVN Brisbane, Australia.
It is without question that adequate oxygen supply to all tissues and cells is essential for life. For oxygen to be delivered to the cells there must be adequate oxygen exchange in the lungs to the blood and therefore adequate pulmonary function is critical.
The Respiratory System
The respiratory system works in conjunction with the blood and cardiovascular system to provide the body with the required exchange of oxygen and carbon dioxide.
External respiration is the gaseous exchange of O2 and CO2 between the environment (air in the lungs) and
the blood. Pulmonary ventilation and diffusion of gases through the alveolar membrane of the lungs as required in order for external respiration to take place.
Internal or tissue respiration is the exchange of O2 and CO2 between the blood in the capillaries and the tissue cells.
The respiratory system is referred to in two parts:
· Upper respiratory system; structures that are outside of the lungs – nose, pharynx, larynx, trachea
· Lower respiratory system; structures within the lungs – Bronchi, Bronchioles, Alveolar ducts and Alveoli
Bronchi and Bronchioles
The primary bronchi are the first branches of the trachea. They are quite large with C-shaped chondral rings like the trachea. They continue to branch, becoming smaller as they progress, resulting in secondary and tertiary bronchi. They are kept tubular by flattened, overlapping, curved cartilages. As branching continues into the smaller bronchioles, the cartilage component to their walls disappears when the diameter has reduced to 1mm or less. The respiratory bronchioles then give rise to the alveolar ducts and alveoli.
After the bronchi enters the lung, each main bronchus divides into even smaller bronchi and then finally into even smaller bronchioles. The bronchioles continue
to subdivide down to the smallest air passages – the microscopic alveolar ducts. The alveolar ducts then end in groups of alveoli – a bit like a bunch of grapes – called alveolar sacs.
· · · · · · · ·
Open mouth breathing Panting
Extension of head and neck
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The bronchial tree is not a rigid tube; the diameter of each one can be adjusted by the smooth muscle fibres in its wall. The autonomic (unconscious) portion of the nervous system controls this smooth muscle.
Bronchodilation - This is when the smooth muscle relaxes (e.g. during times of intense physical activity) allowing the air passageways to dilate to their maximum capacity which helps the respiratory effort move the greatest amount of air back and forth into the alveoli with each breath.
Bronchoconstriction - During times of rest, the smooth muscle partially contracts reducing the size of the air passageways which in turn lessens the workload for the respiratory tract. However, some respiratory irritants and disease can cause severe bronchoconstriction making it difficult for the animal to breathe.
Alveolar ducts and Alveoli
External respiration takes place in the alveoli. This
is when oxygen and carbon dioxide are exchanged between blood and the air in the alveoli. The rest of the respiratory structures just move air in and out of the alveoli.
The alveoli are tiny, thin walled sacs that are surrounded by a network of capillaries. The wall of each alveolus
is made up from the thinnest epithelium in the body
– simple squamous epithelium. The capillaries that surround the alveoli are also made up from simple squamous epithelium.
As these two layers are so thin, they freely allow oxygen and carbon dioxide to diffuse between the air and blood.
Each alveolus is lined with a thin layer of fluid that contains a substance called surfactant. This surfactant helps reduce the surface tension (the attraction of water molecules to each other) of the fluid. This prevents the alveoli from collapsing as air moves in and out during each breath.
Respiratory membrane
This is the alveolar and capillary wall through which gas exchange takes place. The alveoli make up the bulk of pulmonary tissue.
Dyspnoea can be defined as the sensation of difficulty in breathing, which is experienced by patients suffering compromised respiratory function.
The sensation is initiated by either hypoxaemia (low oxygen levels) or hypercapnoea (high carbon dioxide levels).
Signs of Dyspnoea / Respiratory Distress”

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