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A I RWAY ANAT OMY
SPRING 2011 ISSUE
EQUINE MATTERS
16
The pharynx
The nasal passages enter the throat into
an area called the nasopharynx. The oral
cavity enters at the back of the mouth into
the oropharynx. These regions then form the
pharynx. Here nerve activity controls the
movement of food down into the oesophagus,
and the movement of air towards the lungs.
At the back of the nasopharynx are small
slits, which make the openings of the guttural
pouches. Guttural pouches are the
enlargements of a tube that starts at the
horse's inner ear. They are large cavities,
and many of the most important blood vessels
and nerves of the head run along their walls.
Again, their function remains unknown.
The larynx
The larynx lies at the
back of the pharynx
and at the entrance of
the windpipe (the trachea). It is a complex
arrangement of cartilages controlled by
ligaments and nerves. It protects the airway
when horses swallow, so stopping food
entering the windpipe. It controls the amount
of airflow entering the lungs. It is also
involved in producing vocal noises.
The larynx does all this by using a pair
of cartilages, called the arytenoids. These
cartilages can move towards and away from
each other to increase or decrease the size
of the opening into the trachea. When a
horse is eating, the arytenoids will close the
opening. When a horse is exercising, the
arytenoids will be raised out of the way
allowing a resistance free passage for air to
travel into the lungs. A cartilage flap called
the epiglottis holds the horse's soft palate
clear of the breathing hole.
The trachea
Bifurcation of the trachea
into the two bronchi.
The windpipe or trachea is a rigid circular
tube that transports air from the larynx to the
lungs. It is composed of rings of cartilage
joined together by sheets of connective tissue.
This means that it is very difficult for the
trachea to collapse, whatever position the
horse's neck is in. The trachea contains cells
which produce mucus, a substance which
will trap the inhaled dust and dirt, and
specialised cells called cilia which have long
tails. These constantly beat the mucus up into
the pharynx where it is swallowed.
The lungs
As the trachea enters the lungs, it splits into
two bronchi. One bronchi enters each lung.
The bronchi constantly divide into smaller
tubes called bronchioles. These bronchi further
subdivide rather like the branches of a tree.
The smaller airways are again lined with
mucus producing cells and cilia to remove
trapped dust. The bronchioles can expand
and contract slightly to increase or decrease
the size of the airway.
At the end of the bronchioles are the alveoli.
These are tiny sacs with very thin walls, with
a rich blood supply. As the air reaches these
sacs, the oxygen is dragged into the red
blood cells in the capillary walls and
transported around the body. At the same
time, the red blood cells release their load of
carbon dioxide which is a waste product
picked up from the body tissues. The carbon
dioxide is released into the alveoli.
The pleural cavity
The lungs sit within the horse's chest cavity,
lined by a thin membrane called the pleura.
The pleura forms a closed sac around the
lungs. This means the lung space is an area
of negative pressure which allows the lungs
to easily expand and contract.
The lungs are bordered by the ribs around
their outside. The diaphragm is a thick
muscular sheet along the bottom of the
lungs which separates them from the
horse's abdomen.
Breathing
As a horse breathes, the chest wall and ribs
move upwards and outwards, increasing the
width of the chest. The diaphragm contracts,
moving downwards, increasing the depth of
the chest. This allows the lungs to expand as
they fill with air, and they can now fill the
larger chest cavity. Air is drawn into the lungs,
and follows the passageway to the alveoli.
Gas exchange occurs - oxygen is exchanged
for carbon dioxide.
As the horse breathes out, the chest muscles
and the diaphragm both relax. The chest
cavity now gets much smaller. The elastic lung
tissue now recoils. These actions drive air out
of the lungs and back into the atmosphere.
MARK TAB ACHN I CK
Fact...
At maximal exertion a horse can breathe
in as much as 4,500 litres of air in one minute!
COMMON
RESPIRATORY
CONDITIONS
UPPER RESPIRATORY TRACT
Idiopathic Laryngeal Hemiplegia (ILH)
This condition is often known as roaring
or whistling. In ILH, horses lose the nerve
supply to the left arytenoid cartilage.
When horses exercise, they can no longer
move this cartilage back and forward.
It sits within the airway causing the horse
to make a whistling type of noise as they
breathe in at the canter. This obstruction
will decrease the amount of air a horse
takes in at exercise, and so may
decrease their athletic performance.
Dorsal Displacement of the Soft Palate
(DDSP)
DDSP is a condition that occurs at intense
exercise, and is commonest in young
race-horses and eventers. In this condition
the soft palate, which is usually held in
place by the epiglottis, flips out of position
and billows loosely at the back of the
pharynx, often blocking the airway. The
condition is usually transient. Riders report
that the horse was going well at the gallop
until suddenly choking or gurgling and
pulling up. The soft palate will often
return to its usual position very quickly.
LOWER RESPIRATORY TRACT
Recurrent Airway Obstruction (RAO)
This condition used to be called Chronic
Obstructive Pulmonary Disease (COPD).
It is often called heaves. Very commonly,
this is an allergy to dust in the environment
and results from horses being kept in
poorly ventilated stables. Horses suffering
from RAO have fast shallow breathing,
can cough and be quite distressed. It can
be treated by using medications which
open the airways making breathing
easier. Good quality air flow is the key to
management. Always ensure the horse is
kept in a well ventilated stable.
Summer Pasture Associated Obstructive
Pulmonary Disease (SPAOPD)
This condition can be similar to RAO, but
occurs to horses out at grass often during
the summer. Horses show very similar signs
to those with RAO, but are allergic to
pollens in the atmosphere rather than dust.
It can be managed by stabling the horse
away from the inciting pollens.