All About - The Horse's Respiratory System

This system's main task is to take oxygen from the air and pass it into the blood stream.  Without oxygen all activity will cease and the horse will die within a few minutes.  The respiratory system is also responsible for:

• removing carbon dioxide from the blood 
• helping temperature control by breathing out warm air and taking in cool air 
• to eliminate water 
• to communicate by sound 
• sensory - smell and touch via the nostril hairs 
• filter for air-borne invaders

The System.

Getting oxygen in .....

• Air is taken in through the nostrils (horses do not breathe through their mouths).  The nostrils can change shape to form part of a facial expression and will become bigger if the horse needs to take in more air.  The nostrils also blow air out as a snort to communicate and the small hairs (cilia) catch air-borne invaders eg: dust particles. 
• The air then passes into the nasal passages. One for each nostril. These nasal passages are separated from the mouth by the hard palate - as it gets nearer to the throat it becomes the soft palate!  Whilst in the nasal passages the air is warmed by the mucous membranes which cover the turbinate bones.  These bones are wafer-thin and curl so that they have a large surface area.  Air must not strike too cold on the lungs.  By warming this air the body is losing some heat when the air is expelled.  The membranes contain the olfactory nerve endings - which detect smells. 
• The air then passes into the pharynx (which is a common passage for air and food).  The epiglottis closes to cover the respiratory passages when the horse swallows so that food passes down the oesophagus instead. Thus when stomach tubing a horse it is imperative to pass the tube up the nostril.  
• Air passes from here into the larynx (this is also where the voice box is located). The larynx controls the air going in and out and monitors it for foreign objects. 
• The trachea (or windpipe) then takes the air.  It is made up of rings of cartilage and is lined by cilia again to help filter the air.  The lining also produces mucus which traps the foreign particles.  These particles are then wafted back up the trachea to the larynx by the cilia where the mucus (containing the particles) is coughed up and swallowed.

•  The trachea then divides into 2 bronchi at the entrance to the chest, one going to each lung!  Each lung is covered by the pleura, a smooth membrane which prevents friction. 
• Within each lung the bronchi then divide and subdivide into bronchioles which are also protected by cilia and mucus.  
• The bronchioles then divide into little alveoli which end as alveolar sacs.  These look like bunches of grapes!  The sacs have thin walls which are covered with capillaries - see my Circulatory System blog coming up next week.  Basically these are tiny blood vessels where the exchange of oxygen and carbon dioxide take place.  Oxygen is passed out of the air into the capillaries and carbon dioxide and some water vapour are passed out of the blood and into the air!

• These capillaries then take the oxygenated blood into the pulmonary vein which carries it to the heart.  See my Circulatory System blog - next week.

Getting carbon dioxide (and other waste) out ....

• The pulmonary artery brings deoxygenated blood (it has given it's oxygen to the body) back to the lungs.  
• In the lungs the pulmonary artery divides and subdivides, these thin walled capillaries then come into contact with the alveoli.  Here (in a similar process to getting the oxygen in) the carbon dioxide passes out into air which is expelled along the same route as it came in!

Both the oxygen and carbon dioxide are carried in the blood attached to haemoglobin. This will also be in next Friday's blog.

How a horse breathes.

Breathing is the process by which air is taken into the lungs and also where the body tissues use the oxygen.

A horse draws the air in by muscular expansion and contraction of the rib cage (thorax), it is expelled by elastic recoil of the rib cage!  The thorax is separated from the abdomen by the diaphragm.  The diaphragm is dome shaped (with the top towards the front of the horse) it flattens, by muscular contraction, to allow the horse to take a deep breath.  If the stomach or intestines are full the diaphragm is not able to flatten, so limiting the amount of air the horse can take in!  This is why we don't work our horses on a full stomach! (see my feeding blog)

A horse at full exertion will take 5 times the amount of air in that it will at rest.  The lungs are NEVER totally empty.

Understanding a bit about the respiratory system again helps us understand our horses and why we are advised to do certain things!  eg: leaving a horse for 1 - 2 hours after feeding before they are worked!


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Until next time!

Jo