Breathe easy
A trachea 85cm long, 20 litres of air inhaled per breath and no breathing through the mouth. Equine vet Hanja Dickinson MRCVS explains the intricacies of the respiratory system
Learn all about your horse’s respiratory system
HORSES PERFORMING INTENSIVE exercise can breathe more than twice each second and maximal oxygen uptake during exercise is 40 times as great as when the horse is at rest. The respiratory system is important and complex, which means it’s susceptible to developing issues. But fear not, a tweak in how you manage your horse, alongside being vigilant to the most common conditions your horse is susceptible to, can help keep his breathing apparatus in tip-top condition.
Respiration in action
As athletes, horses have to be efficient at providing large amounts of oxygen to the body quickly, and their breathing system is well adapted to allow them to do just this. An average 500kg horse breathes in 6.7 litres of air in each breath, but at maximum effort this can go up to 20 litres. This is a huge volume compared to an average person who breathes in only 400ml of air in one breath. At gallop, each stride is linked to one breath and horses need to maximise the volume of air taken in each time, to provide the level of oxygen required by the body. During canter or gallop, expiration of air occurs as the forelimbs are in stance phase and in contact with the ground; inspiration occurs when they’re in flight. Horses can only breathe through their nose because of the tight seal the epiglottis makes with the soft palate when they’re not eating. So air can only pass further through the respiratory tract when this seal is in place. When a horse breathes in, air first travels through the nostrils, which are able to expand during exercise. Each nostril is divided into two by cartilage, creating a false nostril (a blind-ended sac) and a true nostril that the air travels on through. In the nasal cavity itself are the nasal conchae — a complex maze of scroll-shaped bones. The conchae act to warm and humidify the air and trap any larger particles that may have been inhaled. Here, the nasal cavity communicates with the six pairs of sinuses in the horse’s head, which also help with warming and humidifying the air.
A myriad key structures
The air continues to pass through the pharynx towards the larynx, a complex area of anatomy at the entrance to the trachea and lower airways. It has several laryngeal cartilages, connecting muscles and ligaments, and the epiglottis. The epiglottis is an important structure that moves back to cover the trachea, preventing food from entering the airways during swallowing. If food were to enter the trachea, its sensitive cells would trigger a reflex cough to bring it back up into the pharynx, ensuring the lungs stay clear. The pair of arytenoid cartilages that sit symmetrically at the entrance to the trachea are pulled open and closed with each inhilation and exhilation. If the nerve controlling the muscle that pulls the arytenoid cartilages away is damaged, airflow down the trachea is disturbed and becomes turbulent.
Continuing further down the airways, the humid, warm, cleaner air now passes into the trachea. The trachea of an average horse is 75-85cm long and 5-6cm wide, supported by 50 to 60 cartilaginous rings. These rings are an incomplete circle that keeps the trachea flexible and permits large boluses of food to pass down the oesophagus, but also helps to prevent it from collapsing shut when negative pressure is created during inspiration. The trachea is lined with specialised cells called ciliated mucous epithelium. These are sticky, so that dust particles can be collected and moved back up the ciliated ‘hair’ cells in peristaltic waves — away from the lungs.
The final destination
At the end of the trachea, it splits into two slightly smaller airways called bronchi, into the left and right lung lobes. The bronchi branch into smaller bronchi and eventually into bronchioles, which are even thinner airways without any cartilage support. Both bronchi and some bronchioles contain muscle in their walls, allowing these airways to dilate and widen during exercise so a greater volume of air can reach all areas of the lung. At the end of the bronchioles are many alveoli, which are the very end of the respiratory tract. The alveoli exchange the oxygen delivered in the fresh air in the lungs into the blood, and remove waste gases produced in the body into the lungs to be expelled when the horse breathes out again. Alveolar sacs appear like a bunch of grapes with a large surface area, to allow for the most efficient exchange of gas with each breath. A meshwork of thin blood vessels called capillaries surround the alveoli to collect and deliver the fresh oxygen to the rest of the body. The oxygen is then used in cells in a process called cellular respiration to create the energy used in exercise and daily life.
Looking after the lungs
A dust-free environment will help to reduce exposure to environmental allergens, so swap straw bedding for shavings or other non-organic beddings, soak haynets and dampen dusty feeds. Improve ventilation in your horse’s stable — there should always be a circulation of fresh air, which will minimise stagnant air and reduce the accumulation of dust. Where possible, except in cases of pollen allergy, turn your horse out for a period of time each day. Store hay and bedding materials away from your horse’s stable to decrease the dust created in his environment. Keep his bed clean and avoid deep litter beds, as a build-up of ammonia can irritate the airways, increasing the chances of respiratory disease.
“A process called cellular respiration creates the energy used in exercise and daily life”