| The Respiratory System of The Horse |
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By Stephen Champion, BSc (Hons) BVM&S MRCVS, We can be in no doubt that the horse is a magnificent athlete. This is due in part to its unique musculo-skeletal system, but also to its highly efficient respiratory system. Here, the oxygen from the atmosphere, used to fuel its powerful muscles, is captured by its lungs and transferred to its blood stream, where it is used to create energy. The essential organs of respiration are the horse’s lungs. However, to reach the lungs, air must pass through the nose, pharynx, larynx and trachea (the windpipe) before reaching the lungs. The conspicuous external nose forming a feature of the human face is hardly recognised in the horse, but is merged with the contours of the muzzle. The nostrils can be greatly dilated when the horse is performing strenuous exercise, hence allowing an increased airflow into the respiratory tract. The nasal cavities of the horse contain turbinate bones, which help filter and warm the air flowing through them. They also contain large portions of the embedded upper cheek teeth, which grow down as the horse ages. The larynx forms the connection with the pharynx and the trachea. It is made up of several cartilages joined together and held in place by a complex system of small bones, called the hyoid apparatus. It is common for one of these cartilages to collapse inwards, especially in large breeds. This partially blocks the airway, reducing performance and often causing a pronounced whistle, which your vet will listen for when “vetting” a horse for purchase. The condition is aptly named laryngeal paralysis. The larynx was originally developed as a device to protect the lower airways against the entrance of food and drink. It achieves this by moving forward when the horse swallows. The part of the larynx called the epiglottis comes against the root of the tongue forming a cover over the respiratory passages. Hence food rarely goes “down the wrong way”. The other function of the larynx is in phonation – the production of the horse’s voice, which developed later in its evolutionary history. The trachea and bronchi are a tree-like structure of tubes leading from the larynx to the lungs, and running down the neck. The construction of the trachea prevents it from collapsing and allows it to make the necessary adjustment in length when the horse stretches over a fence! It achieves this by being composed of many cartilage rings held together by strips of muscle. The tissue on the inside of the trachea continually secretes mucus, which catches small particulate matter and sends it back up to the pharynx on a small “conveyor belt” of cells called cilia, where it is swallowed. As the air reaches the end of the trachea it enters the two bronchi and then moves down into smaller bronchioli in the lungs. The lungs of the horse are contained in complicated anatomical structures called the pleurae, which are thin, membranous sacs lining each half of the chest. They are folded, forming a potential space called the pleural cavity, which can become infected when a horse develops a chest infection. This is commonly called pleurisy, and can be very serious. The purpose of the pleurae is to lubricate and facilitate the movement of the lungs within the chest cavity. They also serve to divide the chest into two halves, along the mediastinum. The lungs of the horse serve to bring together the air arriving at them with the blood flowing through them. They are divided into lobes and have a soft, spongy texture. Ultimately they are divided into tiny chambers called alveoli, like microscopic bunches of grapes, where gaseous exchange takes place. Blood flows from the horse’s heart to the lungs in the pulmonary arteries. It is the only occasion in the body where arteries (blood vessels taking blood away from the heart) carry de-oxygenated blood. In the lungs, this blood passes very close the air in the alveoli and picks up the vital oxygen, returning as oxygenated blood (blood containing oxygen) to the heart, through the pulmonary veins. The heart then pumps this oxygenated blood through the aorta and arteries to the tissues that need it, which convert it into energy. The respiratory system of the horse is thus a highly efficient system that can adapt to the demands placed upon it to provide oxygen to the body’s tissues and organs. Only when it is functioning perfectly, can the horse perform to its true potential! |
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