Respiratory
Introduction
The respiratory tract includes the nostrils, nasal passages, sinuses, trachea (wind pipe) and the lungs and is responsible for providing oxygen for the blood to be transported to all the tissues of the body.
Breathing is controlled by involuntary centres in the brain that can control the respiratory rate by detecting changes in carbon dioxide, pH, temperature and blood pressure and then passing this information onto the respiratory muscles. The air that is breathed in ends up in the lung tissue where there are many blood vessels.
Oxygen is moved into the blood and at the same time carbon dioxide is removed to be excreted on expiration. The name for this area of the lung is called the blood gas barrier and is the reason why the heart and circulation and respiratory tract must work in perfect harmony to maximize the exchange of these gases. During rest respiration rates are at around 12 breaths per minute and works very efficiently, however, during exercise the demand for oxygen increases greatly so the body must respond to provide this. There is an increase in the respiratory rate as well as an increase in the amount of air in each breath (tidal volume) and the nostrils will become flared.
The heart will increase its output and diffusion at the blood gas barrier will increase. The amount of haemaglobin (the carrier of oxygen in the blood) in the blood also needs increasing, this is done by the spleen releasing it’s stores into the blood stream. Despite all these changes the respiratory tract is the limiting factor in the exercise levels of a horse as if the tissues could be supplied with more oxygen they could still continue.
Unlike many other animals, including humans, horses are obligate nasal breathers, i.e. can only breath through their nostrils, so are unable to increase the amount of air they can breath in by breathing through their mouths. Any condition anywhere in the respiratory tract can disrupt this fine balance and cause horses to show signs of poor performance and/or respiratory disease, cough, nasal discharge, respiratory noise and laboured breathing.