By the time a horse crosses the finish line in a 5 furlong race, has completed a Grand Prix showjumping round or gone 1/6th of the way round a 3-star cross-country course it will have moved somewhere around 1800 litres of air in and out of the lungs. If you find 1800 litres hard to visualise, then think of 6 baths of air. This equates to moving two buckets of air into and out of the lung every second. The air breathed in (inhaled) during a race will consist of around 380 litres of oxygen (the rest being made up of the gas nitrogen), and the horse will take up into the blood and use around a quarter of this oxygen i.e. 95 litres.
Of the total amount of energy the racehorse needs to get from the starting gate to the finish in the 5 furlong race, around 70% of this will come from aerobic metabolism (also around 70% for showjumping and 90% for cross-country). Aerobic metabolism is essentially the process of getting energy from glucose [carbohydrate] in structures inside the muscle cells known as mitochondria using oxygen. The remainder comes from anaerobic metabolism – this also breaks down glucose to generate energy but this process can work whether oxygen is present or not. The main difference here is that anaerobic is very fast but inefficient, and can be used for only a short period of time due to build up of lactic acid, whilst aerobic is not so fast but very efficient at generating the energy to run.
So, even in a race or jumping round lasting less than a minute, the majority of the energy generated by the muscles must come from using oxygen to "burn” carbohydrates. Even in a barrel race, around 40% of the energy to run still comes from aerobic metabolism. These examples underline the importance of a respiratory system. The harder a horse works, the more oxygen it needs and the more air it must move into and out of the lungs. In fact, these are so tightly coupled that if a horse doubles its speed, it will need to double the amount of air moved into and out of the lungs.
The respiratory system moves air containing oxygen from outside the body to inside the lungs bringing the oxygen as close as possible to the blood in the circulation.
Air moving outside the body passes first through the upper respiratory system including the nostrils, the nasal passages and larynx and then into the trachea (or windpipe).
The horse’s windpipe is around 5-8cm in diameter nearest the larynx, but as it passes deeper in the lung it begins to divide to produce smaller and smaller airways, much like a tree on its side. Each time an airway divides in two, the "daughter” airways are smaller than the "parent” from which they arose. When we get down to the level of the smallest airways, after perhaps 25 divisions, the airways are fractions of a millimetre in size. When the air gets to this point in the chain from nostril to muscle cell, it has to cross from the air space ("alveoli”) into the blood vessel At this stage, the membranes separating the oxygen containing air in the alveoli from the red blood cells in the blood vessel are only the thickness of 1/100th the width of a human hair. The transfer of oxygen from the alveoli across this thin membrane and into the blood does take place by the process of diffusion i.e. the oxygen moves from high (in the air) to lower (in the blood). Incidentally, the total area for oxygen to diffuse across in the horse is equivalent to the area of 10 tennis courts!
Once in the bloodstream the oxygen is bound to haemoglobin (the molecule inside red blood cells that makes blood red) and then the oxygen rich blood is pumped around the body by the heart. At the muscle the reverse process takes place with oxygen leaving the red blood cells and crossing into the muscle cells, again by diffusion, because the oxygen in the blood is higher than in the muscle cells. A final step of diffusion takes place within the muscle cells as the oxygen moves to the areas within the cell where the oxygen content is even lower – inside the mitochondria. So by the time it gets inside the mitochondria, the level of oxygen may only be around 1/80th of that in the air outside the horse.
There are two particularly important facts about the horse’s upper and lower respiratory system. First, with regard to the upper respiratory system, unlike humans, horses can only breathe through their nose. During exercise inspiratory pulmonary resistance approximately doubles. 50% of the total resistance originates in the nasal passages. Because the nasal valve is the narrowest point in the nasal cavity, it is a major contributor to nasal resistance.
Second, with regard to the lower respiratory system, for maximum efficiency of oxygen transfer from the alveoli to the red blood cells the membranes between are very thin- as mentioned above, abou 1/100th the thickness of a human hair. Understanding the importance of thin separating membranes for maximum oxygen transfer efficiency, may now help to understand, perhaps not surprisingly, that these small membranes can rupture under the stress of exercise allowing the red bloods cells (RBCs) to spill from the capillaries into the alveoli, which we term exercise-induced pulmonary haemorrhage (EIPH or "bleeding”).
Several independent clinical studies have now proven that that by reducing nasal resistance at the nasal valve by use of FLAIR® Nasal Strips, directly reduces EIPH occurring due to rupture of the membrane between the alveoli and rbc’s in the lower respiratory tract.
You may already be picking up that efficient oxygen transfer from the airways to the red blood cells is a potentially very important limiting step for a horse’s ability to exercise. In fact, it is documented that some of the best racehorses (especially those racing over middle and longer distances) have large hearts and or a high capacity to use oxygen – something referred to as maximal oxygen uptake or aerobic capacity.
So the primary function of the respiratory system is to bring oxygen in air down into the lungs where it can pass across a thin membrane into the blood and then be pumped around the body.
One of the other important functions of the respiratory system is to get rid of carbon dioxide; a waste product produced within the mitochondria of muscle cells during exercise. This is effectively the same as the process for bringing oxygen in but in reverse. Carbon dioxide moves out of the cells by diffusion as the concentration of carbon dioxide inside the cells is higher than in the bloodstream. When the blood reaches the lungs, the carbon dioxide diffuses out across the membrane and into the airways because the concentration of carbon dioxide in the airways is lower than in the blood. The carbon dioxide is then exhaled (breathed out). Accumulation of carbon dioxide is not a good thing and can itself contribute to the development of fatigue during exercise so its important that as much as possible is exhaled as fast as possible.
The lung is also a very important filter. All the blood in the circulation passes through the lungs when it comes back in veins from being pumped out around the body in arteries. As such, the lung is ideally placed to filter out any small blood clots (thrombi) or gas bubbles (emboli). It may not be great to have a pulmonary embolism (a gas bubble in the lung), but its still highly preferable to this going through the lung and lodging in a coronary (heart) vessel or the brain. The lung also has a better capacity to deal with bubbles and clots than most other organs in the body.
The lung is also able to activate or deactivate certain hormones in the circulation and in some cases the lung acts as an endocrine organ, actually releasing hormones which can have effects on the whole body.
The skin, the lung and the gastro-intestinal tract are the body’s interfaces with the outside world. The lung therefore has a highly developed immune system different to that in other parts of the body with specialised types of white blood cells to deal with things that could be inhaled, such as particles, bacteria, fungi and virus.
Finally, perhaps one of the most important but often overlooked non-respiratory (i.e. not related to moving gases in and out) functions of the respiratory system is in control of body temperature (thermoregulation). If a horse is taken from a cool climate to a warmer climate, say to a temperature of around 85°F (30°C), then one of the first things that can be noticed is an increase in the rate of breathing at rest. Whilst the horse will also open up small blood vessels in the skin in an attempt to lose heat and may also sweat slightly, respiratory heat loss is an important thermoregulatory mechanism for the horse. In fact, we can take an opportunity here to dispel a common horseman’s myth. When horses blow after hard exercise it has commonly been believed that this is because they are trying to get more oxygen into the blood. In fact, from studies on treadmills where we can measure the blood oxygen levels during and after exercise, we know that whilst the blood oxygen level may fall during intense exercise, even as the horse is pulling up the levels return to and in fact go above the normal resting level. The main thing that controls blowing after exercise in horses is how hot they are not the blood oxygen level.
To some extent the horse is still an enigma. There is no other animal that can carry the weight of a person (often representing an extra 10-15% of its own bodyweight) and itself at speeds of up to 35 mph or even more. It may therefore not be surprising that the horse’s respiratory system displays some curiosities, especially when compared to ourselves.
10 Things You Might Not Know About the Horse’s Respiratory System
Exercise Induced Pulmonary Hemorrhage, or "bleeding" in horses, is a health problem that occurs in horses that work hard during activities such as:
Some studies report that horses bleed even when doing mild exercise such as trotting on a treadmill.
Many people believe that if a horse doesn’t show blood at the nostrils it’s not bleeding. Typically, bleeding is a silent injury that can go undetected by trainers and riders because it occurs deep in the lungs and is best detected by lung washes or endoscopy. In addition, blood in the airways has been shown to be an irritant that leads to further bleeding.
Reducing bleeding not only helps a horse perform better in the short term, but may also help in the long term by reducing the possibility of inflammatory airway disease and chronic lung damage due to repeated bleeding.
The first breakthrough in understanding the role of FLAIR®® Strips in reducing EIPH was by a research team at Kansas State University, which included Dr. David Marlin, formerly of the Animal Health Trust, Newmarket, England; and led by Drs. Howard Erickson and David Poole.
In 1999, the Kansas State group published the first data from studies of FLAIR® Strip on exercising Thoroughbreds. The data showed that horses affected by EIPH that wore a nasal strip had fewer blood cells in their airways after exercise when compared to the same horses not wearing the strip.
This group was the first to show that bleeding could be reduced by normalizing airflow with a nasal strip. The benefits of FLAIR® Strips during training are now well known.
Erickson’s team subsequently showed similar protective effects when horses were treated with the injectable drug, furosemide. However, rather than normalizing airflow, furosemide is a potent diuretic that works by reducing blood volume and pulmonary vascular pressures. Unfortunately, the drug reduces blood volume by increasing urine production, which consequently reduces fluid in the tissues and organs of the body. Side effects of furosemide use include dehydration, weight loss, and electrolyte loss, particularly potassium.
Clinical and university studies in equine science and sports medicine research have shown that the use of FLAIR® Strips reduces airway resistance, fatigue, lung stress and bleeding in horses during physical exertion.
Studies have also shown that horses wearing FLAIR® Strips conserve oxygen during exercise and experience a shorter recovery time following exercise.
These studies are summarised below:
"During exercise, the horse is an obligate nasal breather and inspiratory pulmonary resistance approximately doubles with 50% of the total resistance originating within the nasal passages."
"Nasal resistance accounts for approximately 50% of total airway resistance in exercising horses…Because the nasal valve is the narrowest point in the nasal cavity, it is a major contributor to nasal resistance."
"Analysis of these results suggests that the nasal strip increases the diameter of the nasal passage and the stability of the soft-tissue structures in the nose."
"Endoscopic examination of the nasal valve region during application of the nasal strip to the horse’s nose revealed an increase in the area of the nasal valve. The nasal strip tented the skin over the nasal valve, pulled the dorsal conchal fold laterally, and increased the cross-sectional area of the dorsal meatus." N. Edward Robinson, et al., "Effect of commercially available nasal strips on airway resistance in exercising horses", AJVR, 63:8, August 2002, pp 1101-1105.
In the study reported here, horses had decreased airway resistance and less negative inspiratory pressures during intense exercise when wearing the nasal strip." N. Edward Robinson, et al., "Effect of commercially available nasal strips on airway resistance in exercising horses", AJVR, 63:8, August 2002, pp 1101-1105.
"Several invstigations have also shown that the nasal strip significantly reduces EIPH severity in galloping horses, presumably by minimizing the negative airway and alvcolar pressures that impinge on the fragile blood gas barrier. In the current study, the reduction in EIPH severity was similar to that seen in previous submaximal and near-maximal exercise studies. This finding is intriguing and several explanations exist as to why the nasal strip appears to maintain its effectiveness over a range of exercise intensities…Thus, the nasal strip appears to be a viable prophylaxis for EIPH during maximal galloping and was at least as effective as furosemide in the present investigation."
"Previous work has demonstrated a significant reduction in VO2 at a given speed with nasal strips compared with control (approx 5%), likely due to a reduction in the O2 cost of breathing…and this was also the case in the current investigation, as time to fatigue was significantly elevated...".
P. McDonough, et al., "Effect of furosemide and the equine nasal strip on exercise-induced pulmonary haemorrhage and time-to-fatigue in maximally exercising horses,” ECEP, 22:33, pp 1-9, January 2004
"In the field, nearly 400 horses that wore nasal strips were evaluated at the Calder Race Course in Florida in 1999-2000. It was observed that horses with the strip had a win percentage 3.4% higher than horses that did not wear a strip. Horses wearing a nasal strip had a 15% decrease in the interval to the next race (23 days) compared with the race-to-race interval before wearing a nasal strip (29 days)."