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.
Other Functions of the Respiratory System
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.
The Peculiarities of the Horse’s Respiratory System
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
- The horse does not breathe through its mouth and nose like we do. The horse only breathes through its nostrils. The nasal passages in the horse are separated from the oral (mouth) cavity. They do not breathe through their mouths unless they have some injury or abnormality to the soft palate (the structure that separates the mouth from the nasal passages).
- At canter and gallop normal horses take one breath perfectly in time with one stride. This is referred to as respiratory-locomotory coupling. A normal horse may swallow 1-2 times during each minute of exercise, but no more. The amount of time taken to inhale is the same as the time taken to exhale.
- The amount of air moved in and out of the lungs increases in direct proportion to how fast the horses is running. If a horse runs twice as fast it must move twice as much air in and out.
- During exercise, when horses inhale, around 90% of the resistance (obstruction) to air movement is in the airways that are in the head, namely, the nostrils, the nasal passages and the larynx. But when horses are exhaling the majority of resistance to air movement (55%) is in the airways within the lung.
- If you tighten a horse’s girth too much, then it will affect it performance not because of constricting the chest and preventing the lungs from expanding but because it decreases the effectiveness of the muscles around the front of the chest and shoulder that move the forelegs.
- Horses do not breathe by expanding and contracting their chest during canter and gallop. They expand and contract the chest when breathing at rest, when breathing at walk and trot, and perhaps most noticeably when blowing hard after exercise. But during canter and gallop, the air moves in and out along the lines of a syringe with the stiff wall of the syringe representing the chest and the plunger the diaphragm i.e. all air movement during canter gallop comes from movement of the diaphragm.
- Horses hold their breath over jumps and do not breathe again until they land, starting with breathing out.
- You cannot train the respiratory system of the horse. Plenty of books will tell you that you can. A number of scientific studies show the reverse. The amount of air moved in and out by an unfit horse at a fixed speed will be the same 6 months later when that horse is fully fit.
- The blood pressure in the blood vessels within the horse’s lung (referred to as pulmonary blood vessels) during galloping increases around 4-5 times above that at rest.
- This is one of the factors that puts stress on the very thin walls of the blood vessels and leads to some of them rupturing.
To learn more about the Equine Respiratory System, read The Growing Physical Demands of Modern Equestrian Sport, by Dr. David Marlin, Eventing, Issue Two, 2008.