The gas exchange system is isolated from the rest of the body because of two things. First, the air is dirty and filled with viruses, bacteria, mold spores, pollutants, dust and dirt with an occasional insect. These need to be trapped or prevented from entering this gas filtering system. The second is a membrane system that allows gas molecules to pass from the outside air through a thin respiratory membrane, through a narrow blood vessel (capillary) and attach to a blood cell for distribution throughout the body. As with the other body systems, this air exchange system also eliminates waste products from the body (carbon dioxide, hormones, medications) to the outside air.
Billions of years ago, the air on this planet was mostly methane and carbon dioxide. With the development of plants, the oxygen content increased. The one-cell organisms that were precursors to the animals we know today (horses and humans) needed to adapt to this change in gas composition. The development of lungs in mammals was part of this adaptation.
This evolution included the pathways for this air to the lungs from our nostrils (air intakes). It needed to be warmed when the air was cold (turbinates). It needed to be filtered (hairs). It needed moistening (secretions). And on the way by, horses might as well use air to make noises (vocal cords, larynx). It also had to block food from getting in (the epiglottis). It needed to expel foreign objects (cough) and fight off bad guys (inflammation, exudate). Finally, it must help equalize the pressures outside the eardrums (Eustachian tube). The horse developed guttural pouches and a complex sinus system to add some uniqueness.
The air we all breathe today is 78% nitrogen, only about 20% oxygen, and about 2% other gasses. Oxygen above 20% becomes toxic. When the air gained more oxygen billions of years ago, bacteria moved into our cells (called mitochondria), allowing for converting oxygen into energy. A good symbiotic relationship formed because most one-cell organisms died from extra oxygen. Today the multi-cell organism (horse and human) takes in air gasses and delivers oxygen to the mitochondria in the cell, which keeps them alive. The mitochondria, in return, make the energy the body cells need to remain alive.
What goes out in the expelled breath is very interesting. Carbon is in every part of our body (sugar, fat and protein), and when a cell uses fuel to create energy, the waste product for all animals is carbon dioxide (CO2). Interestingly, carbon dioxide (CO2 and 48 atomic mass) is heavier than water (H2O and 18 atomic mass), so most long-term weight loss is the loss of carbon and oxygen through our breath, not the lighter, short-termed water loss. Carbon dioxide, then, is the most abundant material removed.
Other molecules are in expelled breath, water being the most common, adrenaline, epinephrine, and other hormones. Control of the respiratory rate and depth of breathing is by kidney and brain sensors. It is determined by the need for oxygen metabolism and the amount of carbon dioxide accumulating in the body. Just hold your breath or exercise to see this happen in you. Therefore, exercise physiology is important in understanding how to train your horses.
Problems in the respiratory system include trauma (pneumothorax), blockage (sinus cyst, collapsed vocal cord), immune reaction (allergic bronchitis, COPD) and infection (viral, bacterial, fungal) of any part.
Physics and math play an important part in this airflow. The volume of air entering the lungs is directly proportional to the area of a circle. The area of a circle equals π (pi) times radius squared (π r squared). Any slight decrease in radius (inflammation, allergy) leads to a massive reduction in the volume of air delivered to or from the body. Laminar flow also describes the increased friction of fluid or gas passing next to a vessel wall. Liquid or gas moving down the center of the pipe will move faster. Turbulence, created from any blockage within an airway (inflammation, larynx problem), disrupts air flow normally in laminar flow, and the decreased radius decreases volume. Think of a traffic accident disrupting the flow of cars on a highway.
Any disruption of laminar flow or decreased radius will lead to a reduced exchange of air needed to fuel the body and is usually seen as exercise intolerance at best and, at worst, pneumonia and death. Drowning, for example, is the total absence of gas exchange leading to a lack of oxygen for cell metabolism. Without air, the cells die in a few minutes. On the other hand, a 10% decrease in air delivery will reduce the ability to metabolize fuel and not cause death but rather exercise intolerance.
One more thing. The lungs have a microbiota consisting of trillions of bacteria that live in the lungs. They help defend the lining of the lungs and are essential to their health. When bad bacteria enter the lungs, this disrupted microbiota damages the lung tissue. Not much is said about these respiratory bacteria, as most research looks at the gut microbiota. Still, bacteria live everywhere, including inside the lungs, sinuses and nostrils.
I expect research about the protective benefits of the respiratory microbiota will be done, but deep breathing should be considered too. Why? Because you need to think of the lungs like a kitchen sponge filled with tiny air bubbles. When the sponge is squeezed by your hand and then placed in a water bowl, it fills with water upon releasing the squeeze. Take the sponge out of the water and, again, squeeze it hard. This removes almost all the water. The lungs work similarly in that when you fully express the lungs (exhale), they become devoid of air, but the squeezing also allows in all of the permitted blood in the lungs. When you fully inflate the lungs, you fill them with air pressing out all the blood. This massive exchange does two things. First, it completely replaces the old air and blood with fresh air and blood. Second, it prevents unexpanded lung sections from becoming solid from disuse (atelectasis). No exchange of air and blood will occur with atelectasis (permanent loss). Taking deep breaths by exercising the horse hard for a brief period a few times a week will prevent atelectasis caused by disuse or adhesions from inflammation, promote the replacement of the stagnant blood and keeps the good bacteria in the respiratory tract healthy as they are most likely air loving bacteria (aerobic). Air-hating bacteria (anaerobic) would love stagnated blood and solid lung tissue.
So sprint with your horse once in a while and feel the difference.
Here is a short video to see the breath of a horse to get you to start thinking about how critical breathing really is to all of us.